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Full Discussion: a problem about malloc()
Top Forums Programming a problem about malloc() Post 25795 by chenhao_no1 on Monday 5th of August 2002 09:31:56 PM
Old 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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MALLOC_ZONE_MALLOC(3)					   BSD Library Functions Manual 				     MALLOC_ZONE_MALLOC(3)

NAME
malloc_create_zone, malloc_destroy_zone, malloc_default_zone, malloc_zone_from_ptr, malloc_zone_malloc, malloc_zone_calloc, malloc_zone_valloc, malloc_zone_realloc, malloc_zone_memalign, malloc_zone_free -- zone-based memory allocation SYNOPSIS
#include <malloc/malloc.h> malloc_zone_t * malloc_create_zone(vm_size_t start_size, unsigned flags); void malloc_destroy_zone(malloc_zone_t *zone); malloc_zone_t * malloc_default_zone(void); malloc_zone_t * malloc_zone_from_ptr(const void *ptr); void * malloc_zone_malloc(malloc_zone_t *zone, size_t size); void * malloc_zone_calloc(malloc_zone_t *zone, size_t num_items, size_t size); void * malloc_zone_valloc(malloc_zone_t *zone, size_t size); void * malloc_zone_realloc(malloc_zone_t *zone, void *ptr, size_t size); void * malloc_zone_memalign(malloc_zone_t *zone, size_t alignment, size_t size); void malloc_zone_free(malloc_zone_t *zone, void *ptr); DESCRIPTION
The malloc_create_zone() function creates a malloc zone, advising an initial allocation of start_size bytes, and specifying flags that alter the standard behavior of the zone. The returned malloc zone can be used to provide custom allocation and deallocation behavior, and to retrieve additional information about the allocations in that zone. The malloc_destroy_zone() function deallocates all memory associated with objects in zone as well as zone itself. The malloc_default_zone() function returns the default system malloc zone, used by malloc(3), and free(3). The malloc_zone_from_ptr() function returns a pointer to the malloc zone which contains ptr or NULL, if the pointer does not point to an allocated object in any current malloc zone. The malloc_zone_malloc(), malloc_zone_calloc(), malloc_zone_valloc(), malloc_zone_realloc(), malloc_zone_memalign(), and malloc_zone_free() perform the same task on zone as their non-prefixed variants, malloc(3), calloc(3), valloc(3), realloc(3), posix_memalign(3), and free(3) perform on the default system malloc zone. RETURN VALUES
The malloc_create_zone(), malloc_default_zone(), and malloc_zone_from_ptr() functions return a pointer to a malloc_zone_t structure, or NULL if there was an error. The malloc_zone_malloc(), malloc_zone_calloc(), malloc_zone_valloc(), malloc_zone_realloc(), and malloc_zone_memalign() functions return a pointer to allocated memory. If there is an error, they return a NULL pointer. They are not required to set errno. SEE ALSO
malloc(3), posix_memalign(3) BSD
Aug 13, 2008 BSD
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