Sorting lines of text according to first character - C programming

Question

Hello I wanted to make a sorting software in C. That sorts line of text based on the first character of the line. I want to group lines starting of V with Vs. so lines with letters starting with V will be sorted together. Anyone can guide me on how to do this?

f -0.80811 -0.520398 -0.275942

v 0.0146114 -0.0939821 0.00366211
v 0.0133369 -0.0926109 0.00480863
v 0.0142794 -0.0932492 0.00325213



f -0.788618 -0.530816 -0.310349

v 0.0146114 -0.0939821 0.00366211
v 0.0142794 -0.0932492 0.00325213
v 0.0144766 -0.0933648 0.00294873



f -0.726784 -0.686213 0.0299526

v 0.0154684 -0.0952972 0.00472045
v 0.0158173 -0.0956479 0.00515121
v 0.0150361 -0.0947439 0.00690701

Answer 1

This proof of concept does what you need. But it would be much easier to
$ sort file.txt

Disclaimer: This is my first post to stack overflow, so if I did something wrong please tell me. :)

Memory overhead of this is 8MiB(sizeof(char *) * 1024*1024). It also stores the full contents of the file in memory. Consuming approximately ~60Mib of RAM.

#define _POSIX_C_SOURCE 200809L
#include <stdio.h>
#include <stdlib.h>

#define LINES_IN_FILE 32

int compare(const void *ap, const void *bp)
{
    char **a = (char **)ap;
    char **b = (char **)bp;

    return strcmp(*a, *b);
}

int main(void)
{
    FILE *in = fopen("text.txt", "r");
    if (!in)
        return 1;

    char **pbuf, **buf;
    pbuf = buf = malloc(sizeof (char *) * LINES_IN_FILE);

    size_t count = 0, len = 0;
    while (getline(pbuf, &len, in) != -1) {
        pbuf++;
            count++;
        len = 0;
    }

    qsort(buf, count, sizeof(char *), compare);

    /* cleanup and print or whatever */
    return 0;
}

You can speed this up a bit by replacing the strcmp() function with tests to only compare the first bytes, i.e. if(*a[0] > *b[0]) ...