Avr Gcc Warning Assignment Makes Integer From Pointer Without A Cast

Warnings are diagnostic messages that report constructions that are not inherently erroneous but that are risky or suggest there may have been an error.

The following language-independent options do not enable specific warnings but control the kinds of diagnostics produced by GCC.

You can request many specific warnings with options beginning with ‘’, for example to request warnings on implicit declarations. Each of these specific warning options also has a negative form beginning ‘’ to turn off warnings; for example, . This manual lists only one of the two forms, whichever is not the default. For further language-specific options also refer to C++ Dialect Options and Objective-C and Objective-C++ Dialect Options.

Some options, such as and , turn on other options, such as , which may turn on further options, such as . The combined effect of positive and negative forms is that more specific options have priority over less specific ones, independently of their position in the command-line. For options of the same specificity, the last one takes effect. Options enabled or disabled via pragmas (see Diagnostic Pragmas) take effect as if they appeared at the end of the command-line.

When an unrecognized warning option is requested (e.g., ), GCC emits a diagnostic stating that the option is not recognized. However, if the form is used, the behavior is slightly different: no diagnostic is produced for unless other diagnostics are being produced. This allows the use of new options with old compilers, but if something goes wrong, the compiler warns that an unrecognized option is present.

Issue all the warnings demanded by strict ISO C and ISO C++; reject all programs that use forbidden extensions, and some other programs that do not follow ISO C and ISO C++. For ISO C, follows the version of the ISO C standard specified by any option used.

Valid ISO C and ISO C++ programs should compile properly with or without this option (though a rare few require or a option specifying the required version of ISO C). However, without this option, certain GNU extensions and traditional C and C++ features are supported as well. With this option, they are rejected.

does not cause warning messages for use of the alternate keywords whose names begin and end with ‘’. Pedantic warnings are also disabled in the expression that follows . However, only system header files should use these escape routes; application programs should avoid them. See Alternate Keywords.

Some users try to use to check programs for strict ISO C conformance. They soon find that it does not do quite what they want: it finds some non-ISO practices, but not all—only those for which ISO C requires a diagnostic, and some others for which diagnostics have been added.

A feature to report any failure to conform to ISO C might be useful in some instances, but would require considerable additional work and would be quite different from . We don’t have plans to support such a feature in the near future.

Where the standard specified with represents a GNU extended dialect of C, such as ‘’ or ‘’, there is a corresponding base standard, the version of ISO C on which the GNU extended dialect is based. Warnings from are given where they are required by the base standard. (It does not make sense for such warnings to be given only for features not in the specified GNU C dialect, since by definition the GNU dialects of C include all features the compiler supports with the given option, and there would be nothing to warn about.)

Give an error whenever the base standard (see ) requires a diagnostic, in some cases where there is undefined behavior at compile-time and in some other cases that do not prevent compilation of programs that are valid according to the standard. This is not equivalent to , since there are errors enabled by this option and not enabled by the latter and vice versa.

This enables all the warnings about constructions that some users consider questionable, and that are easy to avoid (or modify to prevent the warning), even in conjunction with macros. This also enables some language-specific warnings described in C++ Dialect Options and Objective-C and Objective-C++ Dialect Options.

turns on the following warning flags:

-Waddress -Warray-bounds=1 (only with) -Wbool-compare -Wbool-operation -Wc++11-compat -Wc++14-compat -Wcatch-value (C++ and Objective-C++ only) -Wchar-subscripts -Wcomment -Wduplicate-decl-specifier (C and Objective-C only) -Wenum-compare (in C/ObjC; this is on by default in C++) -Wformat -Wint-in-bool-context -Wimplicit (C and Objective-C only) -Wimplicit-int (C and Objective-C only) -Wimplicit-function-declaration (C and Objective-C only) -Winit-self (only for C++) -Wlogical-not-parentheses -Wmain (only for C/ObjC and unless) -Wmaybe-uninitialized -Wmemset-elt-size -Wmemset-transposed-args -Wmisleading-indentation (only for C/C++) -Wmissing-attributes -Wmissing-braces (only for C/ObjC) -Wmultistatement-macros -Wnarrowing (only for C++) -Wnonnull -Wnonnull-compare -Wopenmp-simd -Wparentheses -Wpointer-sign -Wreorder -Wrestrict -Wreturn-type -Wsequence-point -Wsign-compare (only in C++) -Wsizeof-pointer-div -Wsizeof-pointer-memaccess -Wstrict-aliasing -Wstrict-overflow=1 -Wswitch -Wtautological-compare -Wtrigraphs -Wuninitialized -Wunknown-pragmas -Wunused-function -Wunused-label -Wunused-value -Wunused-variable -Wvolatile-register-var

Note that some warning flags are not implied by . Some of them warn about constructions that users generally do not consider questionable, but which occasionally you might wish to check for; others warn about constructions that are necessary or hard to avoid in some cases, and there is no simple way to modify the code to suppress the warning. Some of them are enabled by but many of them must be enabled individually.

This enables some extra warning flags that are not enabled by . (This option used to be called . The older name is still supported, but the newer name is more descriptive.)

-Wclobbered -Wcast-function-type -Wempty-body -Wignored-qualifiers -Wimplicit-fallthrough=3 -Wmissing-field-initializers -Wmissing-parameter-type (C only) -Wold-style-declaration (C only) -Woverride-init -Wsign-compare (C only) -Wtype-limits -Wuninitialized -Wshift-negative-value (in C++03 and in C99 and newer) -Wunused-parameter (only withor) -Wunused-but-set-parameter (only withor)

The option also prints warning messages for the following cases:

  • A pointer is compared against integer zero with , , , or .
  • (C++ only) An enumerator and a non-enumerator both appear in a conditional expression.
  • (C++ only) Ambiguous virtual bases.
  • (C++ only) Subscripting an array that has been declared .
  • (C++ only) Taking the address of a variable that has been declared .
  • (C++ only) A base class is not initialized in the copy constructor of a derived class.

Warn if an array subscript has type . This is a common cause of error, as programmers often forget that this type is signed on some machines. This warning is enabled by .

Warn about an invalid memory access that is found by Pointer Bounds Checker ().

Warn if feedback profiles do not match when using the option. If a source file is changed between compiling with and with , the files with the profile feedback can fail to match the source file and GCC cannot use the profile feedback information. By default, this warning is enabled and is treated as an error. can be used to disable the warning or can be used to disable the error. Disabling the error for this warning can result in poorly optimized code and is useful only in the case of very minor changes such as bug fixes to an existing code-base. Completely disabling the warning is not recommended.

(C, Objective-C, C++, Objective-C++ and Fortran only)

Suppress warning messages emitted by directives.

Give a warning when a value of type is implicitly promoted to . CPUs with a 32-bit “single-precision” floating-point unit implement in hardware, but emulate in software. On such a machine, doing computations using values is much more expensive because of the overhead required for software emulation.

It is easy to accidentally do computations with because floating-point literals are implicitly of type . For example, in:

float area(float radius) { return 3.14159 * radius * radius; }

the compiler performs the entire computation with because the floating-point literal is a .

Warn if a declaration has duplicate , , or specifier. This warning is enabled by .

Check calls to and , etc., to make sure that the arguments supplied have types appropriate to the format string specified, and that the conversions specified in the format string make sense. This includes standard functions, and others specified by format attributes (see Function Attributes), in the , , and (an X/Open extension, not in the C standard) families (or other target-specific families). Which functions are checked without format attributes having been specified depends on the standard version selected, and such checks of functions without the attribute specified are disabled by or .

The formats are checked against the format features supported by GNU libc version 2.2. These include all ISO C90 and C99 features, as well as features from the Single Unix Specification and some BSD and GNU extensions. Other library implementations may not support all these features; GCC does not support warning about features that go beyond a particular library’s limitations. However, if is used with , warnings are given about format features not in the selected standard version (but not for formats, since those are not in any version of the C standard). See Options Controlling C Dialect.

Option is equivalent to , and is equivalent to . Since also checks for null format arguments for several functions, also implies . Some aspects of this level of format checking can be disabled by the options: , , and . is enabled by .

If is specified, do not warn about format strings that contain NUL bytes.

If is specified, do not warn about excess arguments to a or format function. The C standard specifies that such arguments are ignored.

Where the unused arguments lie between used arguments that are specified with ‘’ operand number specifications, normally warnings are still given, since the implementation could not know what type to pass to to skip the unused arguments. However, in the case of formats, this option suppresses the warning if the unused arguments are all pointers, since the Single Unix Specification says that such unused arguments are allowed.

Warn about calls to formatted input/output functions such as and that might overflow the destination buffer. When the exact number of bytes written by a format directive cannot be determined at compile-time it is estimated based on heuristics that depend on the argument and on optimization. While enabling optimization will in most cases improve the accuracy of the warning, it may also result in false positives.

Level of enabled by employs a conservative approach that warns only about calls that most likely overflow the buffer. At this level, numeric arguments to format directives with unknown values are assumed to have the value of one, and strings of unknown length to be empty. Numeric arguments that are known to be bounded to a subrange of their type, or string arguments whose output is bounded either by their directive’s precision or by a finite set of string literals, are assumed to take on the value within the range that results in the most bytes on output. For example, the call to below is diagnosed because even with both and equal to zero, the terminating NUL character () appended by the function to the destination buffer will be written past its end. Increasing the size of the buffer by a single byte is sufficient to avoid the warning, though it may not be sufficient to avoid the overflow.

void f (int a, int b) { char buf [13]; sprintf (buf, "a = %i, b = %i\n", a, b); }

Level warns also about calls that might overflow the destination buffer given an argument of sufficient length or magnitude. At level , unknown numeric arguments are assumed to have the minimum representable value for signed types with a precision greater than 1, and the maximum representable value otherwise. Unknown string arguments whose length cannot be assumed to be bounded either by the directive’s precision, or by a finite set of string literals they may evaluate to, or the character array they may point to, are assumed to be 1 character long.

At level , the call in the example above is again diagnosed, but this time because with equal to a 32-bit the first directive will write some of its digits beyond the end of the destination buffer. To make the call safe regardless of the values of the two variables, the size of the destination buffer must be increased to at least 34 bytes. GCC includes the minimum size of the buffer in an informational note following the warning.

An alternative to increasing the size of the destination buffer is to constrain the range of formatted values. The maximum length of string arguments can be bounded by specifying the precision in the format directive. When numeric arguments of format directives can be assumed to be bounded by less than the precision of their type, choosing an appropriate length modifier to the format specifier will reduce the required buffer size. For example, if and in the example above can be assumed to be within the precision of the type then using either the format directive or casting the argument to reduces the maximum required size of the buffer to 24 bytes.

void f (int a, int b) { char buf [23]; sprintf (buf, "a = %hi, b = %i\n", a, (short)b); }

If is specified, do not warn about zero-length formats. The C standard specifies that zero-length formats are allowed.

Enable plus additional format checks. Currently equivalent to .

If is specified, also warn if the format string is not a string literal and so cannot be checked, unless the format function takes its format arguments as a .

If is specified, also warn about uses of format functions that represent possible security problems. At present, this warns about calls to and functions where the format string is not a string literal and there are no format arguments, as in . This may be a security hole if the format string came from untrusted input and contains ‘’. (This is currently a subset of what warns about, but in future warnings may be added to that are not included in .)

If is specified, also warn if the format string requires an unsigned argument and the argument is signed and vice versa.

Warn about calls to formatted input/output functions such as and that might result in output truncation. When the exact number of bytes written by a format directive cannot be determined at compile-time it is estimated based on heuristics that depend on the argument and on optimization. While enabling optimization will in most cases improve the accuracy of the warning, it may also result in false positives. Except as noted otherwise, the option uses the same logic .

Level of enabled by employs a conservative approach that warns only about calls to bounded functions whose return value is unused and that will most likely result in output truncation.

Level warns also about calls to bounded functions whose return value is used and that might result in truncation given an argument of sufficient length or magnitude.

If is specified, also warn about formats that may yield only a two-digit year.

Warn about passing a null pointer for arguments marked as requiring a non-null value by the function attribute.

is included in and . It can be disabled with the option.

Warn when comparing an argument marked with the function attribute against null inside the function.

is included in . It can be disabled with the option.

Warn if the compiler detects paths that trigger erroneous or undefined behavior due to dereferencing a null pointer. This option is only active when is active, which is enabled by optimizations in most targets. The precision of the warnings depends on the optimization options used.

Warn about uninitialized variables that are initialized with themselves. Note this option can only be used with the option.

For example, GCC warns about being uninitialized in the following snippet only when has been specified:

int f() { int i = i; return i; }

This warning is enabled by in C++.

Warn when a declaration does not specify a type. This warning is enabled by .

Give a warning whenever a function is used before being declared. In C99 mode ( or ), this warning is enabled by default and it is made into an error by . This warning is also enabled by .

Same as and . This warning is enabled by .

is the same as and is the same as .

Warn when a switch case falls through. For example:

switch (cond) { case 1: a = 1; break; case 2: a = 2; case 3: a = 3; break; }

This warning does not warn when the last statement of a case cannot fall through, e.g. when there is a return statement or a call to function declared with the noreturn attribute. also takes into account control flow statements, such as ifs, and only warns when appropriate. E.g.

switch (cond) { case 1: if (i > 3) { bar (5); break; } else if (i < 1) { bar (0); } else return; default: … }

Since there are occasions where a switch case fall through is desirable, GCC provides an attribute, , that is to be used along with a null statement to suppress this warning that would normally occur:

switch (cond) { case 1: bar (0); __attribute__ ((fallthrough)); default: … }

C++17 provides a standard way to suppress the warning using instead of the GNU attribute. In C++11 or C++14 users can use , which is a GNU extension. Instead of these attributes, it is also possible to add a fallthrough comment to silence the warning. The whole body of the C or C++ style comment should match the given regular expressions listed below. The option argument specifies what kind of comments are accepted:

  • disables the warning altogether.
  • matches regular expression, any comment is used as fallthrough comment.
  • case insensitively matches regular expression.
  • case sensitively matches one of the following regular expressions:
    • case sensitively matches one of the following regular expressions:
      • doesn’t recognize any comments as fallthrough comments, only attributes disable the warning.

      The comment needs to be followed after optional whitespace and other comments by or keywords or by a user label that precedes some or label.

      switch (cond) { case 1: bar (0); /* FALLTHRU */ default: … }

      The warning is enabled by .

      Control if warning triggered by the attribute should be issued. This is is enabled by default. Use to disable it.

      Warn if the return type of a function has a type qualifier such as . For ISO C such a type qualifier has no effect, since the value returned by a function is not an lvalue. For C++, the warning is only emitted for scalar types or . ISO C prohibits qualified return types on function definitions, so such return types always receive a warning even without this option.

      This warning is also enabled by .

      Warn when an attribute is ignored. This is different from the option in that it warns whenever the compiler decides to drop an attribute, not that the attribute is either unknown, used in a wrong place, etc. This warning is enabled by default.

      Warn if the type of is suspicious. should be a function with external linkage, returning int, taking either zero arguments, two, or three arguments of appropriate types. This warning is enabled by default in C++ and is enabled by either or .

      Warn when the indentation of the code does not reflect the block structure. Specifically, a warning is issued for , , , and clauses with a guarded statement that does not use braces, followed by an unguarded statement with the same indentation.

      In the following example, the call to “bar” is misleadingly indented as if it were guarded by the “if” conditional.

      if (some_condition ()) foo (); bar (); /* Gotcha: this is not guarded by the "if". */

      In the case of mixed tabs and spaces, the warning uses the option to determine if the statements line up (defaulting to 8).

      The warning is not issued for code involving multiline preprocessor logic such as the following example.

      if (flagA) foo (0); #if SOME_CONDITION_THAT_DOES_NOT_HOLD if (flagB) #endif foo (1);

      The warning is not issued after a directive, since this typically indicates autogenerated code, and no assumptions can be made about the layout of the file that the directive references.

      This warning is enabled by in C and C++.

      Warn when a declaration of a function is missing one or more attributes that a related function is declared with and whose absence may adversely affect the correctness or efficiency of generated code. For example, in C++, the warning is issued when an explicit specialization of a primary template declared with attribute , , , , , , or is declared without it. Attributes , , and suppress the warning. (see Function Attributes).

      is enabled by .

      For example, since the declaration of the primary function template below makes use of both attribute and the declaration of the explicit specialization of the template is diagnosed because it is missing one of the attributes.

      template <class T> T* __attribute__ ((malloc, alloc_size (1))) allocate (size_t); template <> void* __attribute__ ((malloc)) // missing alloc_size allocate<void> (size_t);

      Warn if an aggregate or union initializer is not fully bracketed. In the following example, the initializer for is not fully bracketed, but that for is fully bracketed. This warning is enabled by in C.

      int a[2][2] = { 0, 1, 2, 3 }; int b[2][2] = { { 0, 1 }, { 2, 3 } };

      This warning is enabled by .

      Warn if a user-supplied include directory does not exist.

      Warn about unsafe multiple statement macros that appear to be guarded by a clause such as , , , , or , in which only the first statement is actually guarded after the macro is expanded.

      For example:

      #define DOIT x++; y++ if (c) DOIT;

      will increment unconditionally, not just when holds. The can usually be fixed by wrapping the macro in a do-while loop:

      #define DOIT do { x++; y++; } while (0) if (c) DOIT;

      This warning is enabled by in C and C++.

      Warn if parentheses are omitted in certain contexts, such as when there is an assignment in a context where a truth value is expected, or when operators are nested whose precedence people often get confused about.

      Also warn if a comparison like appears; this is equivalent to , which is a different interpretation from that of ordinary mathematical notation.

      Also warn for dangerous uses of the GNU extension to with omitted middle operand. When the condition in the : operator is a boolean expression, the omitted value is always 1. Often programmers expect it to be a value computed inside the conditional expression instead.

      For C++ this also warns for some cases of unnecessary parentheses in declarations, which can indicate an attempt at a function call instead of a declaration:

      { // Declares a local variable called mymutex. std::unique_lock<std::mutex> (mymutex); // User meant std::unique_lock<std::mutex> lock (mymutex); }

      This warning is enabled by .

      Warn about code that may have undefined semantics because of violations of sequence point rules in the C and C++ standards.

      The C and C++ standards define the order in which expressions in a C/C++ program are evaluated in terms of sequence points, which represent a partial ordering between the execution of parts of the program: those executed before the sequence point, and those executed after it. These occur after the evaluation of a full expression (one which is not part of a larger expression), after the evaluation of the first operand of a , , or (comma) operator, before a function is called (but after the evaluation of its arguments and the expression denoting the called function), and in certain other places. Other than as expressed by the sequence point rules, the order of evaluation of subexpressions of an expression is not specified. All these rules describe only a partial order rather than a total order, since, for example, if two functions are called within one expression with no sequence point between them, the order in which the functions are called is not specified. However, the standards committee have ruled that function calls do not overlap.

      It is not specified when between sequence points modifications to the values of objects take effect. Programs whose behavior depends on this have undefined behavior; the C and C++ standards specify that “Between the previous and next sequence point an object shall have its stored value modified at most once by the evaluation of an expression. Furthermore, the prior value shall be read only to determine the value to be stored.”. If a program breaks these rules, the results on any particular implementation are entirely unpredictable.

      Examples of code with undefined behavior are , and . Some more complicated cases are not diagnosed by this option, and it may give an occasional false positive result, but in general it has been found fairly effective at detecting this sort of problem in programs.

      The C++17 standard will define the order of evaluation of operands in more cases: in particular it requires that the right-hand side of an assignment be evaluated before the left-hand side, so the above examples are no longer undefined. But this warning will still warn about them, to help people avoid writing code that is undefined in C and earlier revisions of C++.

      The standard is worded confusingly, therefore there is some debate over the precise meaning of the sequence point rules in subtle cases. Links to discussions of the problem, including proposed formal definitions, may be found on the GCC readings page, at http://gcc.gnu.org/readings.html.

      This warning is enabled by for C and C++.

      Do not warn about returning a pointer (or in C++, a reference) to a variable that goes out of scope after the function returns.

      Warn whenever a function is defined with a return type that defaults to . Also warn about any statement with no return value in a function whose return type is not (falling off the end of the function body is considered returning without a value).

      For C only, warn about a statement with an expression in a function whose return type is , unless the expression type is also . As a GNU extension, the latter case is accepted without a warning unless is used.

      For C++, a function without return type always produces a diagnostic message, even when is specified. The only exceptions are and functions defined in system headers.

      This warning is enabled by .

      Warn if shift count is negative. This warning is enabled by default.

      Warn if shift count >= width of type. This warning is enabled by default.

      Warn if left shifting a negative value. This warning is enabled by in C99 and C++11 modes (and newer).

      Warn about left shift overflows. This warning is enabled by default in C99 and C++11 modes (and newer).

      This is the warning level of and is enabled by default in C99 and C++11 modes (and newer). This warning level does not warn about left-shifting 1 into the sign bit. (However, in C, such an overflow is still rejected in contexts where an integer constant expression is required.)

      This warning level also warns about left-shifting 1 into the sign bit, unless C++14 mode is active.

      Warn whenever a statement has an index of enumerated type and lacks a for one or more of the named codes of that enumeration. (The presence of a label prevents this warning.) labels outside the enumeration range also provoke warnings when this option is used (even if there is a label). This warning is enabled by .

      Warn whenever a statement does not have a case.

      Warn whenever a statement has an index of enumerated type and lacks a for one or more of the named codes of that enumeration. labels outside the enumeration range also provoke warnings when this option is used. The only difference between and this option is that this option gives a warning about an omitted enumeration code even if there is a label.

      Warn whenever a statement has an index of boolean type and the case values are outside the range of a boolean type. It is possible to suppress this warning by casting the controlling expression to a type other than . For example:

      switch ((int) (a == 4)) { … }

      This warning is enabled by default for C and C++ programs.

      Warn whenever a statement contains statements between the controlling expression and the first case label, which will never be executed. For example:

      switch (cond) { i = 15; … case 5: … }

      does not warn if the statement between the controlling expression and the first case label is just a declaration:

      switch (cond) { int i; … case 5: i = 5; … }

      This warning is enabled by default for C and C++ programs.

      Warn when and built-in functions are used. These functions changed semantics in GCC 4.4.

      Warn whenever a function parameter is assigned to, but otherwise unused (aside from its declaration).

      To suppress this warning use the attribute (see Variable Attributes).

      This warning is also enabled by together with .

      Warn whenever a local variable is assigned to, but otherwise unused (aside from its declaration). This warning is enabled by .

      To suppress this warning use the attribute (see Variable Attributes).

      This warning is also enabled by , which is enabled by .

      Warn whenever a static function is declared but not defined or a non-inline static function is unused. This warning is enabled by .

      Warn whenever a label is declared but not used. This warning is enabled by .

      To suppress this warning use the attribute (see Variable Attributes).

      Warn when a typedef locally defined in a function is not used. This warning is enabled by .

      Warn whenever a function parameter is unused aside from its declaration.

      To suppress this warning use the attribute (see Variable Attributes).

      Do not warn if a caller of a function marked with attribute (see Function Attributes) does not use its return value. The default is .

      Warn whenever a local or static variable is unused aside from its declaration. This option implies for C, but not for C++. This warning is enabled by .

      To suppress this warning use the attribute (see Variable Attributes).

      Warn whenever a constant static variable is unused aside from its declaration. is enabled by for C, but not for C++. In C this declares variable storage, but in C++ this is not an error since const variables take the place of s.

      To suppress this warning use the attribute (see Variable Attributes).

      This is the warning level that is enabled by for C. It warns only about unused static const variables defined in the main compilation unit, but not about static const variables declared in any header included.

      This warning level also warns for unused constant static variables in headers (excluding system headers). This is the warning level of and must be explicitly requested since in C++ this isn’t an error and in C it might be harder to clean up all headers included.

      Warn whenever a statement computes a result that is explicitly not used. To suppress this warning cast the unused expression to . This includes an expression-statement or the left-hand side of a comma expression that contains no side effects. For example, an expression such as causes a warning, while does not.

      This warning is enabled by .

      All the above options combined.

      In order to get a warning about an unused function parameter, you must either specify (note that implies ), or separately specify .

      Warn if an automatic variable is used without first being initialized or if a variable may be clobbered by a call. In C++, warn if a non-static reference or non-static member appears in a class without constructors.

      If you want to warn about code that uses the uninitialized value of the variable in its own initializer, use the option.

      These warnings occur for individual uninitialized or clobbered elements of structure, union or array variables as well as for variables that are uninitialized or clobbered as a whole. They do not occur for variables or elements declared . Because these warnings depend on optimization, the exact variables or elements for which there are warnings depends on the precise optimization options and version of GCC used.

      Note that there may be no warning about a variable that is used only to compute a value that itself is never used, because such computations may be deleted by data flow analysis before the warnings are printed.

      Warn for invocations of __atomic Builtins, __sync Builtins, and the C11 atomic generic functions with a memory consistency argument that is either invalid for the operation or outside the range of values of the enumeration. For example, since the and built-ins are only defined for the relaxed, release, and sequentially consistent memory orders the following code is diagnosed:

      void store (int *i) { __atomic_store_n (i, 0, memory_order_consume); }

      is enabled by default.

      For an automatic (i.e. local) variable, if there exists a path from the function entry to a use of the variable that is initialized, but there exist some other paths for which the variable is not initialized, the compiler emits a warning if it cannot prove the uninitialized paths are not executed at run time.

      These warnings are only possible in optimizing compilation, because otherwise GCC does not keep track of the state of variables.

      These warnings are made optional because GCC may not be able to determine when the code is correct in spite of appearing to have an error. Here is one example of how this can happen:

      { int x; switch (y) { case 1: x = 1; break; case 2: x = 4; break; case 3: x = 5; } foo (x); }

      If the value of is always 1, 2 or 3, then is always initialized, but GCC doesn’t know this. To suppress the warning, you need to provide a default case with assert(0) or similar code.

      This option also warns when a non-volatile automatic variable might be changed by a call to . The compiler sees only the calls to . It cannot know where will be called; in fact, a signal handler could call it at any point in the code. As a result, you may get a warning even when there is in fact no problem because cannot in fact be called at the place that would cause a problem.

      Some spurious warnings can be avoided if you declare all the functions you use that never return as . See Function Attributes.

      This warning is enabled by or .

      Warn when a directive is encountered that is not understood by GCC. If this command-line option is used, warnings are even issued for unknown pragmas in system header files. This is not the case if the warnings are only enabled by the command-line option.

      Do not warn about misuses of pragmas, such as incorrect parameters, invalid syntax, or conflicts between pragmas. See also .

      This option is only active when is active. It warns about code that might break the strict aliasing rules that the compiler is using for optimization. The warning does not catch all cases, but does attempt to catch the more common pitfalls. It is included in . It is equivalent to

      This option is only active when is active. It warns about code that might break the strict aliasing rules that the compiler is using for optimization. Higher levels correspond to higher accuracy (fewer false positives). Higher levels also correspond to more effort, similar to the way works. is equivalent to .

      Level 1: Most aggressive, quick, least accurate. Possibly useful when higher levels do not warn but still breaks the code, as it has very few false negatives. However, it has many false positives. Warns for all pointer conversions between possibly incompatible types, even if never dereferenced. Runs in the front end only.

      Level 2: Aggressive, quick, not too precise. May still have many false positives (not as many as level 1 though), and few false negatives (but possibly more than level 1). Unlike level 1, it only warns when an address is taken. Warns about incomplete types. Runs in the front end only.

      Level 3 (default for ): Should have very few false positives and few false negatives. Slightly slower than levels 1 or 2 when optimization is enabled. Takes care of the common pun+dereference pattern in the front end: . If optimization is enabled, it also runs in the back end, where it deals with multiple statement cases using flow-sensitive points-to information. Only warns when the converted pointer is dereferenced. Does not warn about incomplete types.

      This option is only active when signed overflow is undefined. It warns about cases where the compiler optimizes based on the assumption that signed overflow does not occur. Note that it does not warn about all cases where the code might overflow: it only warns about cases where the compiler implements some optimization. Thus this warning depends on the optimization level.

      An optimization that assumes that signed overflow does not occur is perfectly safe if the values of the variables involved are such that overflow never does, in fact, occur. Therefore this warning can easily give a false positive: a warning about code that is not actually a problem. To help focus on important issues, several warning levels are defined. No warnings are issued for the use of undefined signed overflow when estimating how many iterations a loop requires, in particular when determining whether a loop will be executed at all.

      Warn about cases that are both questionable and easy to avoid. For example the compiler simplifies to . This level of is enabled by ; higher levels are not, and must be explicitly requested.

      Also warn about other cases where a comparison is simplified to a constant. For example: . This can only be simplified when signed integer overflow is undefined, because overflows to , which is less than zero. (with no level) is the same as .

      Also warn about other cases where a comparison is simplified. For example: is simplified to .

      Also warn about other simplifications not covered by the above cases. For example: is simplified to .

      Also warn about cases where the compiler reduces the magnitude of a constant involved in a comparison. For example: is simplified to . This is reported only at the highest warning level because this simplification applies to many comparisons, so this warning level gives a very large number of false positives.

      Warn for calls to string manipulation functions such as and that are determined to overflow the destination buffer. The optional argument is one greater than the type of Object Size Checking to perform to determine the size of the destination. See Object Size Checking. The argument is meaningful only for functions that operate on character arrays but not for raw memory functions like which always make use of Object Size type-0. The option also warns for calls that specify a size in excess of the largest possible object or at most bytes. The option produces the best results with optimization enabled but can detect a small subset of simple buffer overflows even without optimization in calls to the GCC built-in functions like that correspond to the standard functions. In any case, the option warns about just a subset of buffer overflows detected by the corresponding overflow checking built-ins. For example, the option will issue a warning for the call below because it copies at least 5 characters (the string including the terminating NUL) into the buffer of size 4.

      enum Color { blue, purple, yellow }; const char* f (enum Color clr) { static char buf [4]; const char *str; switch (clr) { case blue: str = "blue"; break; case purple: str = "purple"; break; case yellow: str = "yellow"; break; } return strcpy (buf, str); // warning here }

      Option is enabled by default.

      The option uses type-zero Object Size Checking to determine the sizes of destination objects. This is the default setting of the option. At this setting the option will not warn for writes past the end of subobjects of larger objects accessed by pointers unless the size of the largest surrounding object is known. When the destination may be one of several objects it is assumed to be the largest one of them. On Linux systems, when optimization is enabled at this setting the option warns for the same code as when the macro is defined to a non-zero value.

      The option uses type-one Object Size Checking to determine the sizes of destination objects. At this setting the option will warn about overflows when writing to members of the largest complete objects whose exact size is known. It will, however, not warn for excessive writes to the same members of unknown objects referenced by pointers since they may point to arrays containing unknown numbers of elements.

      The option uses type-two Object Size Checking to determine the sizes of destination objects. At this setting the option warns about overflowing the smallest object or data member. This is the most restrictive setting of the option that may result in warnings for safe code.

      The option uses type-three Object Size Checking to determine the sizes of destination objects. At this setting the option will warn about overflowing any data members, and when the destination is one of several objects it uses the size of the largest of them to decide whether to issue a warning. Similarly to this setting of the option may result in warnings for benign code.

      Warn for calls to bounded string manipulation functions such as , , and that may either truncate the copied string or leave the destination unchanged.

      In the following example, the call to specifies a bound that is less than the length of the source string. As a result, the copy of the source will be truncated and so the call is diagnosed. To avoid the warning use as the bound.

      void append (char *buf, size_t bufsize) { strncat (buf, ".txt", 3); }

      As another example, the following call to results in copying to just the characters preceding the terminating NUL, without appending the NUL to the end. Assuming the result of is necessarily a NUL-terminated string is a common mistake, and so the call is diagnosed. To avoid the warning when the result is not expected to be NUL-terminated, call instead.

      void copy (char *d, const char *s) { strncpy (d, s, strlen (s)); }

      In the following example, the call to specifies the size of the destination buffer as the bound. If the length of the source string is equal to or greater than this size the result of the copy will not be NUL-terminated. Therefore, the call is also diagnosed. To avoid the warning, specify as the bound and set the last element of the buffer to .

      void copy (const char *s) { char buf[80]; strncpy (buf, s, sizeof buf); … }

      In situations where a character array is intended to store a sequence of bytes with no terminating such an array may be annotated with attribute to avoid this warning. Such arrays, however, are not suitable arguments to functions that expect -terminated strings. To help detect accidental misuses of such arrays GCC issues warnings unless it can prove that the use is safe. See Common Variable Attributes.

      Warn for cases where adding an attribute may be beneficial. The attributes currently supported are listed below.

      Warn about functions that might be candidates for attributes , or or . The compiler only warns for functions visible in other compilation units or (in the case of and ) if it cannot prove that the function returns normally. A function returns normally if it doesn’t contain an infinite loop or return abnormally by throwing, calling or trapping. This analysis requires option , which is enabled by default at and higher. Higher optimization levels improve the accuracy of the analysis.

      Warn about function pointers that might be candidates for attributes. Note these are only possible candidates, not absolute ones. GCC guesses that function pointers with attributes that are used in assignment, initialization, parameter passing or return statements should have a corresponding attribute in the resulting type. I.e. the left-hand side of the assignment or initialization, the type of the parameter variable, or the return type of the containing function respectively should also have a attribute to avoid the warning.

      GCC also warns about function definitions that might be candidates for attributes. Again, these are only possible candidates. GCC guesses that attributes might be appropriate for any function that calls a function like or , but this might not always be the case, and some functions for which attributes are appropriate may not be detected.

      Warn about functions that might be candidates for attribute. This is based on static detection and generally will only warn about functions which always leads to a call to another function such as wrappers of C++ or fatal error reporting functions leading to .

      Warn about types with virtual methods where code quality would be improved if the type were declared with the C++11 specifier, or, if possible, declared in an anonymous namespace. This allows GCC to more aggressively devirtualize the polymorphic calls. This warning is more effective with link time optimization, where the information about the class hierarchy graph is more complete.

      Warn about virtual methods where code quality would be improved if the method were declared with the C++11 specifier, or, if possible, its type were declared in an anonymous namespace or with the specifier. This warning is more effective with link-time optimization, where the information about the class hierarchy graph is more complete. It is recommended to first consider suggestions of and then rebuild with new annotations.

      Warn about overriding virtual functions that are not marked with the override keyword.

      Warn about calls to allocation functions decorated with attribute that specify zero bytes, including those to the built-in forms of the functions , , , , and . Because the behavior of these functions when called with a zero size differs among implementations (and in the case of has been deprecated) relying on it may result in subtle portability bugs and should be avoided.

      Warn about calls to functions decorated with attribute that attempt to allocate objects larger than the specified number of bytes, or where the result of the size computation in an integer type with infinite precision would exceed . The option argument may end in one of the standard suffixes designating a multiple of bytes such as and for kilobyte and kibibyte, respectively, and for megabyte and mebibyte, and so on. See Function Attributes.

      This option warns on all uses of in the source.

      This option warns on calls to that are not bounded by a controlling predicate limiting its argument of integer type to at most bytes, or calls to where the bound is unknown. Arguments of non-integer types are considered unbounded even if they appear to be constrained to the expected range.

      For example, a bounded case of could be:

      void func (size_t n) { void *p; if (n <= 1000) p = alloca (n); else p = malloc (n); f (p); }

      In the above example, passing would not issue a warning because the call to is known to be at most 1000 bytes. However, if were passed, the compiler would emit a warning.

      Unbounded uses, on the other hand, are uses of with no controlling predicate constraining its integer argument. For example:

      void func () { void *p = alloca (n); f (p); }

      If were passed, the above would trigger a warning, but this time because of the lack of bounds checking.

      Note, that even seemingly correct code involving signed integers could cause a warning:

      void func (signed int n) { if (n < 500) { p = alloca (n); f (p); } }

      In the above example, could be negative, causing a larger than expected argument to be implicitly cast into the call.

      This option also warns when is used in a loop.

      This warning is not enabled by , and is only active when is active (default for and above).

      See also .

      This option is only active when is active (default for and above). It warns about subscripts to arrays that are always out of bounds. This warning is enabled by .

      This is the warning level of and is enabled by ; higher levels are not, and must be explicitly requested.

      This warning level also warns about out of bounds access for arrays at the end of a struct and for arrays accessed through pointers. This warning level may give a larger number of false positives and is deactivated by default.

      Warn about declarations using the and similar attributes whose target is incompatible with the type of the alias. See Declaring Attributes of Functions.

      Warn about boolean expression compared with an integer value different from /. For instance, the following comparison is always false:

      int n = 5; … if ((n > 1) == 2) { … }

      This warning is enabled by .

      Warn about suspicious operations on expressions of a boolean type. For instance, bitwise negation of a boolean is very likely a bug in the program. For C, this warning also warns about incrementing or decrementing a boolean, which rarely makes sense. (In C++, decrementing a boolean is always invalid. Incrementing a boolean is invalid in C++17, and deprecated otherwise.)

      This warning is enabled by .

      Warn when an if-else has identical branches. This warning detects cases like

      if (p != NULL) return 0; else return 0;

      It doesn’t warn when both branches contain just a null statement. This warning also warn for conditional operators:

      Warn about duplicated conditions in an if-else-if chain. For instance, warn for the following code:

      if (p->q != NULL) { … } else if (p->q != NULL) { … }

      Warn when the ‘’ or ‘’ is called with an argument greater than 0. Such calls may return indeterminate values or crash the program. The warning is included in .

      Do not warn if type qualifiers on pointers are being discarded. Typically, the compiler warns if a variable is passed to a function that takes a parameter. This option can be used to suppress such a warning.

      Do not warn if type qualifiers on arrays which are pointer targets are being discarded. Typically, the compiler warns if a variable is passed to a function that takes a parameter. This option can be used to suppress such a warning.

      Do not warn when there is a conversion between pointers that have incompatible types. This warning is for cases not covered by , which warns for pointer argument passing or assignment with different signedness.

      Do not warn about incompatible integer to pointer and pointer to integer conversions. This warning is about implicit conversions; for explicit conversions the warnings and may be used.

      Do not warn about compile-time integer division by zero. Floating-point division by zero is not warned about, as it can be a legitimate way of obtaining infinities and NaNs.

      Print warning messages for constructs found in system header files. Warnings from system headers are normally suppressed, on the assumption that they usually do not indicate real problems and would only make the compiler output harder to read. Using this command-line option tells GCC to emit warnings from system headers as if they occurred in user code. However, note that using in conjunction with this option does not warn about unknown pragmas in system headers—for that, must also be used.

      Warn if a self-comparison always evaluates to true or false. This warning detects various mistakes such as:

      int i = 1; … if (i > i) { … }

      This warning also warns about bitwise comparisons that always evaluate to true or false, for instance:

      if ((a & 16) == 10) { … }

      will always be false.

      This warning is enabled by .

      Warn about trampolines generated for pointers to nested functions. A trampoline is a small piece of data or code that is created at run time on the stack when the address of a nested function is taken, and is used to call the nested function indirectly. For some targets, it is made up of data only and thus requires no special treatment. But, for most targets, it is made up of code and thus requires the stack to be made executable in order for the program to work properly.

      Warn if floating-point values are used in equality comparisons.

      The idea behind this is that sometimes it is convenient (for the programmer) to consider floating-point values as approximations to infinitely precise real numbers. If you are doing this, then you need to compute (by analyzing the code, or in some other way) the maximum or likely maximum error that the computation introduces, and allow for it when performing comparisons (and when producing output, but that’s a different problem). In particular, instead of testing for equality, you should check to see whether the two values have ranges that overlap; and this is done with the relational operators, so equality comparisons are probably mistaken.

      Warn about certain constructs that behave differently in traditional and ISO C. Also warn about ISO C constructs that have no traditional C equivalent, and/or problematic constructs that should be avoided.

      • Macro parameters that appear within string literals in the macro body. In traditional C macro replacement takes place within string literals, but in ISO C it does not.
      • In traditional C, some preprocessor directives did not exist. Traditional preprocessors only considered a line to be a directive if the ‘’ appeared in column 1 on the line. Therefore warns about directives that traditional C understands but ignores because the ‘’ does not appear as the first character on the line. It also suggests you hide directives like not understood by traditional C by indenting them. Some traditional implementations do not recognize , so this option suggests avoiding it altogether.
      • A function-like macro that appears without arguments.
      • The unary plus operator.
      • The ‘’ integer constant suffix, or the ‘’ or ‘’ floating-point constant suffixes. (Traditional C does support the ‘’ suffix on integer constants.) Note, these suffixes appear in macros defined in the system headers of most modern systems, e.g. the ‘’/‘’ macros in . Use of these macros in user code might normally lead to spurious warnings, however GCC’s integrated preprocessor has enough context to avoid warning in these cases.
      • A function declared external in one block and then used after the end of the block.
      • A statement has an operand of type .
      • A non- function declaration follows a one. This construct is not accepted by some traditional C compilers.
      • The ISO type of an integer constant has a different width or signedness from its traditional type. This warning is only issued if the base of the constant is ten. I.e. hexadecimal or octal values, which typically represent bit patterns, are not warned about.
      • Usage of ISO string concatenation is detected.
      • Initialization of automatic aggregates.
      • Identifier conflicts with labels. Traditional C lacks a separate namespace for labels.
      • Initialization of unions. If the initializer is zero, the warning is omitted. This is done under the assumption that the zero initializer in user code appears conditioned on e.g. to avoid missing initializer warnings and relies on default initialization to zero in the traditional C case.
      • Conversions by prototypes between fixed/floating-point values and vice versa. The absence of these prototypes when compiling with traditional C causes serious problems. This is a subset of the possible conversion warnings; for the full set use .
      • Use of ISO C style function definitions. This warning intentionally is not issued for prototype declarations or variadic functions because these ISO C features appear in your code when using libiberty’s traditional C compatibility macros, and . This warning is also bypassed for nested functions because that feature is already a GCC extension and thus not relevant to traditional C compatibility.

      Warn if a prototype causes a type conversion that is different from what would happen to the same argument in the absence of a prototype. This includes conversions of fixed point to floating and vice versa, and conversions changing the width or signedness of a fixed-point argument except when the same as the default promotion.

      Warn when a declaration is found after a statement in a block. This construct, known from C++, was introduced with ISO C99 and is by default allowed in GCC. It is not supported by ISO C90. See Mixed Declarations.

      Warn whenever a local variable or type declaration shadows another variable, parameter, type, class member (in C++), or instance variable (in Objective-C) or whenever a built-in function is shadowed. Note that in C++, the compiler warns if a local variable shadows an explicit typedef, but not if it shadows a struct/class/enum. Same as .

      Do not warn whenever a local variable shadows an instance variable in an Objective-C method.

      The default for . Warns for any (global) shadowing.

      Warn when a local variable shadows another local variable or parameter. This warning is enabled by .

      Warn when a local variable shadows another local variable or parameter whose type is compatible with that of the shadowing variable. In C++, type compatibility here means the type of the shadowing variable can be converted to that of the shadowed variable. The creation of this flag (in addition to ) is based on the idea that when a local variable shadows another one of incompatible type, it is most likely intentional, not a bug or typo, as shown in the following example:

      for (SomeIterator i = SomeObj.begin(); i != SomeObj.end(); ++i) { for (int i = 0; i < N; ++i) { ... } ... }

      Since the two variable in the example above have incompatible types, enabling only will not emit a warning. Because their types are incompatible, if a programmer accidentally uses one in place of the other, type checking will catch that and emit an error or warning. So not warning (about shadowing) in this case will not lead to undetected bugs. Use of this flag instead of can possibly reduce the number of warnings triggered by intentional shadowing.

      This warning is enabled by .

      Warn whenever an object of larger than bytes is defined.

      Warn if the size of a function frame is larger than bytes. The computation done to determine the stack frame size is approximate and not conservative. The actual requirements may be somewhat greater than even if you do not get a warning. In addition, any space allocated via , variable-length arrays, or related constructs is not included by the compiler when determining whether or not to issue a warning.

      Do not warn when attempting to free an object that was not allocated on the heap.

      Warn if the stack usage of a function might be larger than bytes. The computation done to determine the stack usage is conservative. Any space allocated via , variable-length arrays, or related constructs is included by the compiler when determining whether or not to issue a warning.

      The message is in keeping with the output of .

      • If the stack usage is fully static but exceeds the specified amount, it’s:
        warning: stack usage is 1120 bytes
      • If the stack usage is (partly) dynamic but bounded, it’s:
        warning: stack usage might be 1648 bytes
      • If the stack usage is (partly) dynamic and not bounded, it’s:
        warning: stack usage might be unbounded

      Warn if the loop cannot be optimized because the compiler cannot assume anything on the bounds of the loop indices. With warn if the compiler makes such assumptions.

      When used in combination with and without GNU extensions, this option disables the warnings about non-ISO / format width specifiers , , and used on Windows targets, which depend on the MS runtime.

      There still seems to be a lot of traffic to my V-USB tutorials, so I thought I’d write a short follow-up post on USB keyboards. I already did a USB HID mouse post earlier, so you might want to check that out to understand a bit about HID descriptors and associated V-USB settings (in short, human interface devices send a binary descriptor to PC telling what kind of “reports” they send to the host on user activities).

      As a basic setup, you’ll need a working V-USB circuit with one switch and one LED attached. Here, I’m using ATtiny2313 with the LED wired to PB0 and switch to PB1. The ATtiny is using 20 MHz crystal, so if you’re following my USB tutorial series and have that circuit at hand, remember to change that frequency in before trying this out. Note the cool breadboard header I have, there will be more posts about that one to follow soon!

      USB HID keyboard basics

      A USB keyboard is a normal USB device, very much like a mouse, but it has an interrupt endpoint, which is used to send keyboard events to the host PC (instead of mouse movements). In this tutorial, I’m using a full “boot compliant HID” specification that can have up to six keys pressed down at a time. The bonus side is that these types of devices receive (at least on Windows, probably on Linux, not on OS X) keyboard LED state changes, so we can do cool things with our device, like I did in my USB password generator, where password generation is triggered by repeated CAPS LOCK presses.

      To make the operating system recognize our device as HID keyboard, we need to make mostly the same changes to as we did in my mouse tutorial. Here’s the overview of things you’ll probably need to change from my USB tutorial series:

      #define USB_CFG_HAVE_INTRIN_ENDPOINT 1 #define USB_CFG_IMPLEMENT_FN_WRITE 1 #define USB_CFG_VENDOR_ID 0x42, 0x42 #define USB_CFG_DEVICE_ID 0x31, 0xe1 #define USB_CFG_DEVICE_CLASS 0 #define USB_CFG_DEVICE_SUBCLASS 0 #define USB_CFG_INTERFACE_CLASS 0x03 // HID #define USB_CFG_INTERFACE_SUBCLASS 0x01 // Boot #define USB_CFG_INTERFACE_PROTOCOL 0x01 // Keyboard #define USB_CFG_HID_REPORT_DESCRIPTOR_LENGTH 63

      On the side, the keyboard descriptor is needed. It’s omitted here for brevity, see the project zip for details. What matters is, that the descriptor describes the following data structure that is used to send keypresses to PC:

      typedef struct { uint8_t modifier; uint8_t reserved; uint8_t keycode[6]; } keyboard_report_t; static keyboard_report_t keyboard_report; // sent to PC

      Every time a new button is pressed, we send a report. We also need to send a report when keys are no longer pressed – so to send a single ‘a’, we first send a report containing that character, and then send a report with only zeroes to tell that letter has been released. Forget to do that, and the device floods your system with a’s – pretty hard to reflash! If you happen to make that mistake, here’s how to fix it:

      1. Unplug the device
      2. Fix the code
      3. Clear the command-line and prepare the ‘make flash’ command
      4. Plug the device in and immediately hit ‘enter’ before the PC has time to recognize the flooder
      5. Once the flashing starts, the device resets and will not send any offending letters

      From the report structure you can also see that “modifier keys” such as control, alt and shift are sent separately from “normal keys” – the PC driver does the actual work so that when we send “shift” as a modifier and ‘a’ as a letter, ‘A’ will be output to any program currently active. The keycodes are also specific to the USB HID standard, for example ‘a’ is 4. You can find the full code set from “HID Usage tables” (google it or find it here), chapter 10, page 53.

      Basic HID keyboard code

      To pass as a functional keyboard, a few things need to be implemented in the firmware:

      1. Our device needs to be able to return the keyboard descriptor to PC when it is requested
      2. Repeat rate needs to be set and returned when asked
      3. The host can also ask the keyboard report via , in which case we just send “no keys pressed”
      4. Also, to receive LED state changes, we need to be able to receive some data on. A custom is used to do this.

      Here’s the code to do that and keep track of LED state (and mirror the CAPS LOCK to a LED in PB0):

      volatile static uchar LED_state = 0xff; // received from PC static uchar idleRate; // repeat rate for keyboards usbMsgLen_t usbFunctionSetup(uchar data[8]) { usbRequest_t *rq = (void *)data; if((rq->bmRequestType & USBRQ_TYPE_MASK) == USBRQ_TYPE_CLASS) { switch(rq->bRequest) { case USBRQ_HID_GET_REPORT: // send "no keys pressed" if asked here // wValue: ReportType (highbyte), ReportID (lowbyte) usbMsgPtr = (void *)&keyboard_report; // we only have this one keyboard_report.modifier = 0; keyboard_report.keycode[0] = 0; return sizeof(keyboard_report); case USBRQ_HID_SET_REPORT: // if wLength == 1, should be LED state return (rq->wLength.word == 1) ? USB_NO_MSG : 0; case USBRQ_HID_GET_IDLE: // send idle rate to PC as required by spec usbMsgPtr = &idleRate; return 1; case USBRQ_HID_SET_IDLE: // save idle rate as required by spec idleRate = rq->wValue.bytes[1]; return 0; } } return 0; // by default don't return any data } #define NUM_LOCK 1 #define CAPS_LOCK 2 #define SCROLL_LOCK 4 usbMsgLen_t usbFunctionWrite(uint8_t * data, uchar len) { if (data[0] == LED_state) return 1; else LED_state = data[0]; // LED state changed if(LED_state & CAPS_LOCK) PORTB |= 1 << PB0; // LED on else PORTB &= ~(1 << PB0); // LED off return 1; // Data read, not expecting more }

      Adding a switch to the mix

      After fulfilling these basic requirements, the code in main method stays pretty much the same as in my V-USB tutorials. If all we wanted to do was to monitor CAPS LOCK, we’d be done by now, but let’s make the switch connected to PB1 send a letter “x” when it’s clicked. In my test circuit, I have enabled a pullup resistor at PB1 and wired to switch so that when it’s pressed, PB1 is connected to ground and PB1 bit goes from 1 to 0. So in an ideal world, the main loop would look like this (I’m using a really simple helper function to fill , see the actual code for that):

      while(1) { wdt_reset(); // keep the watchdog happy usbPoll(); if(!(PINB & (1<<PB1))) // button pressed state = STATE_SEND_KEY; if(usbInterruptIsReady() && state != STATE_WAIT) { buildReport('x'); // fill keyboard_report with 'x' pressed usbSetInterrupt((void *)&keyboard_report, sizeof(keyboard_report)); buildReport(NULL); // clear keyboard report usbSetInterrupt((void *)&keyboard_report, sizeof(keyboard_report)); state = STATE_WAIT; } }

      Unfortunately, we don’t live in an ideal world. First of all, it takes some time after sending one report via the interrupt endpoint, so instead of two calls right after each other, we need to have a simple state machine inside the “if(ready)” code that first sends the key and changes state, then sends the “no keys pressed” message and changes the state. That’s rather easy to do:

      if(usbInterruptIsReady() && state != STATE_WAIT && LED_state != 0xff){ switch(state) { case STATE_SEND_KEY: buildReport('x'); state = STATE_RELEASE_KEY; // release next break; case STATE_RELEASE_KEY: buildReport(NULL); state = STATE_WAIT; // go back to waiting break; } }

      Second problem arises from the fact that a physical switch does not toggle cleanly from 1 to 0, but flickers for several times every time when pressed/released. Therefore, the current code would probably send the ‘x’ a dozen times per switch toggle. An easy way to fix this is to have a simple release counter that counts the number of iterations since last switch release – only after enough time has elapsed, is a new event generated. Like this:

      if(!(PINB & (1<<PB1))) { // button pressed // also check if some time has elapsed since last button press if(state == STATE_WAIT && button_release_counter == 255) state = STATE_SEND_KEY; button_release_counter = 0; // now button needs to be released for a while until retrigger } if(button_release_counter < 255) button_release_counter++; // increase release counter

      Now everything should work, more or less. For your enjoyment, I have compiled a nice little zip file also for this project, where you can find both and and a nice makefile to build and flash the project with a single invocation of .

      If you’ve done everything correctly, after flashing the MCU you should have device which toggles the LED in sync with your normal keyboard when you press CAPS LOCK on and off, and sends ‘x’ whenever you toggle the switch. This project is easily expandable to a data logger, full keyboard, or any cool project you can think of. Enjoy!

      Published by

      Joonas Pihlajamaa

      Coding since 1990 in Basic, C/C++, Perl, Java, PHP, Ruby and Python, to name a few. Also interested in math, movies, anime, and the occasional slashdot now and then. Oh, and I also have a real life, but lets not talk about it! View all posts by Joonas Pihlajamaa

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