Using malloc(), realloc(), free() in C

• 

http://en.wikipedia.org/wiki/Malloc

Malloc is a subrouitine for performing dynamic memory allocation in C and C++.

• must be freed by free()
• memory is uninitialized
• not guaranteed to succeed.  Return NULL if fails.

C manages memory statically, automatically, or dynamically.

static-duration variables are allocated in main (fixed) memory and persist for the lifetime of the program.

automatic-duration variables are allocated on the stack and come and go as function are called and return.  Size of allocation is required a compile time (a compile-time constant).

dynamic-duration variables are allocated on the heap in C using malloc.

• use free(ptr) free pointer and returns memory to heap.

http://en.wikipedia.org/wiki/Dynamic_memory_allocation

dynamic memory allocation (also known as heap-based memory allocation) is the allocation of memory storage in a computer program during runtime.

http://en.wikipedia.org/wiki/Heap_overflow

heap overflow 

• a buffer overflow that occurs in the heap data area
• typically contains program data

http://en.wikipedia.org/wiki/Stack-based_memory_allocation

stack-based memory allocation

• a computing architecture where regions of memory have data added or removed in a list-in-first-out manner
• each threat usually has its own stack
• typically faster than heap-based memory allocation
• programmer doesn't need to release the memory used.
• memory size for a given threat is small (a few dozen kilobytes)

http://publications.gbdirect.co.uk/c_book/chapter5/sizeof_and_malloc.html

Has some nice examples.

sizeof(char) is always 1.

malloc(10) // allocates ten chars.

malloc(sizeof(int[10])) // allocates ten ints

If malloc cannot find enough space to satisfy the request, a null pointer is returned.

There is a lot more here on malloc, but that is beyond what I need at the moment.  However, I may need to use there GROW_BY approach to allocating more memory.

http://en.wikipedia.org/wiki/Calloc#calloc

calloc()

• initializes memory block to 0.
• parameter are element count and size of each element.
• should be slower than malloc()

http://en.wikipedia.org/wiki/Realloc#realloc

realloc()

• data is unchanged
• new memory is unallocated
• if new size is smaller than old size, memory is truncated.
• if new returned pointer is set to original pointer, could get a memory if a NULL is returned.

magic.h Man Page

http://swoolley.org/man.cgi/3/libmagic

This can also be found using:

$ man 3 libmagic

or

$ man libmagic

Notice the '3' in front of the search for indicates which section to search in.

libmagic

http://vivithemage.co.uk/blog/?p=105

A real nice article that go me started with using libmagic or magic.h.

<code>#include <stdio.h>

#include <magic.h>

 

int main(void)

{

    char *actual_file = "/file/you/want.yay";

    const char *magic_full;

    magic_t magic_cookie;

    /*MAGIC_MIME tells magic to return a mime of the file, but you can specify different things*/

    magic_cookie = magic_open(MAGIC_MIME);

        if (magic_cookie == NULL) {

            printf("unable to initialize magic library\n");

            return 1;

            }

        printf("Loading default magic database\n");

        if (magic_load(magic_cookie, NULL) != 0) {

            printf("cannot load magic database - %s\n", magic_error(magic_cookie));

            magic_close(magic_cookie);

            return 1;

        }

    magic_full = magic_file(magic_cookie, actual_file);

    printf("%s\n", magic_full);

    magic_close(magic_cookie);

        return 0;

}

</code>

To compile it do: "gcc magic_test.c -o magics -lmagic". Furthermore make sure the libmagic library is installed. "apt-get install libmagic-dev" should do it if you're using a machine with debian/ubuntu.

Below is my magic_buff testing:

<code>

#include <stdio.h>

#include <magic.h>

int main(void)

{

   printf("In main()\n");

   char *actual_file = "hello.c";

   const char *magic_full;

   magic_t magic_cookie;

   int BUF_SIZE = 8388608; // 2**23

   char buf[BUF_SIZE];

   magic_cookie = magic_open(MAGIC_MIME);

   if (magic_cookie == NULL)

   {

      printf("Unable to initialize magic library\n");

      return 1; 

   }

   if (magic_load(magic_cookie, NULL) != 0)

   {

      printf("Cannot load magic database - %s\n", magic_error(magic_cookie));

      magic_close(magic_cookie);

      return 1;

   }

   //magic_full = magic_file(magic_cookie, actual_file);

   magic_full = magic_buffer(magic_cookie, (const void *) &buf, BUF_SIZE);

   printf("%s\n", magic_full);

   magic_close(magic_cookie);

   return 0;

}

</code>