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void *malloc(size_t); |
Calling malloc( n s) allocates memory for an object whose size is n s and returns either a null pointer or a pointer to the allocated memory. A program can implicitly convert the pointer that malloc() returns into a different pointer type.
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p = malloc(sizeof(gadget)); /* imminent problem */ |
quietly assigns p to point to storage too small for a widget. The subsequent assignments to p->i and p->d produce undefined behavior. They'll will most likely produce memory overruns.
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Casting the result of malloc() to the appropriate pointer type enables the compiler to catch subsequent inadvertent pointer conversions. When allocating individual objects, the "appropriate pointer type" is a pointer to the type argument in the sizeof expression passed to malloc(), as in:
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enum { N = 16 };
widget *p;
/* ... */
p = MALLOC_ARRAY(N, widget); /* OK */
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A small collection of macros can provide secure implementations for common uses for the standard memory allocation functions. The omission of a {{REALLOC()}} macro is intentional (see \[[MEM08-A. Use realloc() only to resize dynamically allocated arrays]]). |
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/* allocates a single object using malloc(). */
#define MALLOC(type) ((type *)malloc(sizeof(type)))
/* allocates an array of objects using malloc(). */
#define MALLOC_ARRAY(number, type) \
((type *)malloc(number * sizeof(type)))
/* allocates a single object with a flexible array member using malloc(). */
#define MALLOC_FLEX(stype, number, etype) \
((stype *)malloc(sizeof(stype) + number * sizeof(etype)))
/* allocates an array of objects using calloc(). */
#define CALLOC(number, type) \
((type *)calloc(number, sizeof(type)))
/* reallocates an array of objects using realloc(). */
#define REALLOC_ARRAY(pointer, number, type) \
((type *)realloc(pointer, number * sizeof(type)))
/* reallocates a single object with a flexible array member using realloc(). */
#define REALLOC_FLEX(pointer, stype, number, etype) \
((stype *)realloc(pointer, sizeof(stype) + number * sizeof(etype)))
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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"
\[[Summit 05|AA. C References#Summit 05]\] [Question 7.7|http://c-faq.com/malloc/cast.html], [Question 7.7b|http://c-faq.com/malloc/mallocnocast.html] |
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