setuid to another user and give group members write permissionto files with certain extensions. No idea why I ever needed this.

perm.c

// perm.c - setuid to another user and give group members write permission
// to files with certain extensions. 
// Emil Hernvall, 2008-04-14

#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <sys/types.h>
#include <dirent.h>
#include <string.h>
#include <sys/stat.h>
#include <errno.h>

// The extensions to modify
const char *extensions[] = { "jpg", "jpeg", "png", "gif" };

int main(int argc, char **argv)
{
	if (argc != 2) {
		printf("usage: perm [dir]\n");
		return;
	}

	// Make sure the directory requested exists.
	struct stat probe_res;
	if (stat(argv[1], &probe_res)) {
		perror("stat");
		return 1;
	}

	// Change working directory out of pure laziness
	printf("Changing cwd to %s\n", argv[1]);
	if (chdir(argv[1])) {
		perror("chdir");
		return 1;
	}

	// Change user id to gain the ability to modify the permissions
	if (setuid(geteuid())) {
		perror("setuid");
		return 1;
	}

	// Change the permissions of the directory
	if (chmod(argv[1], S_IRUSR | S_IWUSR | S_IXUSR | S_IRGRP | S_IWGRP | S_IXGRP)) {
		printf("Failed to change permission for %s: %s\n", argv[1], strerror(errno));
		return 1;
	} else {
		printf("Changed permission for %s\n", argv[1]);
	}

	// Open the directory
	DIR* dirh = opendir(".");
	if (dirh == NULL) {
		perror("opendir");
		return 1;
	}

	struct dirent* current;
	int i;
	while ((current = readdir(dirh)) != NULL) {

		// Retrieve the extension of the current file
		char *ext = strchr(current->d_name, '.');
		if (ext == NULL) {
			continue;
		}
	
		// Check for a valid extension
		for (i = 0; i < sizeof(extensions)/sizeof(const char*); i++) {
			if (strcasecmp(extensions[i], ext+1) != 0) {
				continue;
			}

			// Change the permissions.
			if (chmod(current->d_name, S_IRUSR | S_IWUSR | S_IRGRP | S_IWGRP)) {
				printf("Failed to change permission for %s: %s\n", current->d_name, strerror(errno));
			} else {
				printf("Changed permission for %s\n", current->d_name);
			}

			// our work here is done.
			break;
		}
	}

	// Close the directory.
	closedir(dirh);

	return 0;
}