C demo displaying a PNG image on X server using Xlib and libspng libraries

xlib-png-demo.c

/* requires Xlib and libspng            *
 * gcc -o demo demo.c -l X11 -l spng -g */
#include <stdio.h>
#include <string.h>
#include <X11/Xlib.h>
#include <spng.h>

unsigned char *process_png_image(char *filename);

int main(void)
{
    XImage *ximage;

    /* defaults */
    int width = 64;
    int height = 64;
    char *filename = "firefox.png";
    char *path = "/usr/share/icons/hicolor/64x64/apps/";
    char full_path[strlen(path) + strlen(filename) + 1];

    strcpy(full_path, path);
    strcat(full_path, filename);

    /* get everything set up */
    Display *display = XOpenDisplay(NULL);
    Visual *visual = DefaultVisual(display, 0);
    Window window = XCreateSimpleWindow(display, RootWindow(display, 0), 0, 0, width, height, 1, 0, 0);

    /* get image */
    ximage = XCreateImage(display, visual, DefaultDepth(display,DefaultScreen(display)), ZPixmap, 0, process_png_image(full_path), width, height, 32, 0);
    XMapWindow(display, window);

    while (1) {
        /* draw image */
        XPutImage(display, window, DefaultGC(display, 0), ximage, 0, 0, 0, 20, width, height);

        /* draw text */
        XSetForeground(display, DefaultGC(display, 0), 0xffffffff); /* set white foreground for text */
        XDrawString(display, window, DefaultGC(display, 0), 10, 15, full_path, strlen(full_path));
    }

    return 0;
}

unsigned char *process_png_image(char *filename)
{
    FILE *png;
    int i;
    spng_ctx *ctx = NULL;
    unsigned char *image32 = NULL, *p, temp;
    size_t image_size;
    
    png = fopen(filename, "rb");
    if (png == NULL)
        fprintf(stderr, "error while opening %s\n", filename);

    ctx = spng_ctx_new(0);
    if (ctx == NULL)
        fprintf(stderr, "spng_ctx_new() failed\n");

    /* Ignore and don't calculate chunk CRC's */
    spng_set_crc_action(ctx, SPNG_CRC_USE, SPNG_CRC_USE);

    /* Set memory usage limits for storing standard and unknown chunks,
       this is important when reading untrusted files! */
    size_t limit = 1024 * 1024 * 32;
    spng_set_chunk_limits(ctx, limit, limit);

    /* Set source PNG */
    spng_set_png_file(ctx, png); /* or _buffer(), _stream() */

    /* Get image header
     * return non-zero if fail */
    struct spng_ihdr ihdr;
    if (spng_get_ihdr(ctx, &ihdr))
        fprintf(stderr, "spng_get_ihdr() error\n");

    /* get image size */
    if (spng_decoded_image_size(ctx, SPNG_FMT_RGBA8, &image_size))
        fprintf(stderr, "spng_decoded_image_size() error\n");

    /* allocate image */
    image32 = malloc(image_size);

    /* Decode the image in one go
     * return non-zero if fail */
    if (spng_decode_image(ctx, image32, image_size, SPNG_FMT_PNG, 0))
        fprintf(stderr, "spng_decode_image() error\n");

    /* perform a byteswap to swap red and blue colors
     * every pixel is 4 bytes long (RGBA) */
    p = image32;
    for (i = 0; i < image_size; i += 4)
    {
        temp = p[i];
        p[i] = p[i + 2];        /* replace the red byte with the blue byte */
        p[i + 2] = temp;        /* replace the blue byte with the red byte */
    }
    
    return image32;
}