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

int main(){
    int len=7;
    // No errors with this:
    //char s1[len];
    //char s2[len];

    // Valgrind error with this:
    char *s1 = malloc(len * sizeof(char));
    char *s2 = malloc(len * sizeof(char));

    strcpy(s1, "String1");
    strcpy(s2, "String2");

    size_t new_size = strlen(s1) + strlen(s2) + 1;

    strncat(s1, s2, new_size);

    puts(s1);

    free(s1);
    free(s2);
    return 0;
}