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Full Discussion: Malloc to void pointer fails
Top Forums Programming Malloc to void pointer fails Post 302910794 by jim mcnamara on Monday 28th of July 2014 09:12:41 AM
Old 07-28-2014
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.
 

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bsdmalloc(3MALLOC)														bsdmalloc(3MALLOC)

NAME
bsdmalloc - memory allocator SYNOPSIS
cc [ flag ... ] file ... -lbsdmalloc [ library ... ] char *malloc(size); unsigned size; int free( ptr); char *ptr; char *realloc( ptr, size); char *ptr; unsigned size; These routines provide a general-purpose memory allocation package. They maintain a table of free blocks for efficient allocation and coa- lescing of free storage. When there is no suitable space already free, the allocation routines call sbrk(2) to get more memory from the system. Each of the allocation routines returns a pointer to space suitably aligned for storage of any type of object. Each returns a null pointer if the request cannot be completed. The malloc() function returns a pointer to a block of at least size bytes, which is appropriately aligned. The free() function releases a previously allocated block. Its argument is a pointer to a block previously allocated by malloc() or real- loc(). The free() function does not set errno. 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. The malloc() and realloc() functions return a null pointer if there is not enough available memory. They return a non-null pointer if size is 0. These pointers should not be dereferenced. When realloc() returns NULL, the block pointed to by ptr is left intact. Always cast the value returned by malloc() and realloc(). If malloc() or realloc() returns unsuccessfully, errno will be set to indicate the following: ENOMEM size bytes of memory cannot be allocated because it exceeds the physical limits of the system. EAGAIN There is not enough memory available at this point in time to allocate size bytes of memory; but the application could try again later. Using realloc() with a block freed before the most recent call to malloc() or realloc() results in an error. Comparative features of the various allocation libraries can be found in the umem_alloc(3MALLOC) manual page. brk(2), malloc(3C), malloc(3MALLOC), mapmalloc(3MALLOC), umem_alloc(3MALLOC) WARNINGS
Use of libbsdmalloc renders an application non-SCD compliant. The libbsdmalloc routines are incompatible with the memory allocation routines in the standard C-library (libc): malloc(3C), alloca(3C), calloc(3C), free(3C), memalign(3C), realloc(3C), and valloc(3C). 21 Mar 2005 bsdmalloc(3MALLOC)
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