Using rewind() returns an error, removing it allows code to run fine

Question

Summary

I am trying to create a program that converts the contents of a file into a string.

I fseek to the end of the file then ftell to find the length in order to use the length for fread.

Problem is, I have to fseek back to the start of the file in order to read from the beginning, or else it will read starting from the end of the file, which is obviously a problem.

Problem is, using rewind() returns an error for some reason.

Note that removing the rewind() line from my code allows it to run without errors, but (null) is printed because its reading from the end of the file. This is why I'm assuming rewind() is the problem.

Code

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

int main(void) {
    FILE* dictionary = fopen("test.txt", "r");
    fseek(dictionary, 0, SEEK_END);
    int len = ftell(dictionary);
    rewind(dictionary);

    char* txt;
    fread(txt, len+1, 1, dictionary);
    printf("%s", txt);
}

Error Message

shell returned 139

`txt` points nowhere. What you see is a manifestation of _undefined behaviour_ (google that). — Jabberwocky, Dec 07 '20 at 15:07
And `fread()`. Not checking the return value from `fread()` is why you thought your code worked without rewind. Your code didn't work - the `fseek()` call left the file pointer at the end of the file, so the `fread()` call **didn't read anything** and never had to try accessing non-existent memory. — Andrew Henle, Dec 07 '20 at 15:24

ryyker · Accepted Answer · 2020-12-07T15:11:12.463

3

Memory is needed in txt:

Change:

 char* txt;
 fread(txt, len+1, 1, dictionary);
 printf("%s", txt);

To

char* txt = malloc(len+1);
if(txt)
{
    fread(txt, len, 1, dictionary);
    txt[len] = 0;//add terminating NULL
    printf("%s", txt);
    free(txt);
}

edited Dec 07 '20 at 15:11

answered Dec 07 '20 at 15:05

ryyker

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2

I suggest you `fread(txt, len, 1, dictionary);` (the actual length) also need `text[len] = '\0';` – Weather Vane Dec 07 '20 at 15:09
Ok thanks! Also, I thought you needed the len+1 for the file terminating character. @WeatherVane – BrianZhang Dec 07 '20 at 15:14
Yes, you do need to *allocate* the extra length. But text files don't contain string terminators. – Weather Vane Dec 07 '20 at 15:14

score 1 · Answer 2 · answered Dec 07 '20 at 15:08

The root of the problem is that fread() tries to write the data from the file into txt, but txt doesn't point to allocated memory. Here's one way to fix it:

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

int main(void) {
    FILE* dictionary = fopen("test.txt", "r");
    fseek(dictionary, 0, SEEK_END);
    int len = ftell(dictionary);
    rewind(dictionary);

    char *txt = malloc(len);
    fread(txt, len, 1, dictionary);
    printf("%s", txt);
}

I think this worked when rewind() was removed because fread reached the end of file before writing to the invalid pointer in txt.

Using rewind() returns an error, removing it allows code to run fine

2 Answers2