why the memory is not getting alocated to this pls help

[naren26051991]

#include<stdio.h>
void allocate(int *,int);
int main(){
int* p =NULL;
allocate(p,100);
printf("%d",*p);
}
void allocate(int* p ,int x){
p = (int*)malloc(sizeof(int));
*p = x;
return 0;
}

compiled but when running its reporting crash dump

[laserlight]

Consider this program:

#include <stdio.h>

void assign(int x);

int main(void) {
    int x = 0;
    assign(x);
    printf("%d\n", x);
    return 0;
}

void assign(int x) {
    x = 1;
}

Why is the output 0 even though I assigned 1 to x in the assign function?

Once you understand why, it should become clear to you that in your allocate function, this:

p = (int*)malloc(sizeof(int));

does not change the value of p in the main function. Hence, you should either be passing a pointer to a pointer, or returning the pointer.

By the way, since you declared allocate as having void return type, you should not be returning 0. Also, please indent your code properly.

[root4]

This program as such compiles and runs fine.
BUT i guess you try to use "p" after that line "allocate(p,100)" and of course dereferencing NULL fails because your function doesn't actually change the pointer value.
What you want is:

int main(void) {
  ...
  allocate(&p, 100);
  ...
}
void allocate(int **p, int x) {...}

At last don't forget to include stdlib.h for malloc.

PS: I wrote the reply at the same time than laserlight so sorry for the redundant info.

[christop]

The variable 'p' in main is different from 'p' in allocate. Any changes to 'p' in allocate are not seen by main.

There are a few other problems I saw in your code as well:

Your main function doesn't have a return statement.

allocate does have a return value but it shouldn't.

You should not cast the return value of malloc, and you should include <stdlib.h> for malloc's prototype.

[naren26051991]

tell me how actually this thing works

actually i am passing the pointer type variable to allocate(p,100)

i am allocating memory through malloc function and

when i am testing it through printf in the allocate function its working but when i am testing the memory allocated through printf in main function its compiling but again crash dump reported pls explain this concept clearly

[christop]

Both main and allocate have variables named 'p'. They are two different variables. Changing p in allocate does not affect p in main. Function calls in C are pass by value, which means the value of main's p (NULL) is passed in to allocate as its own copy, which is also named p. Changing p in allocate changes only allocate's copy of p.

[laserlight]

Okay, I will say it plainly then: your code fails because you assign the result of malloc to a local variable. As such, the pointer named p in the main function is not changed, i.e., it is still a null pointer.

[iMalc]

Passing a pointer to an int would be fine if you just wanted to modify the value of an int variable in main. However you want to modify the value of a pointer to an int, so you have to pass a pointer to a pointer to an int, in order to modify it.