Issue with memory allocation and file reading

Hello all,

I am trying to read a file into some allocated memory as part of a
program and I am running into a problem.  The program gets a
segmentation fault during fread.  I have previously used this code on
64-bit RHEL4 without any problems but I am not having this issue on 32-
bit RHEL5.  I have simplified the code to it's most basic form and I
am still seeing the issue.  Below is a full program that causes the
error.  If anybody knows what may be causing the problem, help would
be greatly appreciated....

#include <stdio.h>
#include <fcntl.h>

int main()
{
     int data_size = 1024*1024;
     u_char *data_buf;
     FILE *data_file;
     int data_num;

     data_buf = (unsigned char *) valloc(data_size);
     printf("Memory allocated.\n");
     data_file = fopen("data_file","rb");
     printf("File opened.");
     data_num = fread(data_buf, sizeof(u_char), data_size, data_file);
     printf("File read.");
     return 0;
}

Thanks for your help!