Malloc to void pointer fails Post: 302910794

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Top Forums Programming Malloc to void pointer fails Post 302910794 by jim mcnamara on Monday 28th of July 2014 09:12:41 AM

07-28-2014

Registered User

There is a lot of opinion in this thread, which is okay as long as a reader knows that fact.

Generally it is bad idea to cast malloc, because it is not required and may introduce subtle bugs that are hard to find, a discussion:

FAQ > Casting malloc - Cprogramming.com

Consider the use of a debugger, ex: gdb. This will help resolve crashes by examining core files, link:

RMS's gdb Tutorial: Segmentation Fault Example

Finally, allocating heap memory (malloc does this for you in C) is not all that simple.
If you allocate x+1 bytes for a string which should never be long than x, you have to check input carefully, otherwise if the string you enter is too long by a few bytes it probably will not segfault, it will simply trash a neighboring variable. Nasty.

Except for embedded systems (ex ARM), declaring strings longer than needed is less harmful, but still requires checking EVERY input string before parking it in the variable, because this makes an entry for a possible code exploit. Shell code and SQL injection come to mind.

malloc works this way in general:
1. at the beginning of code invocation, the brk() system call allocates pages of memory, and those pages then are controlled by malloc, not directly by your program code, normally. Do not call brk() on your own if you use malloc or functions like strdup which call malloc. Calling brk() directly in code that uses malloc usually results in chaos.

So if you malloc 10 bytes and page size is 8192 bytes (example), then you still have more already allocated memory available.

2. malloc keeps track of what it allocates, pages are in a page table, variables are tracked usually with some kind of descriptor. The descriptor is often a struct that consists of a pointer to the start of the variable (what malloc returns to you), and a length in bytes.

hypothetical example base on Doug Lea's original malloc:

[address of word aligned memory][length].... [word aligned memory]

So, if you increment or decrement the pointer you have (your variable) it no longer references [address of word aligned memory]. This causes free() to get nasty with you.

- finally, some of the suggestions in this thread are just that - there is both art and science in building good code.

jim mcnamara

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LEARN ABOUT PLAN9

malloc

MALLOC(2)							System Calls Manual							 MALLOC(2)

NAME

       malloc, free, realloc, calloc - memory allocator

SYNOPSIS

       #include <u.h>
       #include <libc.h>

       void* malloc(long size)

       void  free(void *ptr)

       void* realloc(void *ptr, long size)

       void* calloc(long nelem, long elsize)

DESCRIPTION

       Malloc  and free provide a simple memory allocation package.  Malloc returns a pointer to a new block of at least size bytes.  The block is
       suitably aligned for storage of any type of object.  No two active pointers from malloc will have the same value.

       The argument to free is a pointer to a block previously allocated by malloc; this space is made available for further  allocation.   It	is
       legal to free a null pointer; the effect is a no-op.

       Realloc	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.  The call realloc(0, size) means the same as

       Calloc allocates space for an array of nelem elements of size elsize.  The space is initialized to zeros.  Free frees such a block.

   Alef
       Except for calloc, these routines are available from Alef; they use the same arena as alloc.  Malloc and realloc execute a check when  they
       fail, rather than return nil.  Memory returned by malloc is cleared.  Realloc does not guarantee new memory is cleared unless ptr is nil.

SOURCE

       /sys/src/libc/port/malloc.c

SEE ALSO

       brk(2)

DIAGNOSTICS

       Malloc, realloc and calloc return 0 if there is no available memory.  Errstr is likely to be set.

BUGS

       The different specification of calloc is bizarre.

       User errors can corrupt the storage arena.  The most common gaffes are (1) freeing an already freed block, (2) storing beyond the bounds of
       an allocated block, and (3) freeing data that was not obtained from the allocator.  When malloc	and  free  detect  such  corruption,  they
       abort.

																	 MALLOC(2)