Question

I am trying to do a pretty simple thing – it is reading a file and then turning it into a char** splitting it into lines. However when I return a struct containing the char** and size i get Segmentation fault. I read here: C segmentation fault before/during return statement that it’s probably “mangled stack”. I still however don’t know what I did to mangle it. This is my code:

#include <stdio.h>
#include <string.h>
#include <stdlib.h>
#include <ctype.h>
#include "comp_words.h"
#define BLOCK 4096

struct sized_str {
    char* str;
    long size;
};

struct sized_arr {
    char** content;
    int size;
};

struct sized_str readfile(char* name) {
    FILE *f;
    long filesize;
    char *buf;
    struct sized_str res;
    int r, p = 0;

    f = fopen(name, "r");
    fseek(f, 0, SEEK_END);
    filesize = ftell(f);
    rewind(f);
    buf = calloc(filesize + 1, sizeof(char));
    while ((r = fread(buf + p, sizeof(char), BLOCK, f))) {
        p += r;
    }
    res.str = buf;
    res.size = filesize + 1;

    return res;
}

struct sized_arr read_dict() {
    struct sized_str file_content;
    struct sized_arr result;
    char *buf, *buf_cpy, *buf_cpy_point, *line, **res;
    int i = 0, j, line_count = 0;

    file_content = readfile("/var/tmp/twl06.txt");
    buf = file_content.str;
    buf_cpy = (char*)malloc(file_content.size * sizeof(char));
    strcpy(buf_cpy, buf);
    buf_cpy_point = buf_cpy;

    while (strtok(buf_cpy_point, "\n\r")) {
        line_count++;
        buf_cpy_point = NULL;
    }

    res = (char**)malloc(sizeof(char*) * line_count);

    while ((line = strtok(buf, "\n\r"))) {
        res[i] = (char*)malloc(sizeof(char) * strlen(line));

        j = 0;
        while ((res[i][j] = tolower(line[j]))) {
            j++;
        }
        buf = NULL;
    }
    free(buf_cpy);
    result.size = line_count;
    result.content = res;

    return result;
}

// ...

int main (int argc, char** argv) {
    struct sized_str input;
    struct sized_arr dict;

    dict = read_dict();

    // ...
    return 0;

The code segfaults while returning from read_dict function.

Answer 1

It looks like you forgot to increment i after storing each line into the result array, so you end up storing all lines into res[0]. But you still set result.size = line_count at the end, so all array elements beyond the first are undefined. An i++ at the end of this loop: while ((line = strtok(buf, "\n\r"))) should fix it.