Append character strings with asprintf(3)

astr.c

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

static int append(char **str, const char *buf, int size);

int main(void) {
  char *str = NULL;
  append(&str, "this", 4);
  append(&str, " is",  3);
  append(&str, " a ",  3);
  append(&str, "test", 4);
  printf("%s\n", str);
  free(str);
  return 0;
}

static int append(char **str, const char *buf, int size) {
  char *nstr;
  if (*str == NULL) {
    nstr = malloc(size + 1);
    memcpy(nstr, buf, size);
    nstr[size] = '\0';
  }
  else {
    if (asprintf(&nstr, "%s%.*s", *str, size, buf) == -1) return -1;
    free(*str);
  }
  *str = nstr;
  return 0;
}