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Implementations conforming to C are required to declare errno in <errno.h>, although some historic implementations failed to do so.
Exceptions
DCL37-C-EX1: Provided that a library function can be declared without reference to any type defined in a header, it is permissible to declare that function without including its header provided that declaration is compatible with the standard declaration.
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Such code is compliant because the declaration matches what stdlib.h would provide and does not redefine the reserved identifier. However, it would not be acceptable to provide a definition for the free() function in this example.
DCL37-C-EX2: For compatibility with other compiler vendors or language standard modes, it is acceptable to create a macro identifier the same as a reserved identifier so long as the behavior is idempotent, as in this example:
| Code Block | ||||
|---|---|---|---|---|
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// Sometimes generated by configuration tools such as autoconf
#define const const
// Allowed compilers with semantically equivalent extension behavior
#define inline __inline |
Risk Assessment
Using reserved identifiers can lead to incorrect program operation.
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