library call from scratch

lib.hex

libcall.c

#include <stdio.h>
#include <sys/types.h>
#include <sys/stat.h>
#include <sys/mman.h>
#include <fcntl.h>
#include <unistd.h>
#include <assert.h>

long filepid()
{
	long (*func)() = NULL;
	long ret;
	int fd = open("lib.hex", O_RDONLY);

	if (fd == -1) {
		perror("open");
		return -1;
	}
	
	func = mmap(NULL, 21, PROT_READ | PROT_EXEC, MAP_PRIVATE, fd, 0);
	if (func == MAP_FAILED) {
		perror("mmap");
		return -1;
	}

	if (close(fd) == -1) {
		perror("close");
		return -1;
	}
	ret = func();
	if (munmap(func, 21) == -1) {
		perror("munmap");
		return -1;
	}
	return ret;
}

int main()
{
	assert(getpid() == filepid());
	puts("Correct");
	return 0;
}

$ hexdump lib.hex 
0000000 4855 e589 27b8 0000 0f00 4805 4589 48f8
0000010 458b 5df8 00c3                         
0000015