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The function calloc allocates memory and clears it to zero. It is declared in stdlib.h.

— Function: void * calloc (size_t count, size_t eltsize)

This function allocates a block long enough to contain a vector of count elements, each of size eltsize. Its contents are cleared to zero before calloc returns.

You could define calloc as follows:

     void *
     calloc (size_t count, size_t eltsize)
       size_t size = count * eltsize;
       void *value = malloc (size);
       if (value != 0)
         memset (value, 0, size);
       return value;

But in general, it is not guaranteed that calloc calls malloc internally. Therefore, if an application provides its own malloc/realloc/free outside the C library, it should always define calloc, too.