Clang 4 documentation


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UndefinedBehaviorSanitizer (UBSan) is a fast undefined behavior detector. UBSan modifies the program at compile-time to catch various kinds of undefined behavior during program execution, for example:

  • Using misaligned or null pointer
  • Signed integer overflow
  • Conversion to, from, or between floating-point types which would overflow the destination

See the full list of available checks below.

UBSan has an optional run-time library which provides better error reporting. The checks have small runtime cost and no impact on address space layout or ABI.

How to build

Build LLVM/Clang with CMake.


Use clang++ to compile and link your program with -fsanitize=undefined flag. Make sure to use clang++ (not ld) as a linker, so that your executable is linked with proper UBSan runtime libraries. You can use clang instead of clang++ if you’re compiling/linking C code.

% cat
int main(int argc, char **argv) {
  int k = 0x7fffffff;
  k += argc;
  return 0;
% clang++ -fsanitize=undefined
% ./a.out runtime error: signed integer overflow: 2147483647 + 1 cannot be represented in type 'int'

You can enable only a subset of checks offered by UBSan, and define the desired behavior for each kind of check:

  • print a verbose error report and continue execution (default);
  • print a verbose error report and exit the program;
  • execute a trap instruction (doesn’t require UBSan run-time support).

For example if you compile/link your program as:

% clang++ -fsanitize=signed-integer-overflow,null,alignment -fno-sanitize-recover=null -fsanitize-trap=alignment

the program will continue execution after signed integer overflows, exit after the first invalid use of a null pointer, and trap after the first use of misaligned pointer.

Available checks

Available checks are:

  • -fsanitize=alignment: Use of a misaligned pointer or creation of a misaligned reference.
  • -fsanitize=bool: Load of a bool value which is neither true nor false.
  • -fsanitize=bounds: Out of bounds array indexing, in cases where the array bound can be statically determined.
  • -fsanitize=enum: Load of a value of an enumerated type which is not in the range of representable values for that enumerated type.
  • -fsanitize=float-cast-overflow: Conversion to, from, or between floating-point types which would overflow the destination.
  • -fsanitize=float-divide-by-zero: Floating point division by zero.
  • -fsanitize=function: Indirect call of a function through a function pointer of the wrong type (Linux, C++ and x86/x86_64 only).
  • -fsanitize=integer-divide-by-zero: Integer division by zero.
  • -fsanitize=nonnull-attribute: Passing null pointer as a function parameter which is declared to never be null.
  • -fsanitize=null: Use of a null pointer or creation of a null reference.
  • -fsanitize=object-size: An attempt to potentially use bytes which the optimizer can determine are not part of the object being accessed. This will also detect some types of undefined behavior that may not directly access memory, but are provably incorrect given the size of the objects involved, such as invalid downcasts and calling methods on invalid pointers. These checks are made in terms of __builtin_object_size, and consequently may be able to detect more problems at higher optimization levels.
  • -fsanitize=return: In C++, reaching the end of a value-returning function without returning a value.
  • -fsanitize=returns-nonnull-attribute: Returning null pointer from a function which is declared to never return null.
  • -fsanitize=shift: Shift operators where the amount shifted is greater or equal to the promoted bit-width of the left hand side or less than zero, or where the left hand side is negative. For a signed left shift, also checks for signed overflow in C, and for unsigned overflow in C++. You can use -fsanitize=shift-base or -fsanitize=shift-exponent to check only left-hand side or right-hand side of shift operation, respectively.
  • -fsanitize=signed-integer-overflow: Signed integer overflow, including all the checks added by -ftrapv, and checking for overflow in signed division (INT_MIN / -1).
  • -fsanitize=unreachable: If control flow reaches __builtin_unreachable.
  • -fsanitize=unsigned-integer-overflow: Unsigned integer overflows.
  • -fsanitize=vla-bound: A variable-length array whose bound does not evaluate to a positive value.
  • -fsanitize=vptr: Use of an object whose vptr indicates that it is of the wrong dynamic type, or that its lifetime has not begun or has ended. Incompatible with -fno-rtti. Link must be performed by clang++, not clang, to make sure C++-specific parts of the runtime library and C++ standard libraries are present.
You can also use the following check groups:
  • -fsanitize=undefined: All of the checks listed above other than unsigned-integer-overflow.
  • -fsanitize=undefined-trap: Deprecated alias of -fsanitize=undefined.
  • -fsanitize=integer: Checks for undefined or suspicious integer behavior (e.g. unsigned integer overflow).

Stack traces and report symbolization

If you want UBSan to print symbolized stack trace for each error report, you will need to:

  1. Compile with -g and -fno-omit-frame-pointer to get proper debug information in your binary.
  2. Run your program with environment variable UBSAN_OPTIONS=print_stacktrace=1.
  3. Make sure llvm-symbolizer binary is in PATH.

Issue Suppression

UndefinedBehaviorSanitizer is not expected to produce false positives. If you see one, look again; most likely it is a true positive!

Disabling Instrumentation with __attribute__((no_sanitize("undefined")))

You disable UBSan checks for particular functions with __attribute__((no_sanitize("undefined"))). You can use all values of -fsanitize= flag in this attribute, e.g. if your function deliberately contains possible signed integer overflow, you can use __attribute__((no_sanitize("signed-integer-overflow"))).

This attribute may not be supported by other compilers, so consider using it together with #if defined(__clang__).

Suppressing Errors in Recompiled Code (Blacklist)

UndefinedBehaviorSanitizer supports src and fun entity types in Sanitizer special case list, that can be used to suppress error reports in the specified source files or functions.

Runtime suppressions

Sometimes you can suppress UBSan error reports for specific files, functions, or libraries without recompiling the code. You need to pass a path to suppression file in a UBSAN_OPTIONS environment variable.


You need to specify a check you are suppressing and the bug location. For example:


There are several limitations:

  • Sometimes your binary must have enough debug info and/or symbol table, so that the runtime could figure out source file or function name to match against the suppression.
  • It is only possible to suppress recoverable checks. For the example above, you can additionally pass -fsanitize-recover=signed-integer-overflow,alignment,vptr, although most of UBSan checks are recoverable by default.
  • Check groups (like undefined) can’t be used in suppressions file, only fine-grained checks are supported.

Supported Platforms

UndefinedBehaviorSanitizer is supported on the following OS:

  • Android
  • Linux
  • FreeBSD
  • OS X 10.6 onwards

and for the following architectures:

  • i386/x86_64
  • ARM
  • AArch64
  • PowerPC64

Current Status

UndefinedBehaviorSanitizer is available on selected platforms starting from LLVM 3.3. The test suite is integrated into the CMake build and can be run with check-ubsan command.

Additional Configuration

UndefinedBehaviorSanitizer adds static check data for each check unless it is in trap mode. This check data includes the full file name. The option -fsanitize-undefined-strip-path-components=N can be used to trim this information. If N is positive, file information emitted by UndefinedBehaviorSanitizer will drop the first N components from the file path. If N is negative, the last N components will be kept.


For a file called /code/library/file.cpp, here is what would be emitted: * Default (No flag, or -fsanitize-undefined-strip-path-components=0): /code/library/file.cpp * -fsanitize-undefined-strip-path-components=1: code/library/file.cpp * -fsanitize-undefined-strip-path-components=2: library/file.cpp * -fsanitize-undefined-strip-path-components=-1: file.cpp * -fsanitize-undefined-strip-path-components=-2: library/file.cpp

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