Question

I’m trying to run function which will return two strings by passing a pointer to them:

#include <stdio.h>

void gen_str(char *str1, char *str2){
   char *s1 = "abcd";
   char *s2 = "defg";
   str1= strdup(s1);
   str2= strdup(s2);
   printf("\n\r str1 %s str2 %s\n\r", str1, str2);
}


int main(void){
   char *s1, *s2;
   gen_str(s1, s2);
   printf("\n\r s1 %s s2 %s\n\r", s1, s2);
   return 0;
}

where output is:

str1 abcd str2 defg


 s1 8, s2 8,

Can someone tell me, what I’m doing wrong? I thought that strdup() will alloc memory for my new strings, and return pointers filled with strings. But actual behavior is different. So I’m asking for help.

Answer 1

You got your pointers confused. The basic type for a C string is already char*, and now you want a pointer to that, i.e. a char**:

void fill_me(char ** ps)
{
  const char * t = "hello";
  *ps = strdup(t);
}

/* elsewhere */
char * s;
fill_me(&s);

Moral: If you want a function to modify an argument passed by the caller, the caller needs to take the address-of (&) something at some point.