Simple su implementation

su.c

#include <stdio.h>
#include <pwd.h>
#include <sys/types.h>
#include <unistd.h>

int main(int argc, char* argv[]) {
	if (argc < 3) {
		fprintf(stderr, "Usage: %s USER COMMAND [ARGS]...\n", argv[0]);
		return -1;
	}

	char* user = argv[1];

	struct passwd* passwd = getpwnam(user);
	if (passwd == NULL) {
		perror("error while getting user");
		return -1;
	}

	if (setuid(passwd->pw_uid) < 0) {
		perror("error while changing user");
		return -1;
	}

	argv[argc] = 0; // args should be null terminated list

	if (execv(argv[2], &argv[2]) < 0) {
		perror("error while executing process");
		return -1;
	}
	return 0;
}