Use a function inside a function from another .c file?

Question

I am trying to change the size of the array from another function. The code compiles but the program crashes... I am stuck, very stuck. New to this, my head is spinning up down left right aaaaah........

array.c

struct array {
    int* data;
    int size;
};

struct array* array_create()
{
    struct array* array = (struct array*) malloc(sizeof(struct array));
    array->data = (int*) malloc(sizeof(int) * 10000);
    array->size = 10000;
    return array;
}

void array_resize(struct array* array, int size)
{
    if (size > array->size) {
        free(array);
        array->data = (int*) malloc(sizeof(int) * size);
    }

    array->size = size;
}

int array_size(struct array* array)
{
    return array->size;
}

array.h

typedef struct array* ARRAY;

ARRAY array_create();
void array_resize(struct array* array, int size);

edit.c

void edit()
{
    array_resize(array, 100); // I can run the code but program crashes
}

main.c

ARRAY array;
array = array_create();
edit();                       // Program crashes

Btw, [do not cast the result of `malloc`](http://stackoverflow.com/questions/605845/do-i-cast-the-result-of-malloc). — pzaenger, Feb 09 '14 at 17:10
Its a good practice after freeing something like `array` to set it to `NULL`. You catch errors easier. — γηράσκω δ' αεί πολλά διδασκόμε, Feb 09 '14 at 17:15

score 1 · Answer 1 · answered Feb 09 '14 at 16:51

1

    free(array);
    array->data = (int*) malloc(sizeof(int) * size);

Here you free array, and dereference it at the next line. That's undefined behavior. But seems to be the cause of your crash.

answered Feb 09 '14 at 16:51

StoryTeller - Unslander Monica

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But if I type `array_resize(array, 100)` in main.c instead of in edit.c it will work perfectly! – user3285214 Feb 09 '14 at 16:55
@user3285214, that's the nature of *undefined behavior*, you can't predict how it will act. – StoryTeller - Unslander Monica Feb 09 '14 at 17:03
I tried putting `free(array)` under the other line of code but the program still crashes... – user3285214 Feb 09 '14 at 17:46
@user3285214, you shouldn't ever access memory you freed, directly or in other scopr. You probably meant to free `array->data` – StoryTeller - Unslander Monica Feb 09 '14 at 17:49
It still crashes... But actually the function array_resize() is pre coded and should not be changed... – user3285214 Feb 09 '14 at 17:52
@user3285214, than seek out whomever pre-coded it and punch them in the face. – StoryTeller - Unslander Monica Feb 09 '14 at 17:58
Ok, well I have tried `free(array->data)` and `free(array)` but both still gives me the crash. I have also tried switching place on the two lines of code you quoted. – user3285214 Feb 09 '14 at 18:03
@user3285214, without seeing exactly what you did I can't tell you where the error is hiding. I suggest you set to `NULL` any memory you free and use debugger to look for freed-memory access. If all is well on that front, your error is not in the code you showed us. – StoryTeller - Unslander Monica Feb 09 '14 at 18:07
I edited the OP, added a function in array.c. The program crashes if I (in edit() in edit.c) call `array_size(array)`. For the love of god , I don't understand why!?!?!?! – user3285214 Feb 09 '14 at 19:07

score 1 · Answer 2 · answered Feb 09 '14 at 16:52

1

This:

void array_resize(struct array* array, int size)
{
    if (size > array->size) {
        free(array);
        array->data = (int*) malloc(sizeof(int) * size);
    }

    array->size = size;
}

Should be free(array->data) not free(array). Or better realloc array->data.

Also, unless you are looking for trouble, declare your functions in your .h files as you do in your .c files, i.e.:

struct array;
struct array *array_create();

and include array.h from array.c.

answered Feb 09 '14 at 16:52

abligh

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Crashes where and how? Compile it with `-Wall` and fix all the warnings. Run it under `gdb` and see where it crashes. Run it under `valgrind`. If you are still stuck, post back the exact code you are running in a compilable state. – abligh Feb 09 '14 at 18:11

Use a function inside a function from another .c file?

2 Answers2

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