Compare two pictures byte by byte in unix c

while ((c=fgetc(img1)) != EOF)

This reads the first byte from img1. Assuming it's not EOF...

fread(&buffer1, 1,1, img1);

This reads the second byte from img1.

fread(&buffer2, 1,1, img2);

This reads the first byte from img2.

if(buffer1 != buffer2)

So now we're comparing the first byte of img2 to the second byte of img1.

If they happen to be equal, we'll go around the loop again, and we'll end up comparing the second byte of img2 to the fourth byte of img1.

It looks like you might have thought that fgetc tested to see if there was another byte available, and that fread then read it. But no. fgetc reads one character from the file and then returns it. If you write

c = fgetc(fp);

it's almost exactly the same as if you wrote

fread(&c, 1, 1, fp);

So a minimal fix to your program would be to get rid of the

fread(&buffer1, 1,1, img1);

line, and change the equality test to

if(c != buffer2)

But that ends up being kind of confusing -- a later reader is likely to wonder, "why is one file being read using fgetc, and the other using fread?"

If I were writing this, I might read one byte from img1 using getc, and one byte from img2 using getc, and compare them. Or, I might use fread to read N bytes from img1 into a character array, and N bytes from img2 into a second character adday, and compare them using the memcmp function.

Also, as Paul Ogilvie mentioned in a comment, since these are binary files, when you call fopen to open them you should specify "rb" mode.

In my opinion, you should convert them into two bitmaps and make a header and matrix comparison. You are having trouble now because headers and tags can be different. You can find many tutorials about this topic, such as this one on Code Project.

As stated in other comments, the fgetc was reading a char off the stream, so the 2 streams are out of sync before the calls to fread. The following uses 2 calls to fgetc and then uses the return values for the comparison.

void compareFiles(char* path1, char* path2){    FILE* img1;     FILE* img2;     if((img1 = fopen(path1,"rb")) == NULL || (img2 = fopen(path2,"rb")) == NULL){         printf("ERROR fopen()\n");         exit(-1);     }       int i=0;    int byte1,byte2;    while (((byte1=fgetc(img1)) != EOF) && ((byte2=fgetc(img2)) != EOF)){        if(byte1 != byte2){            printf("Byte differs\n IMG1: %c, IMG2: %c; Position %d\n",byte1,byte2,i);              i++;         }     }      fclose(img1);    fclose(img2);}