readlines.c

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

void *getlines(char *file);

int main(int argc, char **argv) {
    if (argc != 2) {
        printf("usage: <file>\n");
        return 1;
    }

    char *file = argv[1];
    printf("reading file %s\n", file);

    // Get lines.
    void *lines = getlines(file);
    int *line_count = ((int *)lines)[0];
    printf("line count: %d\n", line_count);

    // Print lines.
    for (int i = 1; i <= line_count; i++) {
        char *line = ((char **)lines)[i];
        printf("line %d: %s\n", i, line);
    }

    return 0;
}

void *getlines(char *file) {
    // Set first pointer to integer containing line count.
    void *lines = malloc(sizeof(void *));
    ((int *)lines)[0] = malloc(sizeof(int));
    // Keep a stack value for line count.
    int line_count = 0;
    ((int *)lines)[0] = line_count;

    FILE *fp = fopen(file, "r");
    char *line = NULL;
    size_t buffer_size = 0;
    ssize_t line_length;

    if (fp == NULL)
        exit(1);

    while (line_length = getline(&line, &buffer_size, fp) != -1) {
        line_length = strlen(line);
        // Increase line count value.
        line_count += 1;
        ((int *)lines)[0] = line_count;
        // Allocate memory for line.
        lines = realloc(lines, sizeof(void *) * (line_count + 2));
        ((char **)lines)[line_count] = malloc(line_length);
        // Copy current line to allocated array.
        memcpy(((char **)lines)[line_count], line, line_length);
        // Set last character to null.
        ((char **)lines)[line_count][line_length - 1] = '\0';
    }

    fclose(fp);
    return lines;
}