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Bionix1441
Bionix1441

Reputation: 2319

Increment content of pointer

I am trying to increment a value in C and return the old value, and I am doing that using a pointer. The problem is that the new value is still 0 even though I am using pointer.

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

int increment (int *mem) {
    int tmp;
    tmp = *mem;
    *mem++;
    return tmp;
}

int main () {
    int a = 0;
    printf("The old value of a is \t %d", increment(&a));
    printf("The new value of a is \t %d", a);
}

Now when I run this method I get the same value for a that is 0; I was expecting 1 in the second printf. I don't know what I am doing wrong here.

Upvotes: 1

Views: 1291

Answers (3)

Dan
Dan

Reputation: 393

In addition to what everyone else has posted about operator precedence, if you are passing in a pointer to an int to be incremented there is no reason to return a copy of the int via tmp. You will have access to the value in mem outside of the function.

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

void increment (int *mem) {
    (*mem)++;
}

int main () {
    int a = 0;
    printf("The old value of a is \t %d", a);
    increment(&a);
    printf("The new value of a is \t %d", a);
}

Upvotes: 0

gsamaras
gsamaras

Reputation: 73424

Change this

*mem++;

to this

(*mem)++;

The problem lies in the operators priority. You may want to read about C Operator precedence.

So, what does your code do? It increments the value of the pointer, since ++ operator is activated first and then * gets activated, having no real effect.

As a result, your code invokes undefined behavior, since you access (and eventually write) into an invalid memory location since the pointer is incremented before the value is written to it.

Upvotes: 9

richerlariviere
richerlariviere

Reputation: 809

Maybe you missed some parentheses?

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

int increment (int *mem) {
    int tmp;
    tmp = *mem;
    (*mem)++; // problem was here.
    return tmp;
}

int main (){
    int a = 0;
    printf("The old value of a is \t %d", increment(&a));
    printf("The new value of a is \t %d", a);
}

Upvotes: 5

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