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This noncompliant example provides definitions for the C standard library functions malloc() and free(). Although this practice is permitted by many traditional implementations of UNIX (e.g.for example, the Dmalloc library), it is disallowed by the C Standard because it is undefined behavior. Even on systems that allow replacing malloc(), doing so without also replacing calloc() and realloc() is likely to cause problems.

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The behavior of a program is undefined  when

a macro definition of errno is suppressed in order to access an actual object, or the program defines an identifier with the name errno. [ISO/IEC 9899:2011]

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