Clang 3.7 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

Build LLVM/Clang with CMake.


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
WARNING: MemorySanitizer: use-of-uninitialized-value
    #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).


MemorySanitizer supports src and fun entity types in Sanitizer special case list, that can be used to relax MemorySanitizer checks for certain source files and functions. All “Use of uninitialized value” warnings will be suppressed and all values loaded from memory will be considered fully initialized.

Report symbolization

MemorySanitizer uses an external symbolizer to print files and line numbers in reports. Make sure that llvm-symbolizer binary is in PATH, or set environment variable MSAN_SYMBOLIZER_PATH to point to it.

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=2 (or simply -fsanitize-memory-track-origins) Clang option. With the code from the example above,

% cat
#include <stdio.h>

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

% clang -fsanitize=memory -fsanitize-memory-track-origins=2 -fno-omit-frame-pointer -g -O2
% ./a.out
WARNING: MemorySanitizer: use-of-uninitialized-value
    #0 0x7f7893912f0b in main
    #1 0x7f789249b76c in __libc_start_main libc-start.c:226

  Uninitialized value was stored to memory at
    #0 0x7f78938b5c25 in __msan_chain_origin
    #1 0x7f7893912ecd in main

  Uninitialized value was created by a heap allocation
    #0 0x7f7893901cbd in operator new[](unsigned long)
    #1 0x7f7893912e06 in main

By default, MemorySanitizer collects both allocation points and all intermediate stores the uninitialized value went through. Origin tracking has proved to be very useful for debugging MemorySanitizer reports. It slows down program execution by a factor of 1.5x-2x on top of the usual MemorySanitizer slowdown.

Clang option -fsanitize-memory-track-origins=1 enabled a slightly faster mode when MemorySanitizer collects only allocation points but not intermediate stores.

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 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 libc++ (as a replacement for libstdc++).

Supported Platforms

MemorySanitizer is supported on

  • Linux x86_64 (tested on Ubuntu 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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