fsize.c

#include <stdio.h>
#include <assert.h>
#include <sys/stat.h>

size_t fsize(const char *pn) {
    struct stat statbuf;
    assert(!stat(pn, &statbuf));
    return statbuf.st_size;
}

int main() {
    printf("%I64u\n", fsize("fsize.c"));
    return 0;
}