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
– Nate Eldredge, Commented Feb 2, 2021 at 0:25

Jonathan Leffler · Accepted Answer · 2021-02-02 00:48:33Z

3

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).

edited Feb 2, 2021 at 0:48

answered Feb 2, 2021 at 0:32

Jonathan Leffler

759k145 gold badges961 silver badges1.3k bronze badges

Sign up to request clarification or add additional context in comments.

Comments

Zoe - Save the data dump · Accepted Answer · 2022-12-30 17:23:56Z

3

The following saves the return value of fread then uses it to printf the characters that have been read, and no more than that.

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

edited Dec 30, 2022 at 17:23

Zoe - Save the data dump

28.4k22 gold badges130 silver badges163 bronze badges

answered Feb 2, 2021 at 0:30

dxiv

17.8k2 gold badges34 silver badges55 bronze badges

4 Comments

Jonathan Leffler Over a year ago

Remarkably similar to my proposed code. Great minds think alike, or something.

dxiv Over a year ago

I am of the opinion that printf doesn't deserve all the bad press that it gets, and this was a chance to show how it can be used right ;-)

Jonathan Leffler Over a year ago

Yes. On the whole, printf() is mostly sane enough. It tends to be scanf() that earns people's ire, and that at least somewhat justifiably. One of the major omissions in scanf() is the analogue of * in printf() — although scanf() does support *, it has a completely different meaning (suppress assignment, not specify size dynamically).

dxiv Over a year ago

Right, and the missing other * has long been one of my pet peeves about scanf.

Collectives™ on Stack Overflow

Eliminating buffer overflow in freads (C)

2 Answers 2

Comments

4 Comments

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

2 Answers 2

Comments

4 Comments

Your Answer

Sign up or log in

Post as a guest

Related