I've been fiddling around with some MD5 code in coreutils,
and discovered this weirdness with gcc:
#include <stdio.h>
void showsize(unsigned char arg[16])
{
unsigned char stack[16];
fprintf(stderr,"[sizeof(arg) = %d]\n",sizeof(arg));
fprintf(stderr,"[sizeof(stack) = %d]\n",sizeof(stack));
}
int main()
{
unsigned char m[16];
showsize(m);
return 0;
}
The output:
[sizeof(arg) = 4]
[sizeof(stack) = 16]
Can anyone explain why the size of arg differs from the array
on the stack?
Is C treating arg as a char* so that sizeof(arg) is sizeof(char*)?
You are right in assuming that arg is being treated as a char*, whereas
stack is treated as a block of 16 bytes. It indeed looks odd, and there
is no good justification for this weirdness. A possible rationale is
that the sizeof operator simply reports the size of any object: and in
the case of arg this means sizeof(char *). Remember that C passes only
values and pointers into funtions; it does not pass the size of an array.
You may think that by writing
void showsize(unsigned char arg[16])
you are supplying the size of the array to the function, but you are
not! In fact the function is seeing only
void showsize(unsigned char *arg)
and it is only a strange syntatical laxness in C that permits the first
form as an alternative to the second.
Chris
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