[SOLVED] Seg-faulting on my simple C sprite type.

arashi256 · 04-05-2014, 01:29 PM

I'm trying to use typedefs and structs in my simple C program. It compiles but it keeps segfaulting when run. It's using Allegro 5 (the al_* functions to load and release bitmaps) but all you really need to know is that I'm making a simple sprite type with x & y co-ordinates and a bitmap for the sprite itself. There is no draw function yet - it just sets things up and shuts it down.

sprite.h

Code:

#ifndef _sprite_h
#define _sprite_h

#include <allegro5/allegro.h>
#include <allegro5/allegro_image.h>

typedef struct {
        double x;
        double y;
        ALLEGRO_BITMAP *bitmap;
        int size_w;
        int size_h;
} SPRITE;

SPRITE* init_sprite(const char *filepath);
void destroy_sprite(SPRITE *s);

#endif

sprite.c

Code:

#define _sprite_c

#include <stdio.h>
#include "sprite.h"

SPRITE* init_sprite(const char *filepath) {
        SPRITE *s = malloc(sizeof(s));
        s->x = 0;
        s->y = 0;
        s->bitmap = al_load_bitmap(filepath);
        if (s->bitmap == NULL) {
                free(s);
                return NULL;
        } else {
                return s;
        }
}

void destroy_sprite(SPRITE *s) {
        if (s != NULL) {
                al_destroy_bitmap(s->bitmap);
                s->bitmap = NULL;
                free(s);
        }
}

#endif

I'm using it in my main() with: -

Code:

#include "sprite.h"
SPRITE *sprite;
sprite = init_sprite("sprite.png");
// Enter main loop and do stuff...
// Then shutdown.
destroy_sprite(sprite);

I'm clearly doing something dreadfully wrong but I don't see what. Can somebody help me out please?

NevemTeve · 04-05-2014, 01:38 PM

As a start, use gdb to decide where the program stops.

Code:

gdb ./myexe
run
...
backtrace

arashi256 · 04-05-2014, 03:14 PM

Okay, as I added some printf() statements to see where it was crashed out and it appears to be when I'm calling destroy_sprite()...I think. Because that's the only call to free() after the game loop finishes.

Code:

*** glibc detected *** ./gameloop: free(): invalid next size (fast): 0x088f9530 ***
======= Backtrace: =========
/lib/i386-linux-gnu/libc.so.6(+0x75ee2)[0xb74beee2]
./gameloop[0x804921e]
./gameloop[0x8049eda]
/lib/i386-linux-gnu/libc.so.6(__libc_start_main+0xf3)[0xb74624d3]
./gameloop[0x8049111]

Is my destroy_sprite() code wrong? What does "free(): invalid next size" mean in this context?

EDIT: I've definitely narrowed it down to the "free(s);" call in destroy_sprite(). If I take out that, I still get a segfault/core dump but I don't see what I'm doing wrong. Every call to malloc() needs a corresponding free(). So if I leave it in, I get the backtrace above. If I take it out (memory leak) I get a core dump.

dwhitney67 · 04-05-2014, 03:56 PM

This statement is wrong:

Code:

SPRITE *s = malloc(sizeof(s));

It should be:

Code:

SPRITE *s = malloc(sizeof(SPRITE));

Because of the error, it is highly probably that in the course of your application, you smashed the heap with errant data writes to the SPRITE pointer.

Note: The sizeof(s) returns the size of the pointer (4 bytes on a 32-bit system, or 8 bytes on a 64-bit system).

arashi256 · 04-05-2014, 04:08 PM

That's fixed it! Thank you - I can't believe I didn't spot that. Ugh.