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3.2.5 Replacing malloc

The GNU C Library supports replacing the built-in malloc implementation with a different allocator with the same interface. For dynamically linked programs, this happens through ELF symbol interposition, either using shared object dependencies or LD_PRELOAD. For static linking, the malloc replacement library must be linked in before linking against libc.a (explicitly or implicitly).

Note: Failure to provide a complete set of replacement functions (that is, all the functions used by the application, the GNU C Library, and other linked-in libraries) can lead to static linking failures, and, at run time, to heap corruption and application crashes.

The minimum set of functions which has to be provided by a custom malloc is given in the table below.

malloc

free

calloc

realloc

These malloc-related functions are required for the GNU C Library to work.¹

The malloc implementation in the GNU C Library provides additional functionality not used by the library itself, but which is often used by other system libraries and applications. A general-purpose replacement malloc implementation should provide definitions of these functions, too. Their names are listed in the following table.

aligned_alloc

malloc_usable_size

memalign

posix_memalign

pvalloc

valloc

In addition, very old applications may use the obsolete cfree function.

Further malloc-related functions such as mallopt or mallinfo will not have any effect or return incorrect statistics when a replacement malloc is in use. However, failure to replace these functions typically does not result in crashes or other incorrect application behavior, but may result in static linking failures.