Passing String Address As Parameter To Pointer To Pointer To Char Function

Question

While I was doing a online quiz (http://www.mycquiz.com/), I came to this question:

#include <stdio.h>
#include <string.h>
#include <stdlib.h>
void myfunc(char** param){
    ++param;
}
int main(){
    char* string = (char*)malloc(64);
    strcpy(string, "hello_World");
    myfunc(&string);
    printf("%s\n", string);
    // ignore memory leak for sake of quiz
    return 0;

My first answer choice was that the string would get modified, whch didn't happen, and I am puzzled why not? Isn't he passing the address of the literal? Also a train of thought came to me that how would these situation vary?

void myfunc(char* param){
        ++param;
}
int main(){
    char* string = (char*)malloc(64);
    strcpy(string, "hello_World");
    myfunc(string);

void myfunc(char* param){
    ++param;
}
int main(){
    char string[12];
    strcpy(string, "hello_World");
    myfunc(string);

None of them seems to actually change the passed string. So if I do need to modify the string how do I pass it by reference?

Thanks.

Mad Physicist · Accepted Answer · 2014-08-29 04:18:38Z

3

In all cases, you are passing a pointer to something to a C function, modifying the pointer, not its contents, and then printing the string in main(). When you pass a pointer, a copy of it is made, so the version you modify in the function does not affect the value in main(). That is why you will not see any change in the string unless you start to modify the contents of the pointer.

If you wish to make changes to the string:

void myfunc(char** param){
        ++(*param);
}

will print ello_World since it will increment the address used by main() to point to the memory containing the string.

void myfunc(char* param){
    ++(*param);
}

will print iello_World since it will modify the first char in the actual piece of memory containing the string.

edited Aug 29, 2014 at 4:18

answered Aug 28, 2014 at 18:04

Mad Physicist

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4 Comments

Jishan Over a year ago

So why does passing string or &string gives the same result? Also changing myfunc(char** param) to myfunc(char* param) has same results. Are these side effects?

R Sahu Over a year ago

I think you meant the first one to be void myfunc(char** param).

Jishan Over a year ago

@RSahu No, I changed the declarations randomly it yielded same resuts.

R Sahu Over a year ago

@Debojyoti, that comment was meant for Mad Physicist.

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