CODEWITHSUNDEEP
cpointers
PandaRaid
PandaRaid

Reputation: 628

C Pointer Assignment

So I was playing around with some pointer today and ran into a rather confusing predicament. I use C a lot but couldn't seem to figure out why this wouldn't work maybe its just one of those days.

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

void subr(int* numero){
    int* newNum = malloc(sizeof(int));
    (*newNum) = 5;
    numero = newNum;
    printf("%d\n", *numero);
}

int main(){
    int* number;
    printf("%p\n", (void *) number);

    subr(number);
    if(number == NULL)
        printf("Not assigned\n");
    else
        printf("%d\n", *number);
}

So I create a pointer in main, pass it into a function which allocates space and then assigns it back to the pointer passed in. Why is it that everytime I run this I get that the pointer is still null?

Upvotes: 0

Views: 112

Answers (1)

Sergey Kalinichenko
Sergey Kalinichenko

Reputation: 726479

The reason that this code does not work is that C is a pass-by-value language. If you wish to modify anything, you have to pass a pointer to it:

void subr(int** numero){
    int* newNum = malloc(sizeof(int));
    (*newNum) = 5;
    *numero = newNum;
    printf("%d\n", **numero);
}

int main(){
    int* number = NULL; // <<== Need to initialize it
    printf("%p\n", (void *) number);

    subr(&number);
    if(number == NULL)
        printf("Not assigned\n");
    else
        printf("%d\n", *number);
    free(number); // <<== Don't forget to free what's malloc-ed
}

Upvotes: 4

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