This document describes how malloc / new calls are routed in the various Chrome platforms.
Bare in mind that the chromium codebase does not always just use malloc(). Some examples:
SharedMemory or DiscardableMemory which, similarly to the above, have their own page-level memory management.The allocator target defines at compile-time the platform-specific choice of the allocator and extra-hooks which services calls to malloc/new. The relevant build-time flags involved are use_allocator and win_use_allocator_shim.
The default choices are as follows:
Windowsuse_allocator: winheap, the default Windows heap. Additionally, static_library (i.e. non-component) builds have a shim layer wrapping malloc/new, which is controlled by win_use_allocator_shim.
The shim layer provides extra security features, such as preventing large allocations that can hit signed vs. unsigned bugs in third_party code.
Linux Desktop / CrOSuse_allocator: tcmalloc, a forked copy of tcmalloc which resides in third_party/tcmalloc/chromium. Setting use_allocator: none causes the build to fall back to the system (Glibc) symbols.
Androiduse_allocator: none, always use the allocator symbols coming from Android's libc (Bionic). As it is developed as part of the OS, it is considered to be optimized for small devices and more memory-efficient than other choices.
The actual implementation backing malloc symbols in Bionic is up to the board config and can vary (typically dlmalloc or jemalloc on most Nexus devices).
Mac/iOSuse_allocator: none, we always use the system's allocator implementation.
In addition, when building for asan / msan / syzyasan valgrind, the both the allocator and the shim layer are disabled.
The allocator target provides both the source files for tcmalloc (where applicable) and the linker flags required for the Windows shim layer. The base target is (almost) the only one depending on allocator. No other targets should depend on it, with the exception of the very few executables / dynamic libraries that don't depend, either directly or indirectly, on base within the scope of a linker unit.
More importantly, no other place outside of /base should depend on the specific allocator (e.g., directly include third_party/tcmalloc). If such a functional dependency is required that should be achieved using abstractions in base (see /base/allocator/allocator_extension.h and /base/memory/)
Why base depends on allocator?
Because it needs to provide services that depend on the actual allocator implementation. In the past base used to pretend to be allocator-agnostic and get the dependencies injected by other layers. This ended up being an inconsistent mess. See the allocator cleanup doc for more context.
Linker unit targets (executables and shared libraries) that depend in some way on base (most of the targets in the codebase) get automatically the correct set of linker flags to pull in tcmalloc or the Windows shim-layer.
This directory contains just the allocator (i.e. shim) layer that switches between the different underlying memory allocation implementations.
The tcmalloc library originates outside of Chromium and exists in ../../third_party/tcmalloc (currently, the actual location is defined in the allocator.gyp file). The third party sources use a vendor-branch SCM pattern to track Chromium-specific changes independently from upstream changes.
The general intent is to push local changes upstream so that over time we no longer need any forked files.
How does the Windows shim layer replace the malloc symbols?
The mechanism for hooking LIBCMT in Windows is rather tricky. The core problem is that by default, the Windows library does not declare malloc and free as weak symbols. Because of this, they cannot be overridden. To work around this, we start with the LIBCMT.LIB, and manually remove all allocator related functions from it using the visual studio library tool. Once removed, we can now link against the library and provide custom versions of the allocator related functionality. See the script preb_libc.py in this folder.