Reading hex value from image in C

Question

I'm trying to read the hex values from an image file using C. In Linux, this code works fine, but with Windows it reads only the first 334 bytes and I don't understand why.

The code to read the file is:

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <sys/stat.h>    
void readHexFile(char* path) {
        FILE *fp;

        if ((fp = fopen (path, "r")) != NULL) {
            struct stat st;
            stat(path, &st);                

            int i;
            int ch;
            for (i = 0; i < st.st_size; i++) {
                ch = fgetc(fp);             
                printf("%x ", ch);
            }

            fclose(fp);
        }
        else {
            return NULL;
        }
}

st.st_size comes from <sys/stat.h> package and contains the right value (the size, in bytes, of the image file)

This image show what my program outputs, and the actual binary content of the file it is reading:

As you see after the sequence of 17, 18, 19 there is also hex values but my program prints ffffffff repeatedly.

There is noting to be seen. See [ask] and provide a [mcve]. And don't post images of text! — too honest for this site, Jan 26 '17 at 14:49
`fgets` is probably returning `EOF`. Probably you are not reading the correct length, or you are not using the correct file. — LPs, Jan 26 '17 at 14:51
show us more of your code: how do you open the file? It's unclear why you are not looping until something `feof(fp)` — Wolf, Jan 26 '17 at 14:53
You should probably also make sure you `fopen()` the file in a binary mode. That is a significant unix/windows difference. Its always binary in unix and linux. — infixed, Jan 26 '17 at 14:55

score 5 · Answer 1 · answered Jan 26 '17 at 14:58

5

You opened the file in a text mode, and not as binary. Different platforms may behave differently.

In this case, Microsoft Windows decided that this plain text file ends at the first occurrence of Ctrl+Z (0x1A), and returns EOF for all fgetc afterwards.

Explicitly state that you want to open the file as binary:

fp = fopen ("yourfile", "rb");

and the problem goes away.

answered Jan 26 '17 at 14:58

Jongware

22,200
8
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Great, but if `0x1A` means `Ctrl+Z`, why are you placing it into parentheses? – Wolf Jan 26 '17 at 15:23

score 1 · Answer 2 · answered Jan 26 '17 at 14:57

1

I think your loop should look like this:

int ch;
while (!feof(fp)) {
    ch = fgetc(fp);
    printf("%x ", ch);
}

It's completely unclear to me why you are using st.st_size here.

answered Jan 26 '17 at 14:57

Wolf

9,679
7
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108

If I use with while cycle it prints until 0x19 but the file is longer and longer – Davide Jan 26 '17 at 14:58
Well, your problem was obviously [using text mode](http://stackoverflow.com/a/41876316/2932052). But what's the reason behind not using `foef`: performance? – Wolf Jan 26 '17 at 15:20
Yes. I used st.st_size to show you wrongs hex. Yes, now I'm already using EOF – Davide Jan 26 '17 at 15:22

score 1 · Answer 3 · edited May 23 '17 at 12:01

On Windows, the character 0x1A (Ctrl+Z) is the EOF character for text mode; see this question.

If you're reading from a binary file like a JPEG, you should do so with first opening the file as binary (fopen mode "rb"), then fread into a pre-allocated buffer, the size of which you would determine with ftell with the file pointer at the end of the file:

size_t i, len;
char *buffer = NULL;
fp = fopen(argv[1], "rb");
if(!fp)
   // handle error

fseek(fp, 0, SEEK_END);
len = ftell(fp);
rewind(fp);

buffer = malloc(len + 1);
if(!buffer)
    // handle error

fread(buffer, 1, len, fp);
for(i = 0; i < len; i++)
{
    printf("%.2X ", buffer[i]);
}

free(buffer);
buffer = NULL;

Reading hex value from image in C

3 Answers3