Odd printf behaviour after calling fread

Question

I am trying to devise a function that will load bitmap images into memory and ultimately display it. I am compiling the code with Watcom 16-bit C compiler with the target set to DOS. I am running the program in DOSBox. The code is as follows:

#ifndef __BITMAP_H__
#define __BITMAP_H__

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

typedef struct DIB
{
    int header_size;
    int px_width;
    int px_height;
}DIB_t;

DIB_t *load_bitmap(char *file_name)
{
    FILE *bmp_file;
    DIB_t *bitmap;
    char garbage[4];
    int c, file_size, pixel_offset;

    bitmap = (DIB_t*)malloc(sizeof bitmap);

    if(!bitmap)
    {
        perror("malloc()");
        return NULL;
    }

    bmp_file = fopen(file_name, "rb");

    if(!bmp_file)
    {
        perror("fopen()");
        return NULL;
    }

    c = fgetc(bmp_file);

    if(c == 'B')
    {
        c = fgetc(bmp_file);

        if(c == 'M')
        {
            fread(&file_size, 4, 1, bmp_file);
            fread(garbage, 1, 4, bmp_file);
            fread(&pixel_offset, 4, 1, bmp_file);
            fread(&bitmap->header_size, 4, 1, bmp_file);
            fread(&bitmap->px_width, 4, 1, bmp_file);
            fread(&bitmap->px_height, 4, 1, bmp_file);
            printf("BMP width: %dpx\nBMP Height: %dpx", bitmap->px_width, bitmap->px_height);
            fclose(bmp_file);
            return bitmap;
        }
    }

    fputs("File format not supported.\n", stderr);
    fclose(bmp_file);
    return NULL;
}
#endif

When you run this program it will output: "BMP width: %dpx\n" but nothing after the newline character?? I find this extremely odd. I have confirmed that none of the operations fail or set the errno and that px_height is actually set to it's appropiate value. Is this something any of you have experience with?

Answer 1

You're only malloc'ing enough space for a pointer with this line

bitmap = (DIB_t*)malloc(sizeof(bitmap));

what you really need is

bitmap = (DIB_t*)malloc(sizeof(DIB_t));

Also, as mrbatch noted, you're using a 16 bit compiler and so attempting to read 4 bytes of data into 2 byte int variables. Make sure the sizeof() your types matches what you're reading, e.g. a long int

So - in general - you are corrupting both your stack and your heap by writing more data than you should, and you can expect your program to behave very oddly :)

Answer 2

In addition to Paul's answer to correct the sizeof for your malloc, try changing your int values to long in their declaration (I'm assuming you're code is correct about the bitmap containing 4 byte values to read). An int in a 16-bit compiler is going to be 2 bytes. You need a long (or equivalently long int) for a 4-byte value.

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As suggested in the comments, even better would be to include inttypes.h and use int32_t instead of int or long to ensure a 32-bit value.