malloc() not allocating enough space

Question

Hello here is my problem

FILE *sourcefile;

if ((sourcefile = fopen(argv[1],"r")) == NULL)  //Opens File
{
    printf("Error: Could not open %s\n",argv[1]);
    return 0;
}

fseek(sourcefile, 0 , SEEK_END);  // Sets file pointer at the end of the file
unsigned int fSize = ftell(sourcefile); // Determines file size by the position of file pointer
fseek(sourcefile, 0 , SEEK_SET); // Sets file pointer at the start of the file

char *buffer = (char *) malloc(fSize); 

if (buffer == NULL)
{
    printf("Error: Not enough system memory\n");
    return 0;
}

printf("%d\n",sizeof(buffer));
printf("%d\n",fSile);

fread (buffer,1,fSize,sourcefile);
fclose (sourcefile);

My code is simply opening a file and loading its contents into memory. The problem is, when I use

char *buffer = (char *) malloc(fSize)

it allocates only 4 bytes and not the full size of the file (i.e. 25 bytes when opening a simple txt with a small sentence). When I print the sizes of buffer and fSize at the end, I get 4 and 25 respectively, so fSize is correct. Any idea why this is happening?

Thanks,

[Do not cast the result of malloc in C](http://stackoverflow.com/questions/605845/do-i-cast-the-result-of-malloc). — Chnossos, May 05 '14 at 00:56
`"%d"` requires an `int` argument. Use `"%zu"` for `size_t` (such as the result of `sizeof`) and `%u"` for `unsigned int`. `fSize` should be of type `long`, since that's what `ftell` returns. You're assuming that `ftell` will return the size of the file; that's true on some systems, but it's not portable. There is no portable way in C to determine the size of a file. As long as you're writing non-portable code, you might as well use `stat()` or your system's equivalent. — Keith Thompson, May 05 '14 at 01:16
and open in binary mode, otherwise it's even less reliable than it was — M.M, May 05 '14 at 01:39

score 4 · Accepted Answer · answered May 05 '14 at 00:37

4

sizeof(buffer) should be 4 bytes on a 32-bit platform. It is a pointer pointing to the buffer that malloc allocated. There is no way to query it for the size of the buffer.

answered May 05 '14 at 00:37

Dithermaster

6,223
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Thanks it was pretty obvious, sometimes i get really stuck. – apboutos May 05 '14 at 00:41

score 0 · Answer 2 · answered May 05 '14 at 00:38

0

sizeof(buffer) is the size of a pointer or 32 bits (4 bytes). It is in actuality allocating enough space.

answered May 05 '14 at 00:38

ojblass

21,146
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132

score 0 · Answer 3 · answered May 05 '14 at 11:27

This is maybe what you want to achieve.

#include <stdio.h>
#include <stdlib.h>
int  main(int argc, char *argv[])
{
    FILE *sourcefile;
    if ((sourcefile = fopen(argv[1],"r")) == NULL)  //Opens File
    {
        printf("Error: Could not open %s\n",argv[1]);
        eturn 0;
    }
    fseek(sourcefile, 0 , SEEK_END);  // Sets file pointer at the end of the file
    unsigned long fSize = ftell(sourcefile); 
    fseek(sourcefile, 0 , SEEK_SET); // Sets file pointer at the start of the file
    fSize -= ftell(sourcefile);  /*This determines file size  */
    char *buffer = (char *) malloc(fSize); 
    if (buffer == NULL)
    {
    printf("Error: Not enough system memory\n");
    return 0;
    }

    printf("%ld\n",sizeof(buffer)); 
    printf("%ld\n",fSize);
    fread (buffer,1,fSize,sourcefile);
    fclose (sourcefile);
    return 1;
}

malloc() not allocating enough space

3 Answers3