Modifying a string in a function in C

Question

I want to change a string in a function. My code is:
void fillStr(char * string);

int main() {
...
char * str;
...
fillStr(str);
printf("%s", str); //the str doesn't contain what I want
return 0;
}

void fillStr (char * string) {
//A successful code with the malloc and the realloc callings for getcharing to the string.
printf("%s", string); //outputs the string
}

Help me please to find out what I did wrong?

Answer 1

Evgeniy Smelov, I've played around with this until it worked:

Seems you need double pointers.

This is my demonstration (input 'Hello'):

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

void fillStr(char **string);

int main() {
char * str;
fillStr(&str);
printf("%s", str);
free(str);
return 0;
}

void fillStr (char ** string) {
*string = (char*)malloc(6);
for(int i=0; i<5; ++i)
(*string)[i] = getchar();
(*string)[5] = '\0';
printf("%s", *string);
}

Answer 2

Martin, the fillStr's code successfully fills the string using the getchar() but it doesn't actually change the str. That's the question.

Answer 3

The function works fine and it's printf() outputs exactly what I want. It gets a pointer on a string and works with it. Otherwise there would be a memory allocation error. The question is why the actual content of the main's str pointer doesn't change?

Answer 4

Martin, (I don't know how to mention someone) yes but in the heap. So the HonFu's advice to use double pointer works fine. Thank you!