#include <errno.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <unistd.h>
 
int main(int argc, char **argv) {
	int opt;
	FILE *fp;
	char *fname = NULL;
	char *brightness = NULL;
	
	while ((opt = getopt(argc, argv, "b:f:")) != -1) {
		switch (opt) {
		case 'b':
			brightness = optarg;
			break;
		case 'f':
			fname = optarg;
			break;
		default:
			fname = brightness = NULL;
		}
	}

	if (fname && brightness) {
		if((fp = fopen(fname, "w+")) != NULL) {
			if (fprintf(fp, "%s", brightness) > 0) {
				fclose(fp);
				exit(EXIT_SUCCESS);
			}
		}
		fprintf(stderr, "%s: %s: '%s'\n", argv[0], strerror(errno), fname);
	} else {
		fprintf(stderr, "Usage: %s -f PATH -b NUM\n"
				"Options: -f\t\tsysfs path\n"
				"         -b\t\tbrightness value\n", argv[0]);
	}
	exit(EXIT_FAILURE);
}

Just for fun and to refresh my C coding skills. Improved version with command line arguments handling and error checking.
Ciao

#include <stdio.h>
#include <fcntl.h>	// Use "open"
#include <unistd.h>	// For "close"

int main() {
	// Open the brightness file.
	FILE *fp = fopen("/sys/class/backlight/acpi_video0/brightness", "w+");
	// Adjust brightness value.
	fprintf(fp, "5");
	// End.
	fclose(fp);
	return 0;
}

Then compile the file, change ownership to root, and set SUID:

$ gcc -o testing testing.c
$ sudo chown root:root testing
$ sudo chmod u+s testing

Now run the executable as regular user and it will change the brightness:

$ ./testing # boom! brightness changes

It was segfaulting because the binary did not have enough permissions to open the file.

A few minutes ago, I tried to translate this into a simple C program.  It compiled fine.  However, when I tried running the program as root, it gave a "Segmentation fault" as the result, and did not change the brightness.

Can anyone offer any pointers?

Here's the code:

#include <stdio.h>
#include <fcntl.h>	// Use "open"
#include <unistd.h>	// For "close"

int main() {
	// Open the brightness file.
	FILE *fp = fopen("/sys/class/backlight/acpi_video0/test", "w+");
	// Adjust brightness value.
	fprintf(fp, "5");
	// End.
	fclose(fp);
	return 0;
}