The variable count stops incrementing at 1 and I don't know why.

#include <stdio.h>
#include <stdlib.h>
#include <stdint.h>

typedef uint8_t  BYTE;

int main(int argc, char* argv[])
    // check if the number of commandlinearguments is legal
    if (argc != 2)
        fprintf(stderr, "Usage: infile\n");
        return 1;

    // open the input file
    FILE* infileptr = fopen(argv[1], "r");

    // check if file exists
    if (infileptr == NULL)
        fprintf(stderr, "Couldn't open file\n");
        return 2;

    // initialize buffer for 512 bytes of the infile
    BYTE buffer[512];

    // initialize counter for filename;
    int count = -0;

    // declare filename
    char filename[7];

    FILE* outfileptr;

    while (fread(buffer, 1, 512, infileptr) == 512)
        // check if new jpeg begins
        if (buffer[0] == 0xff &&
            buffer[1] == 0xd8 &&
            buffer[2] == 0xff &&
            (buffer[3] & 0xf0) == 0xe0)

            // close old file
            if (count > 0)

            // update filename
            sprintf(filename, "%03d.jpg", count);

            // open new file
            outfileptr = fopen(filename, "w");

            // increment file counter

        // copy jpeg if there is an open file
        if (count > 0)
            fwrite(buffer, 512, 1, outfileptr);

    return 0;

2 Answers 2


When a string (char*) is initialized like this char* filename = "000.jpg"; it is a "string literal", or constant, and is stored in read-only memory. The program will seg fault here sprintf(filename, "%03d.jpg", count); when it tries to change the contents of filename. You'll need to declare filename as a char array, or a char* that is allocated with malloc/calloc (and make sure you allocate enough room for the null terminator in either case!).

Another problem brewing: The comment says // initialize buffer for 512 bytes of the infile. But the allocation int buffer[512]; is 512 ints. Review bmp.h (from resize) for hints on how to make the array a "byte type".

  • I've changed the char* to an array and no i have no errors. But I have still no images as output. May 25, 2017 at 15:26
  • answer edited with additional info. May 25, 2017 at 16:09
  • Thank you I've implemented the byte datatype and know I get 2 images. May 25, 2017 at 16:21
  • What's wrong with the filename allocation? May 25, 2017 at 16:46

The problem is that the strings always have a terminating byte \0 so my string is 0 bytes long but I copy it in a 7 byte buffer. And because my counter variable is next to my buffer on the stack i write the terminating byte in my counter variable and set it every iteration to zero.

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