Try including:
#include <memory.h>
#include <string.h>
So, I have no idea. But some suggestions:
You might try copying/pasting this code here from this example just to make sure it works 'as expected'
Maybe try defining some of the macros here? (when I did C programming on zOS, I had to do include some weird macros in order to get stuff to work. I have no reasonable technical explanation for this.)
You could try searching for memset() using "=3.14" (from ispf.) See if any other modules use that function, and then check the headers that they include (or macros that they define - either in the C files or H files) to make it work.
Another thought: before the memset(), try doing putting a printf() in. If you get a syntax error on the same line (only for printf, rather than memset) then you can see if the problem is before line 6 - like a misplaced parenthesis.
Finally, if i recall correctly, I had to compile my individual modules, and then link them manually (unless I wrote a JCL to do this for me.) So you might have to link once to link with your other modules, and then link again against the C library. Not to be pedantic, but: you're fairly certain that you're doing all of the link passes?
I realize that's a lot of hoops to try and you've probably already read the manuals, but maybe there is something useful to try?
Also, and you probably already know this, but this site (for looking up error codes) is infinitely useful. (along with the above links for full-text-searching the manual)
Edit: this page also talks about "built-in functions" - you could try (as stated at the bottom of the page) "#undef memcpy" to use the non-built-in version?
Can you show us your compiler arguments? You need to make sure that you're not pulling in the standard C header files in addition to the metal C ones. Here's an example:
xlc -c -Wc,metal,longname,nosearch,'list(./)' -I. -I /usr/include/metal -I "//'SYS1.SIEAHDRV'" -S -qlanglvl=extended foo.c
as -mrent -mgoff -a=foo.list -o foo.o foo.s
ld -bac=1 -brent -S "//'SYS1.CSSLIB'" -o foo foo.o
Are you missing the closing brace '}' for the function? How about any missing semi-colon line terminators? When missing braces/semi-colons the z/OS C compiler throws some strange/misleading messages sometimes. I don't have it to try out, but I'm assuming Metal does as well.
The issue was with the search order. Although I did search(/usr/metal/include) from with in my JCL I didn't proceed it with a nosearch option, so string.h was getting picked up from the standard system librarys instead of the version included with Metal C. I've pasted my optfile dataset I passed to the CPARM below for refference.
//OPTIONS DD * SO LIST LONG NOXREF CSECT METAL LP64 NOSEARCH search(/usr/include/metal/)