What's wrong with this code, which gets content (chars) from file and stores it in char**?

After this line

workspace_file->flc = realloc(workspace_file->flc, (line+2)*sizeof(char*));

the added pointer has an indeterminate value. So using realloc in this statement

workspace_file->flc[line] = realloc(workspace_file->flc[line], (pos+2)*sizeof(char));

results in undefined behavior.

Pay attention to that in general the function realloc can return a null pointer. So you should use an intermediate pointer to store the result of a call of realloc and if it is not equal to null to assign it to pointers workspace_file->flc and workspace_file->flc[line]. Otherwise you will have at least a memory leak.

In the else clause you extend the number of lines you're storing, but you don't initialize the new line that you're going to write to. So when you try to write the first character of the next line, you're dereferencing an invalid pointer.

workspace_file->flc = realloc(workspace_file->flc, (line+2)*sizeof(char*));
line++;
pos = 0;
workspace_file->flc[line] = malloc(1);    // set up the next line

There are multiple problems in the posted fragment:

• you do not test for fopen success: the program will crash if the file cannot be opened.
• fileInputBuffer should have type int to accommodate for all byte values and the special negative value EOF.
• when you reallocate the array of char*, you do not allocate a new empty line at the end of the array.
• you do not check for memory allocation errors.
• the variable names are too long, making the code harder to read.

Here is a modified version:

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

bool read_file(MyStruct *wp) {
    FILE *fp = fopen(wp->filename, "r");
    wp->flc = NULL;
    wp->len = 0;
    if (fp == NULL) {
        fprintf(stderr, "cannot open %s: %s\n", wp->filename, strerror(errno));
        return false;
    }
    int line = 0;
    int pos = 0;
    wp->flc = malloc(1 * sizeof(char *));
    wp->flc[0] = malloc(1);
    int c;
    while ((c = fgetc(fp)) != EOF) {
        if (c == '\n') {
            wp->flc[line][pos] = '\0';
            line++;
            wp->flc = realloc(wp->flc, (line + 1) * sizeof(char *));
            pos = 0;
            wp->flc[line] = malloc(1);
        } else {
            wp->flc[line][pos] = (char)c;
            pos++;
            wp->flc[line] = realloc(wp->flc[line], pos + 1);
        }
    }
    if (pos) {
        // last line without a trailing newline
        wp->flc[line][pos] = '\0';
        line++;
    } else {
        // free the empty line
        free(wp->flc[line]);
    }
    wp->len = line;
    fclose(fp);
    return true;
}