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libgmalloc(3)            BSD Library Functions Manual            libgmalloc(3)


NAME

     libgmalloc -- (Guard Malloc), an aggressive debugging malloc library


DESCRIPTION

     libgmalloc is a debugging malloc library that can track down insidious
     bugs in your code or library.  If your application crashes when using
     libgmalloc, then you've found a bug.

     libgmalloc is used in place of the standard system malloc, and uses the
     virtual memory system to identify memory access bugs.  Each malloc allo-
     cation is placed on its own virtual memory page (or pages).  By default,
     the returned address for the allocation is positioned such that the end
     of the allocated buffer is at the end of the last page, and the next page
     after that is kept unallocated.  Thus, accesses beyond the end of the
     buffer cause a bad access error immediately.  When memory is freed, libg-
     malloc deallocates its virtual memory, so reads or writes to the freed
     buffer cause a bad access error.  Bugs which had been difficult to iso-
     late become immediately obvious, and you'll know exactly which code is
     causing the problem.

     Guard Malloc is thread-safe and works for all uses of malloc(), Objec-
     tive-C's alloc method, C++'s new operator, and other functions which
     result in allocation in the malloc heap.

     As of Mac OS X 10.5, libgmalloc aligns the start of allocated buffers on
     16-byte boundaries by default, to allow proper use of vector instructions
     (e.g., SSE).  (The use of vector instructions is common, including in
     some Mac OS X system libraries.  The regular system malloc also uses
     16-byte alignment.)  Because of this 16-byte alignment, up to 15 bytes at
     the end of an allocated block may be excess at the end of the page, and
     libgmalloc will not detect buffer overruns into that area by default.
     This default alignment can be changed with environment variables.

     libgmalloc is available in /usr/lib/libgmalloc.dylib.  To use it, set
     this environment variable:

           set DYLD_INSERT_LIBRARIES to /usr/lib/libgmalloc.dylib

     Note:  it is no longer necessary to set DYLD_FORCE_FLAT_NAMESPACE.

     This tells dyld to use Guard Malloc instead of the standard version of
     malloc.  Run the program, and wait for the crash indicating the bad
     access.  When the program crashes, examine it in the debugger to identify
     the cause.

     As of Mac OS X 10.6, libgmalloc can be used with the standard malloc
     stack logging by setting the MallocStackLogging environment variable.
     The malloc_history(1) command can then be used to show backtraces of all
     malloc and free events made when using libgmalloc.


USING libgmalloc WITH THE XCODE DEBUGGER OR LLDB

     Because the goal of libgmalloc is to "encourage" your application to
     crash if memory access errors occur, it is best to run your application
     under a debugger such as the Xcode IDE's debugger, or lldb at the command
     line.

     To use Guard Malloc with the Xcode debugger, choose Edit Scheme... from
     the Scheme popup.  Click on the Diagnostics tab then turn on the Enable
     Guard Malloc check box.  Then when launching the target application,
     Xcode automatically sets the DYLD_INSERT_LIBRARIES environment variable
     properly.  Xcode retains that setting with that executable.  To set any
     of the additional environment variables described below, click on the
     Arguments tab in the Scheme editor and add them in the Environment Vari-
     ables section.

     If you're using lldb from the command line, use lldb's "settings set tar-
     get.env-vars VAR=VALUE" command to set the environment variables.  Or
     simply use the "env VAR=VALUE" command alias.


EXAMPLE

     % cat gmalloctest.c
     #include <stdlib.h>
     #include <stdio.h>

     int main(int argc, char **argv) {
       unsigned *buffer = (unsigned *)malloc(sizeof(unsigned) * 100);
       unsigned i;

       for (i = 0; i < 200; i++) {
         buffer[i] = i;
       }

       for (i = 0; i < 200; i++) {
         printf ("%d  ", buffer[i]);
       }
     }

     % cc -g -o gmalloctest gmalloctest.c
     % lldb gmalloctest
     Current executable set to 'gmalloctest' (x86_64).
     (lldb) env DYLD_INSERT_LIBRARIES=/usr/lib/libgmalloc.dylib
     (lldb) process launch
     Process 7895 launched: '/private/tmp/testit/gmalloctest' (x86_64)
     GuardMalloc[gmalloctest-7895]: Allocations will be placed on 16 byte boundaries.
     GuardMalloc[gmalloctest-7895]:  - Some buffer overruns may not be noticed.
     GuardMalloc[gmalloctest-7895]:  - Applications using vector instructions (e.g., SSE) should work.
     GuardMalloc[gmalloctest-7895]: version 27
     Process 7895 stopped
     * thread #1: tid = 0x6880e, 0x0000000100000eda gmalloctest`main(argc=1, argv=0x00007fff5fbffa00) + 74 at gmalloctest.c:9, stop reason = EXC_BAD_ACCESS (code=1, address=0x100342000)
         frame #0: 0x0000000100000eda gmalloctest`main(argc=1, argv=0x00007fff5fbffa00) + 74 at gmalloctest.c:9
        6      unsigned i;
        7
        8      for (i = 0; i < 200; i++) {
     -> 9        buffer[i] = i;
        10     }
        11
        12     for (i = 0; i < 200; i++) {
     (lldb) print i
     (unsigned int) $0 = 100
     (lldb) print &buffer[i]
     (unsigned int *) $1 = 0x0000000100342000

     Once you have the backtrace, you can examine that line of source code to
     see what variable was accessed, and determine why that address was
     invalid memory.  In the example above, notice that it crashes when it
     tries to write one character beyond the end of the malloc'ed buffer it's
     operating on, causing a bad access error when accessing the protected
     page following the string.

     These sorts of problems may seem minor, especially when the application
     normally behaves correctly.  However, they're usually the hallmark of
     intermittent bugs or unexplained crashes in long running programs.  In
     normal use, the bug in the example program might have caused no problems
     at all... or it might have trashed the following buffer, leading occa-
     sionally to corrupted data.  If the application had been referencing
     freed memory, the program might have worked fine until the one time where
     the freed memory was immediately reused and modified.


ENVIRONMENT

     libgmalloc's behavior can be changed with several additional environment
     variables:

     MALLOC_LOG_FILE <f>            Create or append messages to the given
                                    file path <f> instead of writing to the
                                    standard error. This can be set to
                                    /dev/null to completely suppress all out-
                                    put if necessary.

     MALLOC_PROTECT_BEFORE          If this flag is set, then libgmalloc tries
                                    harder to detect buffer underruns.
                                    Specifically, libgmalloc places the start
                                    of the allocated buffer at the beginning
                                    of a virtual memory page, then protects
                                    the page before.  Buffer underruns then
                                    cause an error.  The behavior without this
                                    variable set is to place the end of the
                                    buffer at the end of the last page of the
                                    allocation, and protect the page after.

     MALLOC_FILL_SPACE              This flag causes libgmalloc to fill the
                                    buffer with 0x55 upon creation.  This can
                                    help catch uninitialized memory problems.

     MALLOC_ALLOW_READS             This flag allows the guard page after the
                                    buffer to be readable so that reads past
                                    the ends of buffers do not cause the pro-
                                    gram to crash.  With the MALLOC_PRO-
                                    TECT_BEFORE flag set, this command instead
                                    sets the guard page before the buffer to
                                    be readable.

     MALLOC_VECTOR_SIZE             This option is the default alignment, as
                                    of Mac OS X 10.5.  With this option, Guard
                                    Malloc places allocations on 16 byte
                                    boundaries, because vector instructions
                                    (e.g., SSE) require buffers to be on 16
                                    byte boundaries.  (The use of vector
                                    instructions is becoming more common in
                                    some Mac OS X system libraries.)

     MALLOC_WORD_SIZE               This flag specifies that Guard Malloc
                                    should place allocations on word (4-byte)
                                    boundaries, with the end of the buffer on
                                    the last 4 bytes of the page.  This option
                                    is useful because Carbon assumes that
                                    pointers are word aligned, and without the
                                    word alignment, any program relying on
                                    Cocoa or Carbon would immediately crash.

     MALLOC_STRICT_SIZE             This flag specifies that Guard Malloc
                                    should always align all allocations on
                                    single-byte boundaries such that the last
                                    byte of the buffer is at the end of the
                                    page.  This will immediately catch even
                                    one-byte buffer overruns, but applications
                                    that use Carbon or Cocoa, or vector
                                    instructions, may not run properly with
                                    this option.

     MALLOC_PERMIT_INSANE_REQUESTS  GuardMalloc tries to protect against
                                    requests for insane amounts of memory by
                                    instructing the program to trap (if run-
                                    ning under the debugger) if more than
                                    100MB is requested.  If this environment
                                    variable is set, then the check is dis-
                                    abled.

     MALLOC_MAXIMUM_VM              To test how a process handles running out
                                    of memory, set this variable to the maxi-
                                    mum size, in bytes, of the allocations for
                                    the process (including the extra overhead
                                    from rounding allocations up to a full
                                    page size).  When this limit is hit,
                                    attempts to allocate additional memory
                                    return NULL.  If MALLOC_PER-
                                    MIT_INSANE_REQUESTS is not set it will
                                    also trap (if running under the debugger).

     MALLOC_CHECK_HEADER            This flag is enabled by default, which
                                    causes Guard Malloc to check the validity
                                    of a magic number in the malloc block
                                    header when a block is freed or reallo-
                                    cated.  To turn off this checking, set
                                    this environment variable to NO or 0.

     MallocStackLogging             If this flag is set, then standard system
                                    malloc stack logging is enabled.  The
                                    malloc_history(1) command can then be used
                                    to show backtraces of all malloc and free
                                    events made when using libgmalloc.


MEMORY VALUES USED BY GUARD MALLOC

     It's often useful to understand how Guard Malloc uses memory when debug-
     ging.  Guard Malloc writes strange byte sequences to catch certain prob-
     lems.  If the MALLOC_FILL_SPACE environment variable is set, newly allo-
     cated buffers will be filled with the value 0x55 in hopes of catching
     references to uninitialized memory.

     The space right before the buffer is dedicated to header information.  If
     MALLOC_PROTECT_BEFORE was set, the header immediately follows the buffer.
     The header is 16 bytes in 32-bit processes and 32 bytes in 64-bit pro-
     cesses and is organized as:

     magic number (0xdeadbeef in 32-bit, or 0xdeadbeefdeadbeef in 64-bit)
     size of buffer + size of header
     thread id
     magic number again


CAVEATS

     libgmalloc doesn't come without some weaknesses.  First, because each
     allocation requires at least two pages of virtual memory, in 32-bit pro-
     cesses only about 500,000 malloc allocations could conceivably exist
     before the process runs out of virtual memory.  The extravagant use of
     virtual memory will also cause much more swapping, so the program will
     run much slower than usual -- usually two orders of magnitude (100x).

     In addition, the extra pressure on the virtual memory system when running
     an application with Guard Malloc can cause top(1) to update its output
     more slowly.

     Don't forget -- if there's a memory bug in your program, the program will
     crash in Guard Malloc.  This is a feature!


SEE ALSO

     malloc_history(1)

Mac OS X                         Mar. 12, 2013                        Mac OS X

OS X 10.10 - Generated Wed Nov 5 06:05:41 CST 2014
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