what does the "const void*" mean in memmove?

c constants strcpy memmove
Alcott picture Alcott · Sep 16, 2011 · Viewed 7.7k times · Source

The second arg in the prototypes for memmove/memcpy/strcpy are similar: For example:

void *memmove(void *dest, const void *src, size_t n); //const void*
char *strcpy(char *dest, const char *src); //const char*

But apparently, if dest and src overlap, then src's content will be altered, violating the const void/char *?

Answer

Steve Jessop picture Steve Jessop · Sep 16, 2011

const void* means that the referand will not be modified through that pointer.

If there are other, non-const pointers to the same object (also known as "aliasing"), then of course it can still be modified through those. In the scenario you describe, that other pointer is dest.

By the way, in the case of strcpy, behavior is undefined if the regions overlap, and in C99 the signature is char *strcpy(char * restrict s1, const char * restrict s2);. But for memmove, aliasing is OK. By giving it overlapping regions you've given it "permission" to modify the dest region, and it will do that.

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