Checking if the correct array length was malloced in C

You can provide your own version of malloc(), calloc(), realloc(), and free() for your tests by re-defining the standard function names.

With gcc, I'd create an object file with the replacement functions

gcc -c myfunctions.c -omyfunctions.o

and compile the test files with a redefiniton and link to the replacement

gcc myfunctions.o -Dmalloc=mymalloc -Dcalloc=mycalloc -Drealloc=myrealloc -Dfree=myfree testfile.c

instead of what the students use

gcc testfile.c

The implementation of the my*() functions shouldn't be too difficult. Basically they check the parameters and call the original functions.

/* myfunctions.c */
#include <stdlib.h>
void *mymalloc(size_t s) {
    /* test s */
    return malloc(s);
}
void *mycalloc(size_t n, size_t s) {
    /* test n and s */
    return calloc(n, s);
}
void *myrealloc(void *p, size_t s) {
    /* test p and s */
    return realloc(p, s);
}
void myfree(void *p) {
    /* test p */
    free(p);
}