strtok() segmentation fault on empty string input

Question

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

#define MAX_INPUT_LENGTH 1024

char *readline(FILE* fp, int max_length)
{
    //The size is extended by the input with the value of the provisional
    char *str;
    int ch;
    int len = 0;
    int current_max = max_length;

    str = (char*)malloc(sizeof(char)*current_max);
    if(!str)
        return str;

    while((ch = fgetc(fp))!=EOF)
    {
        /* Separate newline handling from EOF handling */
        if((char)ch == '\n')
        {
            str[len] = '\0';
            return str;
        }
        str[len++] = ch;

        if(len == current_max)
        {
            current_max = current_max + max_length;
            str = realloc(str, sizeof(char)*current_max);
            if(!str)
                return str;
        }
    }
    str[len] = '\0';

    return str;
}


int main(void)
{
    char *input, *token;

    input = readline(stdin, MAX_INPUT_LENGTH);

    printf("BEFORE\n");
    token = strtok(input, " "); //Segmentation Fault Here
    printf("AFTER"); //control never reaches here on " " or "\n" input.
}

In the above snippet, I am trying to tokenize strings on whitspace as delimiter. Whenever I give input as a newline(press ENTER) or a sequence of whitespaces, the strtok() call segfaults. From what I understand, it should return NULL which I should be handling in sanity check later, but that is not really happening.

Please help me, what am I missing here ?

[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, Sep 09 '16 at 17:12
You mind creating a [___MCVE___](http://stackoverflow.com/help/mcve)? — Sourav Ghosh, Sep 09 '16 at 17:14
@SouravGhosh it's a success because, I do check for that in the original code and for non-empty string input like "hello world", the program works fine as expected. — bawejakunal, Sep 09 '16 at 17:14
Or are you trying to *use* the value of `token` without checking whether it is NULL? — rici, Sep 09 '16 at 17:17
@SouravGhosh MCVE created. I checked on Ideone, it's not reproducing the same error, but don't know why on my machine it's giving a segmentation fault. — bawejakunal, Sep 09 '16 at 17:22
What is the actual error message? Your updated question is not causing any fault here, but `token == NULL`. Are you *sure* this is your code, that you are not using `token`? — Weather Vane, Sep 09 '16 at 17:32

score 1 · Answer 1 · answered Sep 09 '16 at 17:28

Your analysis of where the fault is occurring is incorrect

printf("AFTER"); //control never reaches here on " " or "\n" input.

Actually, control does reach there. You just don't see the "AFTER" message because there's no newline or flush there. Change the code to this and the mystery will go away:

printf("AFTER\n");

I'll bet you couldn't replicate the fault with your MCVE because it does reach the end of main, which flushes output. You forgot the "V" in MCVE. You need to verify that it replicates the problem.

strtok() segmentation fault on empty string input

1 Answers1