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If the size of the space requested is zero, the behavior is implementation defined: either a null NULL pointer is returned, or the behavior is as if the size were some nonzero value, except that the returned pointer shall not be used to access an object.
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However, this commonly recommended idiom has problems with zero length allocations. If the value of nsize in this example is 0, the standard allows the option of either returning a null NULL pointer or returning a pointer to an invalid (e.g., zero-length) object. In cases where the realloc() function frees the memory but returns a null NULL pointer, execution of the code in this example results in a double free.
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The realloc() function for gcc 3.4.6 with libc 2.3.4 returns a non-NULL pointer to a zero-sized object (the same as malloc(0)). However, the realloc() function for both Microsoft Visual Studio Version 7.1 and gcc version 4.1.0 return a null NULL pointer, resulting in a double free on the call to free() in this example.
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Search for vulnerabilities resulting from the violation of this rule on the CERT website.
References
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\[[ISO/IEC 9899-1999|AA. C References#ISO/IEC 9899-1999]\] Section 7.20.3, "Memory Management Functions" \[[Seacord 05|AA. C References#Seacord 05]\] Chapter 4, "Dynamic Memory Management" |