08-05-2002
a problem about malloc()
1 . Thanks everyone who read the post.
2 . the programe is that :
#include <stdio.h>
#include <string.h>
void do_it(char *p)
{
p = (char *) malloc(100);
(void )strcpy(p,"1234");
}
int main(void)
{
char *p;
do_it(p);
(void )printf("p = %s \n",p);
return 0;
}
cc -g a.c -o a.out
./a.out
it will show that "memory fault(coredump)"
3 . Why ? I think that malloc() get memory from heap , and the heap can be seen at everywhere in the programe .In other words , it is "global" .
4 Then I write another programe. it works correctly .
#include <stdio.h>
#include <string.h>
char *do_it(void)
{
return ((char *) malloc(100));
}
int main(void)
{
char *p;
p = do_it();
strcpy(p, "asdfa");
(void) printf("p = %s \n",p);
}
5 . Then , the point 4 declare that point 3 is correct , but it is not we want .
6 . Then I think the OS has changed the value p when do_it() function return
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LEARN ABOUT REDHAT
__malloc_hook
MALLOC_HOOK(3) Linux Programmer's Manual MALLOC_HOOK(3)
NAME
__malloc_hook, __malloc_initialize_hook, __memalign_hook, __free_hook, __realloc_hook, __after_morecore_hook - malloc debugging variables
SYNOPSIS
#include <malloc.h>
void *(*__malloc_hook)(size_t size, const void *caller);
void *(*__realloc_hook)(void *ptr, size_t size, const void *caller);
void *(*__memalign_hook)(size_t alignment, size_t size, const void *caller);
void (*__free_hook)(void *ptr, const void *caller);
void (*__malloc_initialize_hook)(void);
void (*__after_morecore_hook)(void);
DESCRIPTION
The GNU C library lets you modify the behavior of malloc(), realloc(), and free() by specifying appropriate hook functions. You can use
these hooks to help you debug programs that use dynamic memory allocation, for example.
The variable __malloc_initialize_hook points at a function that is called once when the malloc implementation is initialized. This is a
weak variable, so it can be overridden in the application with a definition like the following:
void (*__malloc_initialize_hook)(void) = my_init_hook;
Now the function my_init_hook() can do the initialization of all hooks.
The four functions pointed at by __malloc_hook, __realloc_hook, __memalign_hook, __free_hook have a prototype like the functions malloc(),
realloc(), memalign(), free(), respectively, except that they have a final argument caller that gives the address of the caller of mal-
loc(), etc.
The variable __after_morecore_hook points at a function that is called each time after sbrk() was asked for more core.
EXAMPLE
Here a short example how to use these variables.
#include <stdio.h>
#include <malloc.h>
/* Prototypes for our hooks. */
static void my_init_hook(void);
static void *my_malloc_hook(size_t, const void *);
/* Variables to save original hooks. */
static void *(*old_malloc_hook)(size_t, const void *);
/* Override initialising hook from the C library. */
void (*__malloc_initialize_hook) (void) = my_init_hook;
static void
my_init_hook(void) {
old_malloc_hook = __malloc_hook;
__malloc_hook = my_malloc_hook;
}
static void *
my_malloc_hook (size_t size, const void *caller) {
void *result;
/* Restore all old hooks */
__malloc_hook = old_malloc_hook;
/* Call recursively */
result = malloc (size);
/* Save underlying hooks */
old_malloc_hook = __malloc_hook;
/* `printf' might call `malloc', so protect it too. */
printf ("malloc(%u) called from %p returns %p0,
(unsigned int) size, caller, result);
/* Restore our own hooks */
__malloc_hook = my_malloc_hook;
return result;
}
SEE ALSO
mallinfo(3), malloc(3), mtrace(3), mcheck(3)
GNU
2002-07-20 MALLOC_HOOK(3)