This is an example of taking an array of data and writing it to "path" as a PNG file. This example shows how to properly premultiply alpha in the image as required by CGBitmapContextCreate.

gistfile1.c

//"bytes" is a pointer to a const array of image data that we can't modify, so we need to copy 
//the data to a new buffer before multiplying alpha

//Premultiply the alpha and create a new image
UInt8 *premultipliedBuffer = calloc(totalBytes, 1);
memcpy(premultipliedBuffer, bytes, totalBytes);
for (int i = 0; i < totalBytes; i += 4)
{
    UInt8 red = premultipliedBuffer[i];
    UInt8 green = premultipliedBuffer[i + 1];
    UInt8 blue = premultipliedBuffer[i + 2];
    UInt8 alpha = premultipliedBuffer[i + 3];
    float alphaFactor = alpha / 255.0;
    
    UInt8 premultipliedRed = (UInt8)nearbyintf(alphaFactor * red);
    UInt8 premultipliedGreen = (UInt8)nearbyintf(alphaFactor * green);
    UInt8 premultipliedBlue = (UInt8)nearbyintf(alphaFactor * blue);
    
    premultipliedBuffer[i] = premultipliedRed;
    premultipliedBuffer[i + 1] = premultipliedGreen;
    premultipliedBuffer[i + 2] = premultipliedBlue;
}

CGColorSpaceRef colorSpace = CGColorSpaceCreateWithName(kCGColorSpaceSRGB);

#pragma clang diagnostic push
#pragma clang diagnostic ignored "-Wconversion"
CGContextRef bitmapContext = CGBitmapContextCreate(
                                                   premultipliedBuffer,
                                                   imageSize.width,
                                                   imageSize.height,
                                                   8,
                                                   4 * imageSize.width,
                                                   colorSpace,
                                                   kCGImageAlphaPremultipliedLast);
#pragma clang diagnostic pop

CGImageRef cgImage = CGBitmapContextCreateImage(bitmapContext);

CFRelease(bitmapContext);
CFRelease(colorSpace);
free(premultipliedBuffer);


//Now write out the new image
CFURLRef outputUrl = (__bridge CFURLRef)[NSURL fileURLWithPath:path];
CGImageDestinationRef destination = CGImageDestinationCreateWithURL(outputUrl, kUTTypePNG, 1, NULL);
CGImageDestinationAddImage(destination, cgImage, NULL);

CGImageDestinationFinalize(destination);
CFRelease(destination);