Clang 3.3 documentation


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MemorySanitizer is a detector of uninitialized reads. It consists of a compiler instrumentation module and a run-time library.

Typical slowdown introduced by MemorySanitizer is 3x.

How to build

Follow the clang build instructions. CMake build is supported.


Simply compile and link your program with -fsanitize=memory flag. The MemorySanitizer run-time library should be linked to the final executable, so make sure to use clang (not ld) for the final link step. When linking shared libraries, the MemorySanitizer run-time is not linked, so -Wl,-z,defs may cause link errors (don’t use it with MemorySanitizer). To get a reasonable performance add -O1 or higher. To get meaninful stack traces in error messages add -fno-omit-frame-pointer. To get perfect stack traces you may need to disable inlining (just use -O1) and tail call elimination (-fno-optimize-sibling-calls).

% cat
#include <stdio.h>

int main(int argc, char** argv) {
  int* a = new int[10];
  a[5] = 0;
  if (a[argc])
  return 0;

% clang -fsanitize=memory -fno-omit-frame-pointer -g -O2

If a bug is detected, the program will print an error message to stderr and exit with a non-zero exit code. Currently, MemorySanitizer does not symbolize its output by default, so you may need to use a separate script to symbolize the result offline (this will be fixed in future).

% ./a.out 2>log
% projects/compiler-rt/lib/asan/scripts/ / < log | c++filt
==30106==  WARNING: MemorySanitizer: UMR (uninitialized-memory-read)
    #0 0x7f45944b418a in main
    #1 0x7f45938b676c in __libc_start_main libc-start.c:226

By default, MemorySanitizer exits on the first detected error.


In some cases one may need to execute different code depending on whether MemorySanitizer is enabled. __has_feature can be used for this purpose.

#if defined(__has_feature)
#  if __has_feature(memory_sanitizer)
// code that builds only under MemorySanitizer
#  endif


Some code should not be checked by MemorySanitizer. One may use the function attribute no_sanitize_memory to disable uninitialized checks in a particular function. MemorySanitizer may still instrument such functions to avoid false positives. This attribute may not be supported by other compilers, so we suggest to use it together with __has_feature(memory_sanitizer). Note: currently, this attribute will be lost if the function is inlined.

Origin Tracking

MemorySanitizer can track origins of unitialized values, similar to Valgrind’s –track-origins option. This feature is enabled by -fsanitize-memory-track-origins Clang option. With the code from the example above,

% clang -fsanitize=memory -fsanitize-memory-track-origins -fno-omit-frame-pointer -g -O2
% ./a.out 2>log
% projects/compiler-rt/lib/asan/scripts/ / < log | c++filt
==14425==  WARNING: MemorySanitizer: UMR (uninitialized-memory-read)
==14425== WARNING: Trying to symbolize code, but external symbolizer is not initialized!
    #0 0x7f8bdda3824b in main
    #1 0x7f8bdce3a76c in __libc_start_main libc-start.c:226
  raw origin id: 2030043137
  ORIGIN: heap allocation:
    #0 0x7f8bdda4034b in operator new[](unsigned long)
    #1 0x7f8bdda3814d in main
    #2 0x7f8bdce3a76c in __libc_start_main libc-start.c:226

Origin tracking has proved to be very useful for debugging UMR reports. It slows down program execution by a factor of 1.5x-2x on top of the usual MemorySanitizer slowdown.

Handling external code

MemorySanitizer requires that all program code is instrumented. This also includes any libraries that the program depends on, even libc. Failing to achieve this may result in false UMR reports.

Full MemorySanitizer instrumentation is very difficult to achieve. To make it easier, MemorySanitizer runtime library includes 70+ interceptors for the most common libc functions. They make it possible to run MemorySanitizer-instrumented programs linked with uninstrumented libc. For example, the authors were able to bootstrap MemorySanitizer-instrumented Clang compiler by linking it with self-built instrumented libcxx (as a replacement for libstdc++).

In the case when rebuilding all program dependencies with MemorySanitizer is problematic, an experimental MSanDR tool can be used. It is a DynamoRio-based tool that uses dynamic instrumentation to avoid false positives due to uninstrumented code. The tool simply marks memory from instrumented libraries as fully initialized. See for more information.

Supported Platforms

MemorySanitizer is supported on

  • Linux x86_64 (tested on Ubuntu 10.04 and 12.04);


  • MemorySanitizer uses 2x more real memory than a native run, 3x with origin tracking.
  • MemorySanitizer maps (but not reserves) 64 Terabytes of virtual address space. This means that tools like ulimit may not work as usually expected.
  • Static linking is not supported.
  • Non-position-independent executables are not supported. Therefore, the fsanitize=memory flag will cause Clang to act as though the -fPIE flag had been supplied if compiling without -fPIC, and as though the -pie flag had been supplied if linking an executable.
  • Depending on the version of Linux kernel, running without ASLR may be not supported. Note that GDB disables ASLR by default. To debug instrumented programs, use “set disable-randomization off”.

Current Status

MemorySanitizer is an experimental tool. It is known to work on large real-world programs, like Clang/LLVM itself.

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