malloc Directory Reference

lib/malloc: Wrappers and utilities for memory management.

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Files
file	malloc.c [code]
	Wrappers for C malloc code, and replacements for items that may be missing.
file	malloc.h [code]
	Headers for util_malloc.c.
file	map_anon.c [code]
	Manage anonymous mappings.
file	map_anon.h [code]
	Headers for map_anon.c.

Detailed Description

lib/malloc: Wrappers and utilities for memory management.

Tor imposes a few light wrappers over C's native malloc and free functions, to improve convenience, and to allow wholescale replacement of malloc and free as needed.

You should never use 'malloc', 'calloc', 'realloc, or 'free' on their own; always use the variants prefixed with 'tor_'. They are the same as the standard C functions, with the following exceptions:

• tor_free(NULL) is a no-op.
• tor_free() is a macro that takes an lvalue as an argument and sets it to NULL after freeing it. To avoid this behavior, you can use tor_free_() instead.
• tor_malloc() and friends fail with an assertion if they are asked to allocate a value so large that it is probably an underflow.
• It is always safe to tor_malloc(0), regardless of whether your libc allows it.
• tor_malloc(), tor_realloc(), and friends are never allowed to fail. Instead, Tor will die with an assertion. This means that you never need to check their return values. See the next subsection for information on why we think this is a good idea.

We define additional general-purpose memory allocation functions as well:

• tor_malloc_zero(x) behaves as calloc(1, x), except the it makes clear the intent to allocate a single zeroed-out value.
• tor_reallocarray(x,y) behaves as the OpenBSD reallocarray function. Use it for cases when you need to realloc() in a multiplication-safe way.

And specific-purpose functions as well:

• tor_strdup() and tor_strndup() behaves as the underlying libc functions, but use tor_malloc() instead of the underlying function.
• tor_memdup() copies a chunk of memory of a given size.
• tor_memdup_nulterm() copies a chunk of memory of a given size, then NUL-terminates it just to be safe.

Why assert on allocation failure?

Why don't we allow tor_malloc() and its allies to return NULL?

First, it's error-prone. Many programmers forget to check for NULL return values, and testing for malloc() failures is a major pain.

Second, it's not necessarily a great way to handle OOM conditions. It's probably better (we think) to have a memory target where we dynamically free things ahead of time in order to stay under the target. Trying to respond to an OOM at the point of tor_malloc() failure, on the other hand, would involve a rare operation invoked from deep in the call stack. (Again, that's error-prone and hard to debug.)

Third, thanks to the rise of Linux and other operating systems that allow memory to be overcommitted, you can't actually ever rely on getting a NULL from malloc() when you're out of memory; instead you have to use an approach closer to tracking the total memory usage.

Conventions for your own allocation functions.

Whenever you create a new type, the convention is to give it a pair of x_new() and x_free_() functions, named after the type.

Calling x_free(NULL) should always be a no-op.

There should additionally be an x_free() macro, defined in terms of x_free_(). This macro should set its lvalue to NULL. You can define it using the FREE_AND_NULL macro, as follows:

#define x_free(ptr) FREE_AND_NULL(x_t, x_free_, (ptr))