GIFLIB

# Debian / Ubuntu
sudo apt install libgif-dev
# Fedora
sudo dnf install giflib-devel
# macOS
brew install giflib
# Windows (vcpkg)
vcpkg install giflib

#include <gif_lib.h>
#include <stdio.h>

int main(void) {
    int err = 0;
    GifFileType *gif = DGifOpenFileName("in.gif", &err);
    if (!gif) {
        fprintf(stderr, "open: %s\n", GifErrorString(err));
        return 1;
    }
    if (DGifSlurp(gif) == GIF_ERROR) {   // reads all frames, de-interlaces
        fprintf(stderr, "decode: %s\n", GifErrorString(gif->Error));
        DGifCloseFile(gif, &err);
        return 1;
    }
    printf("canvas %d x %d, %d frames\n", gif->SWidth, gif->SHeight, gif->ImageCount);

    for (int i = 0; i < gif->ImageCount; i++) {
        SavedImage *frame = &gif->SavedImages[i];
        GifImageDesc *d = &frame->ImageDesc;
        GraphicsControlBlock gcb;
        DGifSavedExtensionToGCB(gif, i, &gcb);  // fills defaults when there's no GCB

        ColorMapObject *cmap = d->ColorMap ? d->ColorMap : gif->SColorMap;
        printf("frame %d: %dx%d at (%d,%d), delay %d0 ms, disposal %d, transparent %d\n",
               i, d->Width, d->Height, d->Left, d->Top,
               gcb.DelayTime, gcb.DisposalMode, gcb.TransparentColor);

        // RGB of the top-left pixel of this frame
        if (cmap) {
            GifColorType c = cmap->Colors[frame->RasterBits[0]];
            printf("  first pixel: %d %d %d\n", c.Red, c.Green, c.Blue);
        }
    }
    DGifCloseFile(gif, &err);
    return 0;
}

#include <stdlib.h>
#include <string.h>

// canvas: SWidth*SHeight*4 bytes, cleared to transparent before frame 0
void draw_frame(GifFileType *gif, int i, unsigned char *canvas) {
    SavedImage *f = &gif->SavedImages[i];
    GifImageDesc *d = &f->ImageDesc;
    ColorMapObject *cmap = d->ColorMap ? d->ColorMap : gif->SColorMap;
    GraphicsControlBlock gcb;
    DGifSavedExtensionToGCB(gif, i, &gcb);
    if (!cmap) return;

    for (int y = 0; y < d->Height; y++) {
        for (int x = 0; x < d->Width; x++) {
            int cx = d->Left + x, cy = d->Top + y;
            if (cx >= gif->SWidth || cy >= gif->SHeight) continue;
            int idx = f->RasterBits[y * d->Width + x];
            if (idx == gcb.TransparentColor || idx >= cmap->ColorCount) continue;
            unsigned char *p = &canvas[(cy * gif->SWidth + cx) * 4];
            p[0] = cmap->Colors[idx].Red;
            p[1] = cmap->Colors[idx].Green;
            p[2] = cmap->Colors[idx].Blue;
            p[3] = 255;
        }
    }
    // after displaying: DISPOSE_BACKGROUND clears the frame rectangle to transparent,
    // DISPOSE_PREVIOUS restores the canvas saved before this frame was drawn
}

#include <gif_lib.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>

// frames[i] holds w*h palette indices into palette (256 colors)
int write_animation(const char *path, int w, int h, int nframes,
                    unsigned char **frames, GifColorType *palette, int delay_cs) {
    int err = 0;
    GifFileType *gif = EGifOpenFileName(path, false, &err);
    if (!gif) {
        fprintf(stderr, "open: %s\n", GifErrorString(err));
        return 1;
    }
    gif->SWidth = w;
    gif->SHeight = h;
    gif->SColorResolution = 8;
    gif->SBackGroundColor = 0;
    gif->SColorMap = GifMakeMapObject(256, palette);

    for (int i = 0; i < nframes; i++) {
        SavedImage *img = GifMakeSavedImage(gif, NULL);  // zeroed
        img->ImageDesc.Width = w;
        img->ImageDesc.Height = h;
        img->RasterBits = malloc((size_t)w * h);          // freed by EGifSpew
        memcpy(img->RasterBits, frames[i], (size_t)w * h);

        if (i == 0) {
            unsigned char loop[3] = {1, 0, 0};  // sub-block 1, loop count 0 = forever
            GifAddExtensionBlock(&img->ExtensionBlockCount, &img->ExtensionBlocks,
                                 APPLICATION_EXT_FUNC_CODE, 11, (unsigned char *)"NETSCAPE2.0");
            GifAddExtensionBlock(&img->ExtensionBlockCount, &img->ExtensionBlocks,
                                 CONTINUE_EXT_FUNC_CODE, sizeof(loop), loop);
        }
        GraphicsControlBlock gcb = {DISPOSE_DO_NOT, false, delay_cs, NO_TRANSPARENT_COLOR};
        EGifGCBToSavedExtension(&gcb, gif, i);
    }

    // writes the file, then frees everything and closes it (GIFLIB 6 signature)
    if (EGifSpew(gif, &err) == GIF_ERROR) {
        fprintf(stderr, "write: %s\n", GifErrorString(err));
        return 1;
    }
    return 0;
}

for (int i = 0; i < gif->ImageCount; i++) {
    SavedImage *f = &gif->SavedImages[i];
    for (int k = 0; k < f->ExtensionBlockCount; k++) {
        ExtensionBlock *eb = &f->ExtensionBlocks[k];
        if (eb->Function == COMMENT_EXT_FUNC_CODE)
            printf("comment: %.*s\n", eb->ByteCount, (char *)eb->Bytes);
    }
}