C: Initializing a struct and assigning it to pointer in function parameter

Question

Because of an assignment at Uni, I face the weird problem of having to initialize a struct to a pointer provided as a function parameter. Everything appears fine inside of that init function, but once I try to access the objects's values inside a different function, they just appear to be empty.

As I'm horrible at explaining things, I got a small example with actual and expected output:

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

struct foo {
    int bar;
};

void init_foo(struct foo* f) {
    f = malloc(sizeof(struct foo));
    f->bar = 5;
    printf("bar0: %d\n", f->bar);
}

void print_foo(struct foo* f) {
    printf("bar1: %d\n", f->bar);
}

int main() {
    struct foo f;
    init_foo(&f);
    print_foo(&f);
}

Acutal output:

bar0: 5
bar1: 0

Expected output:

bar0: 5
bar1: 5

I haven't dealt with C all that much before, so if someone could provide an explanation and/or a solution to this, I'd be very thankful.

Remy Lebeau · Accepted Answer · 2019-10-26 00:04:24Z

You need to get rid of the malloc(), it doesn't belong since main() already allocated memory for its local f variable. init_foo() just needs to populate the existing memory. By calling malloc(), you are changing the foo* pointer inside of init_foo() to point at different memory, ignoring the original memory that was passed in.

Try this instead:

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

struct foo {
    int bar;
};

void init_foo(struct foo* f) {
    f->bar = 5;
    printf("bar0: %d\n", f->bar);
}

void print_foo(struct foo* f) {
    printf("bar1: %d\n", f->bar);
}

int main() {
    struct foo f;
    init_foo(&f);
    print_foo(&f);
}

On the other hand, if you want init_foo() to allocate memory for the struct, then do this instead:

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

struct foo {
    int bar;
};

void init_foo(struct foo** f) {
    *f = malloc(sizeof(struct foo));
    if (f) {
        (*f)->bar = 5;
        printf("bar0: %d\n", (*f)->bar);
    }
}

void free_foo(struct foo* f) {
    free(f);
}

void print_foo(struct foo* f) {
    printf("bar1: %d\n", f->bar);
}

int main() {
    struct foo *f;
    init_foo(&f);
    if (f) {
        print_foo(f);
        free_foo(f);
    }
}

Or this:

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

struct foo {
    int bar;
};

struct foo* init_foo() {
    struct foo *f = malloc(sizeof(struct foo));
    if (f) {
        f->bar = 5;
        printf("bar0: %d\n", f->bar);
    }
    return f;
}

void free_foo(struct foo* f) {
    free(f);
}

void print_foo(struct foo* f) {
    printf("bar1: %d\n", f->bar);
}

int main() {
    struct foo *f = init_foo(&f);
    if (f) {
        print_foo(f);
        free_foo(f);
    }
}

Thanks for the additional knowledge, but just removing the malloc was sufficient in my case :)

Remy Lebeau · Accepted Answer · 2019-10-26 00:05:17Z

2

In main(), f is an allocated version of the foo struct. You passed a pointer to that allocated structure to init_foo().

There is no need for f to be malloc()'ed inside of init_foo(). f already points to an allocated structure.

I think if you just remove the malloc(), it should work.

edited Oct 26, 2019 at 0:05

Remy Lebeau

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answered Oct 25, 2019 at 23:57

Henry F

483 bronze badges

5 Comments

d0n.key Over a year ago

Thank you very much :) (You were the first one to answer this, so I'll accept as soon as I can)

Remy Lebeau Over a year ago

"You were the first one to answer this" - only by 7 seconds ;-)

d0n.key Over a year ago

@RemyLebeau Gotta support the newcomers :P

Henry F Over a year ago

Ha. Thanks. I've been writing "C" for way too long. I think my favorite language is PHP these days.

d0n.key Over a year ago

@HenryF I should have accepted the other answer then. What a monster would favor PHP over anything?!

Scott Hunter · Accepted Answer · 2019-10-25 23:57:43Z

2

Don't malloc inside init_foo; space was already allocated by the caller. So you're filling up the space you malloced, but not the space pointed to by the pointer passed to the function.

answered Oct 25, 2019 at 23:57

Scott Hunter

50k12 gold badges64 silver badges107 bronze badges

Comments

Chris · Accepted Answer · 2019-10-25 23:59:05Z

1

You are eclipsing struct foo* f.

You have a stack based structure struct foo f defined in main. You pass a pointer to it, to the init_foo function. However you then immediately replace that structure pointer with an allocated structure, and proceed to fill that structure in. The original struct foo* f which was passed into the init_foo function was therefore not modified.

You then leak the memory by exiting the init_foo function and then you print the uninitilized structure.

You do not need the memory allocation, remove it all. Simply initialize directly into the struct foo* f you pass into the init_foo function.

answered Oct 25, 2019 at 23:59

Chris

2,8032 gold badges20 silver badges24 bronze badges

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