Segmentation fault on malloc for double pointer

Question

I'm making a program that reads in text from a file, delimits it into alphanumerical words, and so on. I'm running into an issue where, I've read from the file into a string, and I'm trying to delimit that string into a 2d array where each word is stored in an array. From there I will go on to order them.

However, I am getting a segfault when I malloc for words[n]. For a test file that I'm using, it has ~400 characters. No word is longer than say 10 characters, so (j - i + 1) wouldn't be a huge allocation that might cause a stack overflow (unless I'm misunderstanding my allocation).

I can explain my code for the function if need be, it's a manipulation of a strsplit() function that I wrote.

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

int main(int argc, char** argv) {
    if (argc != 2) {
        printf("Please provide 1 argument.\n");
        return 0;
    }

    FILE *file = fopen(argv[1], "r");
    fseek(file, 0, SEEK_END);
    size_t length = ftell(file); // Getting file size

    char buff, **words, buffer[length + 1];
    int i = 0, j = 0, n = 0, h = 0;

    buffer[length] = '\0';
    rewind(file);
    fread(buffer, 1, length, file); // Read file into buffer, close file
    fclose(file);

    // Deliminate to 2-D array of words
    while (buffer[i] != '\0') {
        while (isalnum(buffer[j]))
            j++;
        i = j;
        while (isalnum(buffer[j]))
            j++;

        words[n] = (char *)malloc(sizeof(char) * (j - i + 1));
        while (i < j)
            words[n][h++] = buffer[i++];

        n += 1, h = 0;
    }
}

It's worth noting that the file input works, and the algorithm to delimit works (it correctly captures the first word from the file from i to j). Do I need to malloc for **words? I don't think that's the issue, but I wouldn't know how many words are in a sample file so I would have to malloc some large amount.

John3136 · Accepted Answer · 2018-02-19 23:06:52Z

5

You never actually set the value of words to anything. words needs some memory before you can populate words[n].

You could do char** words = malloc(MAX_NUM_OF_WORDS * sizeof(char*)) or have a stack based collection: char* words[MAX_NUM_OF_WORDS]

Free tip for the day: using file without a NULL check is not good practice.

answered Feb 19, 2018 at 23:06

John3136

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5 Comments

DanBan Over a year ago

Damn, that's what I thought it might be. I didn't want to set some arbitrary MAX_NUM_OF_WORDS but I supposed there's no way around it. And I agree, this program is only in early stages (there's still a lot more to do) and I'm in testing phase, using only a specific file to get my functions working. I'll add in the NULL check. Thanks!

Rorschach Over a year ago

@DanBan you can resize it as you go with realloc

DanBan Over a year ago

@jenesaisquoi So every time it runs through the loop I realloc and add space for a word before I add it? That wouldn't affect the current contents of it would it?

Rorschach Over a year ago

@DanBan you could do that, but I would limit the system calls by reallocation larger chunks at a time. The contents won't be affected

John3136 Over a year ago

Either go with a different structure (perhaps a linked list? depends what you want to do with the words at the end). If it really needs to be an array then as @jenesaisquoi says: allocate in "chunks" (pretty much implementing a C++ vector ;-)

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