malloc function

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Old 09-30-2002
Originally posted by rajashekaran
p= (int*)malloc(sizeof(int)*580);

/*my reply*/

since by mentioning a integer pointer and storing the integers
by incrementing the pointer value
then what is the purpose of malloc?
u can decalre it as
in t *p;
several integers can be stored by incrementing the value of p,
hence what is the diffrence between this declaration and
malloc function.
this is an basic question according to my knowledge.
please answer this. [/B]
The problem with the original code is that you are allocating that space for use by your program. You were only allocating a single address space and then advancing the pointer, potentially stepping on other programs and system processes.

By allocating the entire chunk you can freely increase the pointer within the chunk without having to worry about stepping on another processing and core'ing the machine.
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mapmalloc(3MALLOC)                                      Memory Allocation Library Functions                                     mapmalloc(3MALLOC)

mapmalloc - memory allocator SYNOPSIS
cc [ flag ... ] file ... -lmapmalloc [ library ... ] #include <stdlib.h> void *malloc(size_t size); void *calloc(size_t nelem, size_t elsize); void free(void * ptr); void *realloc(void *ptr, size_t size); DESCRIPTION
The collection of malloc functions in this library use mmap(2) instead of sbrk(2) for acquiring new heap space. The functions in this library are intended to be used only if necessary, when applications must call sbrk(), but need to call other library routines that might call malloc. The algorithms used by these functions are not sophisticated. There is no reclaiming of memory. The malloc() and free() functions provide a simple general-purpose memory allocation package. The malloc() function returns a pointer to a block of at least size bytes suitably aligned for any use. The argument to free() is a pointer to a block previously allocated by malloc(), calloc() or realloc(). If ptr is a NULL pointer, no action occurs. Undefined results will occur if the space assigned by malloc() is overrun or if some random number is handed to free(). The calloc() function allocates space for an array of nelem elements of size elsize. The space is initialized to zeros. The realloc() function changes the size of the block pointed to by ptr to size bytes and returns a pointer to the (possibly moved) block. The contents will be unchanged up to the lesser of the new and old sizes. If the new size of the block requires movement of the block, the space for the previous instantiation of the block is freed. If the new size is larger, the contents of the newly allocated portion of the block are unspecified. If ptr is NULL, realloc() behaves like malloc() for the specified size. If size is 0 and ptr is not a null pointer, the space pointed to is freed. Each of the allocation functions returns a pointer to space suitably aligned (after possible pointer coercion) for storage of any type of object. The malloc() and realloc() functions will fail if there is not enough available memory. Entry points for malloc_debug(), mallocmap(), mallopt(), mallinfo(), memalign(), and valloc() are empty routines, and are provided only to protect the user from mixing malloc() functions from different implementations. RETURN VALUES
If there is no available memory, malloc(), realloc(), and calloc() return a null pointer. When realloc() returns NULL, the block pointed to by ptr is left intact. If size, nelem, or elsize is 0, a unique pointer to the arena is returned. ATTRIBUTES
See attributes(5) for descriptions of the following attributes: +-----------------------------+-----------------------------+ | ATTRIBUTE TYPE | ATTRIBUTE VALUE | +-----------------------------+-----------------------------+ |MT-Level |Safe | +-----------------------------+-----------------------------+ SEE ALSO
brk(2), getrlimit(2), mmap(2), realloc(3C), malloc(3MALLOC), attributes(5) SunOS 5.10 20 Feb 2004 mapmalloc(3MALLOC)