For example : let's assume that program uses a code like this: #include<iostream> void convertToASCII() { FILE *bmp; FILE *ASCII; bmp=fopen("1.bmp","r"); ASCII=fopen("1.bin","w"); char c; int x; while( (c = fgetc( bmp )) != EOF ) { for( x = 0; x < 8; x++ ) fputc( '0' + !!(c&(1<<x)), ASCII ); } } int main() { convertToASCII(); } you are looking for the function that converts back the bin file to bmp i guess.. as far as i understood your question is this.. am i right?