How-to append strings using snprintf (without strcat/strncat).

append-strings-with-snprintf.c

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

#define LOC_MAXLEN 13

int main (void)
{
  char dest[LOC_MAXLEN];
  snprintf(dest, LOC_MAXLEN, "%s%s", "abc", "def");

  printf("%s\n", dest);
  
  /* append new string using length of previously added string */
  snprintf(dest + strlen(dest), LOC_MAXLEN - strlen(dest), "%s", "ghi");
  printf("%s\n", dest);

  /* repeat that */
  snprintf(dest + strlen(dest), LOC_MAXLEN - strlen(dest), "%s", "jkl");
  printf("%s\n", dest);
  
  return 0;
}

One other approach to consider is to have a dedicated tracking variable tracking the number of characters written.

This removes the overhead of strlen.

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

#define LOC_MAXLEN 13

int main (void)
{
  char dest[LOC_MAXLEN] = {0};
  unsigned int destcount = 0;

  destcount += snprintf(dest + destcount, LOC_MAXLEN - destcount, "%s%s", "abc", "def");
  printf("%s\n", dest);
  
  /* append new string using length of previously added string */
  destcount += snprintf(dest + destcount, LOC_MAXLEN - destcount, "%s", "ghi");
  printf("%s\n", dest);

  /* repeat that */
  destcount += snprintf(dest + destcount, LOC_MAXLEN - destcount, "%s", "jkl");
  printf("%s\n", dest);
  
  return 0;
} 

Expected output:

abcdef
abcdefghi
abcdefghijkl