c - unite two strings by copying one by one the characters of the second at the end of the first

Question

this program it suppose to print Hello World but guess what exited, segmentation fault why is that happening ?

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

char f(char *a, char *b) 
{
    int i , m, n;
    m = strlen(a);
    n = strlen(b);
    for (i = 0; i<=n; i++)
    {
        a[m+i] = b[i];
    }
}

int main() {
char*str1 = "hello ";
char*str2 = "world!";
str1=f(str1, str2);
printf("%s", str1);
return 0;
}

Answer 1

You are not allowed to modify string literals. Use arrays with enough elements instead for strings to be modified.

Also assigning the return value of f to str1 is a bad idea because no return statement is executed in the function f and using its return value invokes undefined behavior. The return type should be changed to void if you are not going to return anything.

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

void f(char *a, char *b) 
{
    int i , m, n;
    m = strlen(a);
    n = strlen(b);
    for (i = 0; i<=n; i++)
    {
        a[m+i] = b[i];
    }
}

int main() {
    char str1[16] = "hello ";
    char*str2 = "world!";
    f(str1, str2);
    printf("%s", str1);
    return 0;
}