Question

Here is my code

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


int main() {

 char f[] = "First";
 char s[] = "Second";
 char *tmp = malloc(strlen(f) + strlen(s) + 2);
 strcpy(tmp, f);
 strcpy(tmp, s);
 printf("%s", tmp);
 free(tmp);
 return 0;
}

I’m trying to concatenate f and s. The problem is that tmp contains only “Second” as a array.
What I miss here

Answer 1

If you insist on using strcpy, your code should be slightly modified:

int main() {
    const char *f = "First";
    const char *s = "Second";
    char *tmp = malloc(strlen(f) + strlen(s) + 1);
    strcpy(tmp, f);
    strcpy(tmp+strlen(f), s);
    printf("%s", tmp);
    free(tmp);
    return 0;
}

You should consider using strncpy instead of strcpy for safety reasons. Also, strcat is a more conventional function for concatenating C string.

EDIT Here is an example of using strncpy instead of strcpy

#define MAX 1024

int main() {
    const char *f = "First";
    const char *s = "Second";
    size_t len_f = min(strlen(f), MAX);
    size_t len_s = min(strlen(s), MAX);
    size_t len_total = len_f + len_s;
    char *tmp = malloc(len_total + 1);
    strncpy(tmp, f, len_f);
    strncpy(tmp+len_f, s, len_s);
    tmp[len_total] = '\0';
    printf("%s", tmp);
    free(tmp);
    return 0;
}