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
allocation 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, libgmalloc deallocates its virtual memory, so reads or writes
to the freed buffer cause a bad access error. Bugs which had been difficult to isolate 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(), Objective-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 Variables section.
If you're using lldb from the command line, use lldb's "settings set target.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 prob-
lems at all... or it might have trashed the following buffer, leading occasionally to corrupted data. If the application had been referenc-
ing 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 output 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 buf-
fer 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 program to crash. With the MALLOC_PROTECT_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 over-
runs, 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 running under the debugger) if more than 100MB is requested. If this environment variable is set, then
the check is disabled.
MALLOC_MAXIMUM_VM To test how a process handles running out of memory, set this variable to the maximum 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_PERMIT_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 mal-
loc block header when a block is freed or reallocated. To turn off this checking, set this environment vari-
able 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 debugging. Guard Malloc writes strange byte sequences to catch certain
problems. If the MALLOC_FILL_SPACE environment variable is set, newly allocated 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 processes 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
processes 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