manual: Document replacing malloc [BZ #20424]

ChangeLog

@@ -1,3 +1,10 @@
+2017-04-21  Florian Weimer  <fweimer@redhat.com>
+
+	[BZ #20424]
+	* manual/memory.texi (Replacing malloc): New section.
+	(Allocating Storage For Program Data): Reference it.
+	(The GNU Allocator): Likewise.
+
 2017-04-20  Joseph Myers  <joseph@codesourcery.com>
 
 	* stdlib/Versions (__strtod_internal): List explicitly, not as

manual/memory.texi

@@ -167,6 +167,7 @@ special to @theglibc{} and GNU Compiler.
 * Unconstrained Allocation::    The @code{malloc} facility allows fully general
 		 		 dynamic allocation.
 * Allocation Debugging::        Finding memory leaks and not freed memory.
+* Replacing malloc::            Using your own @code{malloc}-style allocator.
 * Obstacks::                    Obstacks are less general than malloc
 				 but more efficient and convenient.
 * Variable Size Automatic::     Allocation of variable-sized blocks
@@ -299,6 +300,9 @@ A more detailed technical description of the GNU Allocator is maintained in
 the @glibcadj{} wiki. See
 @uref{https://sourceware.org/glibc/wiki/MallocInternals}.
 
+It is possible to use your own custom @code{malloc} instead of the
+built-in allocator provided by @theglibc{}.  @xref{Replacing malloc}.
+
 @node Unconstrained Allocation
 @subsection Unconstrained Allocation
 @cindex unconstrained memory allocation
@@ -1898,6 +1902,67 @@ from line 33 in the source file @file{/home/drepper/tst-mtrace.c} four
 times without freeing this memory before the program terminates.
 Whether this is a real problem remains to be investigated.
 
+@node Replacing malloc
+@subsection Replacing @code{malloc}
+
+@cindex @code{malloc} replacement
+@cindex @code{LD_PRELOAD} and @code{malloc}
+@cindex alternative @code{malloc} implementations
+@cindex customizing @code{malloc}
+@cindex interposing @code{malloc}
+@cindex preempting @code{malloc}
+@cindex replacing @code{malloc}
+@Theglibc{} supports replacing the built-in @code{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 @code{LD_PRELOAD}.  For static
+linking, the @code{malloc} replacement library must be linked in before
+linking against @code{libc.a} (explicitly or implicitly).
+
+@strong{Note:} Failure to provide a complete set of replacement
+functions (that is, all the functions used by the application,
+@theglibc{}, 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
+@code{malloc} is given in the table below.
+
+@table @code
+@item malloc
+@item free
+@item calloc
+@item realloc
+@end table
+
+These @code{malloc}-related functions are required for @theglibc{} to
+work.@footnote{Versions of @theglibc{} before 2.25 required that a
+custom @code{malloc} defines @code{__libc_memalign} (with the same
+interface as the @code{memalign} function).}
+
+The @code{malloc} implementation in @theglibc{} provides additional
+functionality not used by the library itself, but which is often used by
+other system libraries and applications.  A general-purpose replacement
+@code{malloc} implementation should provide definitions of these
+functions, too.  Their names are listed in the following table.
+
+@table @code
+@item aligned_alloc
+@item malloc_usable_size
+@item memalign
+@item posix_memalign
+@item pvalloc
+@item valloc
+@end table
+
+In addition, very old applications may use the obsolete @code{cfree}
+function.
+
+Further @code{malloc}-related functions such as @code{mallopt} or
+@code{mallinfo} will not have any effect or return incorrect statistics
+when a replacement @code{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.
+
 @node Obstacks
 @subsection Obstacks
 @cindex obstacks