make line feed terminated string

make_buffer.c

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

auto const src = "This is a test. This is a test. This is a test. This is a test.";
ssize_t len;
char* dst;

void copy_append () {
  memcpy (dst, src, len);
  dst [len] = '\n';
}

void use_sprintf () { sprintf (dst, "%s\n", src); }

int main (int argc, char **argv) {

  printf ("%d\n", argc);

  if (argc < 2) return 1;

  auto make_buffer = copy_append;

  printf ("copy_append = %p\n", copy_append);
  printf ("use_sprintf = %p\n", use_sprintf);
  printf ("%c\n", argv [1][0]);

  switch (argv [1][0]) {
    case 's': make_buffer = use_sprintf; break;
    case 'c': break;
    default: return 2;
  }

  printf ("make_buffer = %p\n", make_buffer);

  len = strlen (src);
  
  for (auto i = 0; i < 30000000; i++) {
    dst = (char*) malloc (len+2);
    make_buffer ();
    free (dst);
  }
  return 0;
}

make_buffer_out.txt

$ g++ -std=c++11 -O2 -o make_buffer make_buffer.c 

#using sprintf
$ time ./make_buffer s
2
copy_append = 0x4007d0
use_sprintf = 0x400810
s
make_buffer = 0x400810

real	0m11.506s
user	0m11.510s
sys	0m0.003s

#using copy/append
$ time ./make_buffer c
2
copy_append = 0x4007d0
use_sprintf = 0x400810
c
make_buffer = 0x4007d0

real	0m3.488s
user	0m3.483s
sys	0m0.000s