Is it possible to modify the content of a struct pointer inside a function?

user_17555862 0
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I am C begginer, I was trying to create a function that modify the content of a struct pointer, but it couldn't make it, instead, the content remains the same.

Here my code:

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

typedef struct
{
  int age;
  int code;
}person;

void enter(person *struct_pointer);

void main(void)
{
  person *person_1 = NULL;

  enter(person_1);
  printf("CODE: %i\n", person_1->code);
  free(person_1);
}

void enter(person *struct_pointer)
{
 struct_pointer = malloc(sizeof(*struct_pointer));
 struct_pointer->age = 10;
 struct_pointer->code = 5090;
}

In the example above when I print code of person_1 it does not print nothing, so I assume is because person_1 is still pointing to NULL.

Can someone pls explain how can I do this, and if it cannot be made why.

Thanks

function-declaration pointers pass-by-reference struct c
2 Answers

To change an object (pointers are objects) in a function you need to pass it to the function by reference.

In C passing by reference means passing an object indirectly through a pointer to it. Thus dereferencing the pointer the function has a direct access to the original object.

So your function should be declared and defined the following way

void enter(person **struct_pointer)
{
    *struct_pointer = malloc(sizeof(**struct_pointer));
    if ( *struct_pointer )
    {
        ( *struct_pointer )->age = 10;
        ( *struct_pointer )->code = 5090;
    }
}

and called like

enter( &person_1 );

Otherwise in case of this function declaration

void enter(person *struct_pointer);

the function will deal with a copy of the value of the passed pointer and changing the copy within the function will not influence on the original pointer.

Pay attention to that according to the C Standard the function main without parameters shall be declared like

int main( void )
user_28772410

You can modify the contents of the struct. It doesn't work for you because you are creating a new struct in the enter function rather than editing the original. Just remove the first line (the one with malloc) and instead allocate the struct in the declaration of the person_1variable.

user_143567210
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