is_gif.c

#include <stdio.h>

// Every GIF must have this first 6 bytes
char *gif_header = "GIF89a";

int main(int argc, char *argv[])
{
    // check for correct number of arguments
    if (argc != 2)
    {
        printf("Usage: ./is_gif filename\n");
        return 1;
    }

    // Attempt to open the file
    FILE *f = fopen(argv[1], "r");
    // If the file can't be opened, quit
    if (!f)
    {
        printf("Can't open file.\n");
        return 1;
    }

    // Read the first 6 bytes from file
    char buffer[6];
    // Gets 6 chunks of 1 byte each from file
    int bytes = fread(buffer, 1, 6, f);

    // If there weren't 6 bytes, file is too small
    if (bytes != 6)
    {
        printf("File is too small\n");
        return 1;
    }

    // Compare each byte with the gif header
    for (int i = 1; i < 6; i++)
    {
        if (buffer[i] != gif_header[i])
        {
            printf("Not a GIF file.\n");
            return 0;
        }
    }
    printf("Is a GIF file.\n");
    return 0;
}