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MALLOC !!!MALLOC NAME SYNOPSIS DESCRIPTION RETURN VALUE CONFORMING TO SEE ALSO NOTES ---- !!NAME calloc, malloc, free, realloc - Allocate and free dynamic memory !!SYNOPSIS __#include __ ''nmemb''__, size_t__ ''size''__); void *malloc(size_t__ ''size''__); void free(void__ ''*ptr''__); void *realloc(void__ ''*ptr''__, size_t__ ''size''__); __ !!DESCRIPTION __calloc()__ allocates memory for an array of ''nmemb'' elements of ''size'' bytes each and returns a pointer to the allocated memory. The memory is set to zero. __malloc()__ allocates ''size'' bytes and returns a pointer to the allocated memory. The memory is not cleared. __free()__ frees the memory space pointed to by ''ptr'', which must have been returned by a previous call to __malloc()__, __calloc()__ or __realloc()__. Otherwise, or if __free(__''ptr''__)__ has already been called before, undefined behaviour occurs. If ''ptr'' is __NULL__, no operation is performed. __realloc()__ changes the size of the memory block pointed to by ''ptr'' to ''size'' bytes. The contents will be unchanged to the minimum of the old and new sizes; newly allocated memory will be uninitialized. If ''ptr'' is __NULL__, the call is equivalent to __malloc(size)__; if size is equal to zero, the call is equivalent to __free(__''ptr''__)__''.'' Unless ''ptr'' is __NULL__, it must have been returned by an earlier call to __malloc()__, __calloc()__ or __realloc()__. !!RETURN VALUE For __calloc()__ and __malloc()__, the value returned is a pointer to the allocated memory, which is suitably aligned for any kind of variable, or __NULL__ if the request fails. __free()__ returns no value. __realloc()__ returns a pointer to the newly allocated memory, which is suitably aligned for any kind of variable and may be different from ''ptr'', or __NULL__ if the request fails or if size was equal to 0. If __realloc()__ fails the original block is left untouched - it is not freed or moved. !!CONFORMING TO ANSI-C !!SEE ALSO brk(2) !!NOTES The Unix98 standard requires __malloc()__, __calloc()__, and __realloc__() to set ''errno'' to ENOMEM upon failure. Glibc assumes that this is done (and the glibc versions of these routines do this); if you use a private malloc implementation that does not set ''errno'', then certain library routines may fail without having a reason in ''errno''. Crashes in __malloc()__, __free()__ or __realloc()__ are almost always related to heap corruption, such as overflowing an allocated chunk or freeing the same pointer twice. Recent versions of Linux libc (later than 5.4.23) and GNU libc (2.x) include a malloc implementation which is tunable via environment variables. When __MALLOC_CHECK___ is set, a special (less efficient) implementation is used which is designed to be tolerant against simple errors, such as double calls of __free()__ with the same argument, or overruns of a single byte (off-by-one bugs). Not all such errors can be proteced against, however, and memory leaks can result. If __MALLOC_CHECK___ is set to 0, any detected heap corruption is silently ignored; if set to 1, a diagnostic is printed on stderr; if set to 2, __abort()__ is called immediately. This can be useful because otherwise a crash may happen much later, and the true cause for the problem is then very hard to track down. Linux follows an optimistic memory allocation strategy. This means that when __malloc()__ returns non-NULL there is no guarantee that the memory really is available. In case it turns out that the system is out of memory, one or more processes will be killed by the infamous OOM killer. ----
5 pages link to
realloc(3)
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