Eliminating buffer overflow in freads (C)

Question

Simply problem, but slightly weird because of requirements.

Basically, I am trying to prevent buffer overflow when I am reading in file use file I/O because I am reading the file with a buffer size of 32. I feel like this should have been answered somewhere, but for the life of me my searching is not turning it up.

A simplified version of my code is here:

#include <stdio.h>                
#include <string.h>               
#define BUFFER_SIZE 32              
 
int main(int argc, char *argv[]) {
    FILE * read_file;
       
    char buffer[BUFFER_SIZE];
    read_file = fopen("test.txt","r");
    
    size_t num_rec = BUFFER_SIZE;
    while(fread(buffer, 1,num_rec, read_file) > 0) {
        printf("%s", buffer);
    }
    fclose(read_file);
    
    return 0;
}

Say I am trying to read a test.txt that has the contents:

This is a test file. 
The file is a test. 
I am having an overflow problem.
My C is not the best.

I get a output like this:

This is a test file.                                                                                                                                                               
The file is a test.                                                                                                                                                                
I am having an overflow problem.                                                                                                                                                   
My C is not the best.w problem.                                                                                                                                                    
My C is not the best

I understand that the simplest way of solving this is reading in 1 char at a time not 32, however, is there a way to solve this while still reading in 32 chars at a time?

printf expects a null terminated string, but you never arranged for buffer to be null terminated. — Nate Eldredge

Jonathan Leffler Jonathan Leffler · Accepted Answer · 2021-02-02T00:32:11

The fread() function reads binary data and doesn't add null bytes. You need to tell printf() how many bytes to print, which should be the number returned by fread().

size_t nbytes;
while((nbytes = fread(buffer, 1, num_rec, read_file)) > 0)
    printf("%.*s", (int)nbytes, buffer);

Note that fread() returns a size_t, but the .* operation in printf() requires an int; hence the cast (though it would be possible to save the value from fread() in an int and use that without the cast).

Eliminating buffer overflow in freads (C)

2 Answers