fread fills string with random character in c

Question

I've been struggling with fread. Even though I'm adding the end of string character '\0', sometimes there is a random character at the end of the string.

Here is my code.

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

uint8_t get_acks(char ** acks);

int main(){
    ...
    char *acks = NULL;
    uint8_t r = get_acks(&acks);
    // test r ...
}


uint8_t get_acks(char ** acks){

    FILE *fp;
    fp = fopen(FILENAME, "r");
    // test fp

    fseek(fp, 0, SEEK_END);
    long len = ftell(fp);
    fseek(fp, 0, SEEK_SET);

    *acks = malloc(sizeof(char)*(len+1));

    fread(*acks, sizeof(char), len, fp);

    acks[len-1] = '\0';

    printf("acks in get_acks: %s", *acks);

    return 0;
}

I've also tried *acks[len-1] = '\0' but the program crashes (nothing is printed from that point on)

Here you have an output example:

acks in get_acks: 1, 2, 3, 4 09:09▒

Here is the file content:

cat new_acks.txt: 1, 2, 3, 4

[Please see this discussion on why not to cast the return value of `malloc()` and family in `C`.](http://stackoverflow.com/q/605845/2173917). — Sourav Ghosh, May 08 '17 at 09:23
@MichaelWalz thank you. I have that test in my code. I didn't put it here to minimize the number of lines. — lmbcerqueira, May 08 '17 at 09:23
Why are you using `uint8_t` as your length? The return type of `ftell` is `long`. You should use a type that can contain all of the possible return values from `ftell`. In particular, `-1` which, if put into a `uint8_t` will look like a length of 255 instead of a failure. — JeremyP, May 08 '17 at 09:31
Also, this code is invalid because the return type isn't compatible with the implicit `int`/`get_acks` is missing prototype. — Antti Haapala -- Слава Україні, May 08 '17 at 09:48

score 3 · Accepted Answer · edited May 08 '17 at 09:33

3

acks is double pointer holding address of acks from main

You need to change

acks[len - 1] = '\0';

to

(*acks)[len] = '\0';

So that buffer you allocated in get_acks gets \0 at the end

edited May 08 '17 at 09:33

JeremyP

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answered May 08 '17 at 09:26

Pras

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True, It should be (*acks)[len] = 0; – Pras May 08 '17 at 09:31
I took the liberty of fixing it. – JeremyP May 08 '17 at 09:33
@JeremyP Thanks! – Pras May 08 '17 at 13:59

fread fills string with random character in c

1 Answers1