Writing bmp in C. see http://pastebin.com/qCRHiPEZ

write_bmp.c

#include <stdlib.h>
#include <fcntl.h>
#include <unistd.h>
#include <stdio.h>

//define pixelformat of windows bitmap, notice the unusual ordering of colors
typedef struct {
	unsigned char B;
	unsigned char G;
	unsigned char R;
} pixel;

//supply an array of pixels[height][width] <- notice that height comes first
int writeBMP(char* filename, unsigned int width, unsigned int height, pixel** pixels) {
	int fd = open(filename, O_WRONLY|O_CREAT|O_TRUNC,S_IRUSR|S_IWUSR|S_IRGRP|S_IWGRP|S_IROTH|S_IWOTH);  
	static unsigned char header[54] = {
		0x42, 0x4d, //BM
		0x00, 0x00, 0x00, 0x00, //File Size, fill later
		0x00, 0x00, 0x00, 0x00, //reserved, filled with zeros
		0x36, 0x00, 0x00, 0x00, //pixel offset 0x36 = 54
		0x28, 0x00, 0x00, 0x00, //BITMAPINFOHEADER
		0x00, 0x00, 0x00, 0x00, //width
		0x00, 0x00, 0x00, 0x00, //height
		0x01, 0x00, //number of color planes
		0x18, 0x00, //bits per pixel 0x18 = 24
		0x00, 0x00, 0x00, 0x00, //no compression
		0x00, 0x00, 0x00, 0x00, //size of raw data in pixel array, fill later
		0x13, 0x0b, 0x00, 0x00, //Horizontal Resolution
		0x13, 0x0b, 0x00, 0x00, //Vertical Resolution
		0x00, 0x00, 0x00, 0x00, //Number of colors
		0x00, 0x00, 0x00, 0x00 //important colors
	}; //rest is zeroes
	unsigned int pixelBytesPerRow = width*sizeof(pixel);
	unsigned int paddingBytesPerRow = (4-(pixelBytesPerRow%4))%4;
	unsigned int* sizeOfFileEntry = (unsigned int*) &header[2];
	*sizeOfFileEntry = 54 + (pixelBytesPerRow+paddingBytesPerRow)*height;  
	unsigned int* widthEntry = (unsigned int*) &header[18];    
	*widthEntry = width;
	unsigned int* heightEntry = (unsigned int*) &header[22];    
	*heightEntry = height;
	write(fd, header, 54);
	static unsigned char zeroes[3] = {0,0,0}; //for padding    
	for (int row = 0; row < height; row++) {
		write(fd,pixels[row],pixelBytesPerRow);
		write(fd,zeroes,paddingBytesPerRow);
	}
	close(fd);
	return 0;
}