SDL_Surface* SDL_CreateRGBSurfaceWithFormatFrom(void*  pixels,
                                                int    width,
                                                int    height,
                                                int    depth,
                                                int    pitch,
                                                Uint32 format)

// This example shows how to create a SDL_Surface* with the data loaded
// from an image file with stb_image.h (https://github.com/nothings/stb/)

// the color format you request stb_image to output,
// use STBI_rgb if you don't want/need the alpha channel
int req_format = STBI_rgb_alpha;
int width, height, orig_format;
unsigned char* data = stbi_load("./test.png", &width, &height, &orig_format, req_format);
if (data == NULL) {
    SDL_Log("Loading image failed: %s", stbi_failure_reason());
    exit(1);
}

int depth, pitch;
Uint32 pixel_format;
if (req_format == STBI_rgb) {
    depth = 24;
    pitch = 3*width; // 3 bytes per pixel * pixels per row
    pixel_format = SDL_PIXELFORMAT_RGB24;
} else { // STBI_rgb_alpha (RGBA)
    depth = 32;
    pitch = 4*width;
    pixel_format = SDL_PIXELFORMAT_RGBA32;
}

SDL_Surface* surf = SDL_CreateRGBSurfaceWithFormatFrom((void*)data, width, height,
                                                       depth, pitch, pixel_format);

if (surf == NULL) {
    SDL_Log("Creating surface failed: %s", SDL_GetError());
    stbi_image_free(data);
    exit(1);
}

// ... do something useful with the surface ...
// ...

// when you don't need the surface anymore, free it..
SDL_FreeSurface(surf);
// .. *and* the data used by the surface!
stbi_image_free(data);