Re: [glib] malloc and bdwgc

[Thomas Martitz <kugel rockbox org>]

Am 08.01.2016 um 12:23 schrieb 张海:
> Thank you very much for pointing me to __libc_* aliases.
>
> However as I was trying to use bdwgc, I didn't figure out whether
> bdwgc will infinite loop as I failed to identify the call to memory
> allocation in its source :(
>
> bdwgc itself has a --enable-redirect-malloc configure option, and by
> greping the source with REDIRECT_MALLOC, I found that to correctly
> redefine malloc and free one need to interfere with redefining dlopen,
> threads and etc, and it seems quite hard to incorporate those lines of
> code into my program. In this case I will need to end up building a
> custom bdwgc with --enable-redirect-malloc.

I'm not sure. You don't have to touch any of the libraries you link if 
you implement malloc in the main program. If you define malloc() in a 
program, that *all* dynamically linked libraries should use the malloc() 
defined in the program. This is because, during dynamic link, unresolved 
references are resolved in load-order, and the main program is 
conceptually loaded first.

Thus, even the C library should call that, even if it provides malloc 
itself, because if global symbols are symbols are defined multiple 
times, the first symbol found is used (again, coming from the application).

The following example shows that the application's malloc is used even 
for memory allocated internally in the C library (here by asprintf()).
$ cat test.c
#include <stdlib.h>
#include <stdio.h>


extern void *__libc_malloc(size_t);
void *malloc(size_t n)
{
  printf("Sup!?\n");
  return __libc_malloc(n);
}

int main()
{
  char *p = NULL;
  printf("%s\n", (asprintf(&p, "%s\n", "hello world"), p));
}

$ gcc test.c && ./a.out
Sup!?
Sup!?
hello world


PS: This should equally work if you use a third party library to 
implement malloc. However, in this case it's important that you link it 
first before any other libraries (especially before libc, but I think 
gcc does that by default).

Best regards