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:

  • Array subscript out of bounds, where the bounds can be statically determined

  • Bitwise shifts that are out of bounds for their data type

  • Dereferencing misaligned or null pointers

  • 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 the -fsanitize=undefined option. Make sure to use clang++ (not ld) as a linker, so that your executable is linked with proper UBSan runtime libraries, unless all enabled checks use trap mode. You can use clang instead of clang++ if you’re compiling/linking C code.

% cat test.cc
int main(int argc, char **argv) {
  int k = 0x7fffffff;
  k += argc;
  return 0;
% clang++ -fsanitize=undefined test.cc
% ./a.out
test.cc:3:5: runtime error: signed integer overflow: 2147483647 + 1 cannot be represented in type 'int'

You can use -fsanitize=... and -fno-sanitize= to enable and disable one check or one check group. For an individual check, the last option that enabling or disabling it wins.

# Enable all checks in the "undefined" group, but disable "alignment".
% clang -fsanitize=undefined -fno-sanitize=alignment a.c

# Enable just "alignment".
% clang -fsanitize=alignment a.c

# The same. -fno-sanitize=undefined nullifies the previous -fsanitize=undefined.
% clang -fsanitize=undefined -fno-sanitize=undefined -fsanitize=alignment a.c

For most checks (checks), the instrumented program prints a verbose error report and continues execution upon a failed check. You can use the following options to change the error reporting behavior:

  • -fno-sanitize-recover=...: print a verbose error report and exit the program;

  • -fsanitize-trap=...: execute a trap instruction (doesn’t require UBSan run-time support). If the signal is not caught, the program will typically terminate due to a SIGILL or SIGTRAP signal.

For example:

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

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.

% clang++ -fsanitize=undefined -fsanitize-trap=all a.cc

All checks in the “undefined” group are put into trap mode. Since no check needs run-time support, the UBSan run-time library it not linked. Note that some other sanitizers also support trap mode and -fsanitize-trap=all enables trap mode for them.

% clang -fsanitize-trap=undefined -fsanitize-recover=all a.c

-fsanitize-trap= and -fsanitize-recover= are a no-op in the absence of a -fsanitize= option. There is no unused command line option warning.

Available checks

Available checks are:

  • -fsanitize=alignment: Use of a misaligned pointer or creation of a misaligned reference. Also sanitizes assume_aligned-like attributes.

  • -fsanitize=bool: Load of a bool value which is neither true nor false.

  • -fsanitize=builtin: Passing invalid values to compiler builtins.

  • -fsanitize=bounds: Out of bounds array indexing, in cases where the array bound can be statically determined. The check includes -fsanitize=array-bounds and -fsanitize=local-bounds. Note that -fsanitize=local-bounds is not included in -fsanitize=undefined.

  • -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. Because the range of representable values for all floating-point types supported by Clang is [-inf, +inf], the only cases detected are conversions from floating point to integer types.

  • -fsanitize=float-divide-by-zero: Floating point division by zero. This is undefined per the C and C++ standards, but is defined by Clang (and by ISO/IEC/IEEE 60559 / IEEE 754) as producing either an infinity or NaN value, so is not included in -fsanitize=undefined.

  • -fsanitize=function: Indirect call of a function through a function pointer of the wrong type.

  • -fsanitize=implicit-unsigned-integer-truncation, -fsanitize=implicit-signed-integer-truncation: Implicit conversion from integer of larger bit width to smaller bit width, if that results in data loss. That is, if the demoted value, after casting back to the original width, is not equal to the original value before the downcast. The -fsanitize=implicit-unsigned-integer-truncation handles conversions between two unsigned types, while -fsanitize=implicit-signed-integer-truncation handles the rest of the conversions - when either one, or both of the types are signed. Issues caught by these sanitizers are not undefined behavior, but are often unintentional.

  • -fsanitize=implicit-integer-sign-change: Implicit conversion between integer types, if that changes the sign of the value. That is, if the original value was negative and the new value is positive (or zero), or the original value was positive, and the new value is negative. Issues caught by this sanitizer are not undefined behavior, but are often unintentional.

  • -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=nullability-arg: Passing null as a function parameter which is annotated with _Nonnull.

  • -fsanitize=nullability-assign: Assigning null to an lvalue which is annotated with _Nonnull.

  • -fsanitize=nullability-return: Returning null from a function with a return type annotated with _Nonnull.

  • -fsanitize=objc-cast: Invalid implicit cast of an ObjC object pointer to an incompatible type. This is often unintentional, but is not undefined behavior, therefore the check is not a part of the undefined group. Currently only supported on Darwin.

  • -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=pointer-overflow: Performing pointer arithmetic which overflows, or where either the old or new pointer value is a null pointer (or in C, when they both are).

  • -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=unsigned-shift-base: check that an unsigned left-hand side of a left shift operation doesn’t overflow. Issues caught by this sanitizer are not undefined behavior, but are often unintentional.

  • -fsanitize=signed-integer-overflow: Signed integer overflow, where the result of a signed integer computation cannot be represented in its type. This includes all the checks covered by -ftrapv, as well as checks for signed division overflow (INT_MIN/-1), but not checks for lossy implicit conversions performed before the computation (see -fsanitize=implicit-conversion). Both of these two issues are handled by -fsanitize=implicit-conversion group of checks.

  • -fsanitize=unreachable: If control flow reaches an unreachable program point.

  • -fsanitize=unsigned-integer-overflow: Unsigned integer overflow, where the result of an unsigned integer computation cannot be represented in its type. Unlike signed integer overflow, this is not undefined behavior, but it is often unintentional. This sanitizer does not check for lossy implicit conversions performed before such a computation (see -fsanitize=implicit-conversion).

  • -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 float-divide-by-zero, unsigned-integer-overflow, implicit-conversion, local-bounds and the nullability-* group of checks.

  • -fsanitize=undefined-trap: Deprecated alias of -fsanitize=undefined.

  • -fsanitize=implicit-integer-truncation: Catches lossy integral conversions. Enables implicit-signed-integer-truncation and implicit-unsigned-integer-truncation.

  • -fsanitize=implicit-integer-arithmetic-value-change: Catches implicit conversions that change the arithmetic value of the integer. Enables implicit-signed-integer-truncation and implicit-integer-sign-change.

  • -fsanitize=implicit-conversion: Checks for suspicious behavior of implicit conversions. Enables implicit-unsigned-integer-truncation, implicit-signed-integer-truncation, and implicit-integer-sign-change.

  • -fsanitize=integer: Checks for undefined or suspicious integer behavior (e.g. unsigned integer overflow). Enables signed-integer-overflow, unsigned-integer-overflow, shift, integer-divide-by-zero, implicit-unsigned-integer-truncation, implicit-signed-integer-truncation, and implicit-integer-sign-change.

  • -fsanitize=nullability: Enables nullability-arg, nullability-assign, and nullability-return. While violating nullability does not have undefined behavior, it is often unintentional, so UBSan offers to catch it.


The null, alignment, object-size, local-bounds, and vptr checks do not apply to pointers to types with the volatile qualifier.

Minimal Runtime

There is a minimal UBSan runtime available suitable for use in production environments. This runtime has a small attack surface. It only provides very basic issue logging and deduplication, and does not support -fsanitize=vptr checking.

To use the minimal runtime, add -fsanitize-minimal-runtime to the clang command line options. For example, if you’re used to compiling with -fsanitize=undefined, you could enable the minimal runtime with -fsanitize=undefined -fsanitize-minimal-runtime.

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.


The default log file for diagnostics is “stderr”. To log diagnostics to another file, you can set UBSAN_OPTIONS=log_path=....

Silencing Unsigned Integer Overflow

To silence reports from unsigned integer overflow, you can set UBSAN_OPTIONS=silence_unsigned_overflow=1. This feature, combined with -fsanitize-recover=unsigned-integer-overflow, is particularly useful for providing fuzzing signal without blowing up logs.

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 (Ignorelist)

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 operating systems:

  • Android

  • Linux

  • NetBSD

  • FreeBSD

  • OpenBSD

  • macOS

  • Windows

The runtime library is relatively portable and platform independent. If the OS you need is not listed above, UndefinedBehaviorSanitizer may already work for it, or could be made to work with a minor porting effort.

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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