Find images susceptible of being used as wallpaper. The hard way, in C.

wallpapers.c

#define _BSD_SOURCE
#define _POSIX_C_SOURCE 200809L
#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <string.h>
#include <dirent.h>
#include <endian.h>
#include <stdint.h>
#include <sys/stat.h>
#include <sys/types.h>
#include <sys/wait.h>
#include <fcntl.h>

#define GEOMETRY "1366x768"
#define MAX_ENTRIES 9999
#define MIN_RATIO 4.0/3.0
#define MAX_RATIO 20.0/9.0
#define MIN_HEIGHT 720

int read_size_jpg(int * dst_height, float *dst_ratio, int fd,
        const char * file_name) 
{
    uint8_t buf[4];

    // Find the SOFx (start of frame marker, FF C0)
    // This function accepts SOF0 and SOF2 markers.
    off_t current_marker = 2;
    while (1) {
        if (pread(fd, buf, 4, current_marker) != 4 || buf[0] != 0xFF) {
            fprintf(stderr, "Corrupted file: %s\n", file_name);
            return -1;
        }
        if (buf[1] == 0xC0 || buf[1] == 0xC2) {
            // Found!
            break;
        } else {
            int sectionLength = be16toh(*((uint16_t*) (buf+2)));
            current_marker += 2 + sectionLength;
        }
    }

    // Read width and height from SOFx
    // Structure: FF Cx ll ll dd hh hh ww ww
    // l = length of section
    // d = depth (always 8 bit)
    // w = width
    // h = height
    if (pread(fd, buf, 4, current_marker + 5) != 4) {
        fprintf(stderr, "Corrupted file: %s\n", file_name);
        return -1;
    }

    uint16_t width, height;
    width = be16toh(*(((uint16_t*) buf) + 1));
    height = be16toh(*(((uint16_t*) buf) + 0));
    *dst_height = height;
    *dst_ratio = (float)width / height;

    return 0;
}

int read_size_png(int * dst_height, float *dst_ratio, int fd,
        const char * file_name) 
{
    char buf[8];
    if (pread(fd, buf, 8, 0x10) != 8) { // width
        fprintf(stderr, "Corrupted file: %s\n", file_name);
        return -1;
    }

    uint32_t width, height;
    width = be32toh(*((uint32_t*) buf));
    height = be32toh(*((uint32_t*) buf+1));
    *dst_height = height;
    *dst_ratio = (float)width / height;

    return 0;
}

int read_size(int * dst_height, float *dst_ratio, const char * file_name) {
    int fd = open(file_name, O_RDONLY);
    if (!fd) {
        perror(file_name);
        return -1;
    }

    const char * extension = strrchr(file_name, '.');
    if (!extension) {
        goto skip;
    }
    int ret;
    if (!strcmp(extension, ".png")) {
        ret = read_size_png(dst_height, dst_ratio, fd, file_name);
    } else if (!strcmp(extension, ".jpg") || !strcmp(extension, ".jpeg")) {
        ret = read_size_jpg(dst_height, dst_ratio, fd, file_name);
    } else {
skip:
        fprintf(stderr, "Skiping %s\n", file_name);
        ret = -1;
    }

    close(fd);
    return ret;
}

int image_qualifies(int height, float ratio) {
    return ratio >= MIN_RATIO && ratio <= MAX_RATIO && height >= MIN_HEIGHT;
}

int main(int argc, char** argv) {
    DIR *directory = opendir(".");
    if (!directory) {
        perror("opendir");
        exit(1);
    }

    char found_entries[MAX_ENTRIES][255];
    int found_entries_count = 0;

    struct dirent * dir_entry;
    while ((dir_entry = readdir(directory)) != NULL) {
        int height;
        float ratio;
        if (read_size(&height, &ratio, dir_entry->d_name) != -1
                && image_qualifies(height, ratio)) {
            printf("%s\n", dir_entry->d_name);

            if (found_entries_count > MAX_ENTRIES) {
                printf("Too many entries.\n");
                break;
            }
            strcpy(found_entries[found_entries_count++], dir_entry->d_name);
        }
    }
    closedir(directory);

    // Open image viewer
    int pid = fork();
    if (pid == 0) {
        char * args[MAX_ENTRIES+6] = {
            "feh", "--geometry", GEOMETRY, "--image-bg", "black"
        };
        int i_arg, i_entry;
        for (i_arg = 5, i_entry = 0;
             i_entry < found_entries_count; 
             i_entry++, i_arg++)
        {
            args[i_arg] = found_entries[i_entry];
        }
        args[i_arg] = NULL;

        if (execvp("feh", args) == -1) {
            perror("feh");
            exit(1);
        }
    } else if (pid > 0) {
        while (wait(NULL) != pid)
            ;
    } else {
        perror("fork");
        exit(1);
    }

    return 0;
}