Clang 4 documentation


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Sanitizer tools have a very simple code coverage tool built in. It allows to get function-level, basic-block-level, and edge-level coverage at a very low cost.

How to build and run

SanitizerCoverage can be used with AddressSanitizer, LeakSanitizer, MemorySanitizer, UndefinedBehaviorSanitizer, or without any sanitizer. Pass one of the following compile-time flags:

  • -fsanitize-coverage=func for function-level coverage (very fast).
  • -fsanitize-coverage=bb for basic-block-level coverage (may add up to 30% extra slowdown).
  • -fsanitize-coverage=edge for edge-level coverage (up to 40% slowdown).

You may also specify -fsanitize-coverage=indirect-calls for additional caller-callee coverage.

At run time, pass coverage=1 in ASAN_OPTIONS, LSAN_OPTIONS, MSAN_OPTIONS or UBSAN_OPTIONS, as appropriate. For the standalone coverage mode, use UBSAN_OPTIONS.

To get Coverage counters, add -fsanitize-coverage=8bit-counters to one of the above compile-time flags. At runtime, use *SAN_OPTIONS=coverage=1:coverage_counters=1.


% cat -n
     1  #include <stdio.h>
     2  __attribute__((noinline))
     3  void foo() { printf("foo\n"); }
     5  int main(int argc, char **argv) {
     6    if (argc == 2)
     7      foo();
     8    printf("main\n");
     9  }
% clang++ -g -fsanitize=address -fsanitize-coverage=func
% ASAN_OPTIONS=coverage=1 ./a.out; ls -l *sancov
-rw-r----- 1 kcc eng 4 Nov 27 12:21 a.out.22673.sancov
% ASAN_OPTIONS=coverage=1 ./a.out foo ; ls -l *sancov
-rw-r----- 1 kcc eng 4 Nov 27 12:21 a.out.22673.sancov
-rw-r----- 1 kcc eng 8 Nov 27 12:21 a.out.22679.sancov

Every time you run an executable instrumented with SanitizerCoverage one *.sancov file is created during the process shutdown. If the executable is dynamically linked against instrumented DSOs, one *.sancov file will be also created for every DSO.


The format of *.sancov files is very simple: the first 8 bytes is the magic, one of 0xC0BFFFFFFFFFFF64 and 0xC0BFFFFFFFFFFF32. The last byte of the magic defines the size of the following offsets. The rest of the data is the offsets in the corresponding binary/DSO that were executed during the run.

A simple script $LLVM/projects/compiler-rt/lib/sanitizer_common/scripts/ is provided to dump these offsets.

% print a.out.22679.sancov a.out.22673.sancov read 2 PCs from a.out.22679.sancov read 1 PCs from a.out.22673.sancov 2 files merged; 2 PCs total

You can then filter the output of through addr2line --exe ObjectFile or llvm-symbolizer --obj ObjectFile to get file names and line numbers:

% print a.out.22679.sancov a.out.22673.sancov 2> /dev/null | llvm-symbolizer --obj a.out

Sancov Tool

A new experimental sancov tool is developed to process coverage files. The tool is part of LLVM project and is currently supported only on Linux. It can handle symbolization tasks autonomously without any extra support from the environment. You need to pass .sancov files (named <module_name>.<pid>.sancov and paths to all corresponding binary elf files. Sancov matches these files using module names and binaries file names.

USAGE: sancov [options] <action> (<binary file>|<.sancov file>)...

Action (required)
  -print                    - Print coverage addresses
  -covered-functions        - Print all covered functions.
  -not-covered-functions    - Print all not covered functions.
  -symbolize                - Symbolizes the report.

  -blacklist=<string>         - Blacklist file (sanitizer blacklist format).
  -demangle                   - Print demangled function name.
  -strip_path_prefix=<string> - Strip this prefix from file paths in reports

Coverage Reports (Experimental)

.sancov files do not contain enough information to generate a source-level coverage report. The missing information is contained in debug info of the binary. Thus the .sancov has to be symbolized to produce a .symcov file first:

sancov -symbolize my_program.123.sancov my_program > my_program.123.symcov

The .symcov file can be browsed overlayed over the source code by running tools/sancov/ script that will start an HTTP server.

How good is the coverage?

It is possible to find out which PCs are not covered, by subtracting the covered set from the set of all instrumented PCs. The latter can be obtained by listing all callsites of __sanitizer_cov() in the binary. On Linux, can do this for you. Just supply the path to binary and a list of covered PCs:

% print a.out.12345.sancov > covered.txt read 2 64-bit PCs from a.out.12345.sancov 1 file merged; 2 PCs total
% missing a.out < covered.txt found 3 instrumented PCs in a.out read 2 PCs from stdin 1 PCs missing from coverage

Edge coverage

Consider this code:

void foo(int *a) {
  if (a)
    *a = 0;

It contains 3 basic blocks, let’s name them A, B, C:

| \
|  B
| /

If blocks A, B, and C are all covered we know for certain that the edges A=>B and B=>C were executed, but we still don’t know if the edge A=>C was executed. Such edges of control flow graph are called critical. The edge-level coverage (-fsanitize-coverage=edge) simply splits all critical edges by introducing new dummy blocks and then instruments those blocks:

| \
D  B
| /


When coverage_bitset=1 run-time flag is given, the coverage will also be dumped as a bitset (text file with 1 for blocks that have been executed and 0 for blocks that were not).

% clang++ -fsanitize=address -fsanitize-coverage=edge
% ASAN_OPTIONS="coverage=1:coverage_bitset=1" ./a.out
% ASAN_OPTIONS="coverage=1:coverage_bitset=1" ./a.out 1
% head *bitset*
==> a.out.38214.bitset-sancov <==
==> a.out.6128.bitset-sancov <==

For a given executable the length of the bitset is always the same (well, unless dlopen/dlclose come into play), so the bitset coverage can be easily used for bitset-based corpus distillation.

Caller-callee coverage

(Experimental!) Every indirect function call is instrumented with a run-time function call that captures caller and callee. At the shutdown time the process dumps a separate file called caller-callee.PID.sancov which contains caller/callee pairs as pairs of lines (odd lines are callers, even lines are callees)

a.out 0x4a2e0c
a.out 0x4a6510
a.out 0x4a2e0c
a.out 0x4a87f0

Current limitations:

  • Only the first 14 callees for every caller are recorded, the rest are silently ignored.
  • The output format is not very compact since caller and callee may reside in different modules and we need to spell out the module names.
  • The routine that dumps the output is not optimized for speed
  • Only Linux x86_64 is tested so far.
  • Sandboxes are not supported.

Coverage counters

This experimental feature is inspired by AFL‘s coverage instrumentation. With additional compile-time and run-time flags you can get more sensitive coverage information. In addition to boolean values assigned to every basic block (edge) the instrumentation will collect imprecise counters. On exit, every counter will be mapped to a 8-bit bitset representing counter ranges: 1, 2, 3, 4-7, 8-15, 16-31, 32-127, 128+ and those 8-bit bitsets will be dumped to disk.

% clang++ -g -fsanitize=address -fsanitize-coverage=edge,8bit-counters
% ASAN_OPTIONS="coverage=1:coverage_counters=1" ./a.out
% ls -l *counters-sancov
... a.out.17110.counters-sancov
% xxd *counters-sancov
0000000: 0001 0100 01

These counters may also be used for in-process coverage-guided fuzzers. See include/sanitizer/coverage_interface.h:

// The coverage instrumentation may optionally provide imprecise counters.
// Rather than exposing the counter values to the user we instead map
// the counters to a bitset.
// Every counter is associated with 8 bits in the bitset.
// We define 8 value ranges: 1, 2, 3, 4-7, 8-15, 16-31, 32-127, 128+
// The i-th bit is set to 1 if the counter value is in the i-th range.
// This counter-based coverage implementation is *not* thread-safe.

// Returns the number of registered coverage counters.
uintptr_t __sanitizer_get_number_of_counters();
// Updates the counter 'bitset', clears the counters and returns the number of
// new bits in 'bitset'.
// If 'bitset' is nullptr, only clears the counters.
// Otherwise 'bitset' should be at least
// __sanitizer_get_number_of_counters bytes long and 8-aligned.
__sanitizer_update_counter_bitset_and_clear_counters(uint8_t *bitset);

Tracing basic blocks

Experimental support for basic block (or edge) tracing. With -fsanitize-coverage=trace-bb the compiler will insert __sanitizer_cov_trace_basic_block(s32 *id) before every function, basic block, or edge (depending on the value of -fsanitize-coverage=[func,bb,edge]). Example:

% clang -g -fsanitize=address -fsanitize-coverage=edge,trace-bb
% ASAN_OPTIONS=coverage=1 ./a.out

This will produce two files after the process exit: trace-points.PID.sancov and trace-events.PID.sancov. The first file will contain a textual description of all the instrumented points in the program in the form that you can feed into llvm-symbolizer (e.g. a.out 0x4dca89), one per line. The second file will contain the actual execution trace as a sequence of 4-byte integers – these integers are the indices into the array of instrumented points (the first file).

Basic block tracing is currently supported only for single-threaded applications.

Tracing PCs

Experimental feature similar to tracing basic blocks, but with a different API. With -fsanitize-coverage=trace-pc the compiler will insert __sanitizer_cov_trace_pc() on every edge. With an additional ...=trace-pc,indirect-calls flag __sanitizer_cov_trace_pc_indirect(void *callee) will be inserted on every indirect call. These callbacks are not implemented in the Sanitizer run-time and should be defined by the user. So, these flags do not require the other sanitizer to be used. This mechanism is used for fuzzing the Linux kernel ( and can be used with AFL.

Tracing PCs with guards

Another experimental feature that tries to combine the functionality of trace-pc, 8bit-counters and boolean coverage.

With -fsanitize-coverage=trace-pc-guard the compiler will insert the following code on every edge:

if (guard_variable)

Every edge will have its own guard_variable (uint32_t).

The compler will also insert a module constructor that will call

// The guards are [start, stop).
// This function may be called multiple times with the same values of start/stop.
__sanitizer_cov_trace_pc_guard_init(uint32_t *start, uint32_t *stop);

Similarly to trace-pc,indirect-calls, with trace-pc-guards,indirect-calls __sanitizer_cov_trace_pc_indirect(void *callee) will be inserted on every indirect call.

The functions __sanitizer_cov_trace_pc_* should be defined by the user.


#include <stdint.h>
#include <stdio.h>
#include <sanitizer/coverage_interface.h>

// This callback is inserted by the compiler as a module constructor
// into every compilation unit. 'start' and 'stop' correspond to the
// beginning and end of the section with the guards for the entire
// binary (executable or DSO) and so it will be called multiple times
// with the same parameters.
extern "C" void __sanitizer_cov_trace_pc_guard_init(uint32_t *start,
                                                    uint32_t *stop) {
  static uint64_t N;  // Counter for the guards.
  if (start == stop || *start) return;  // Initialize only once.
  printf("INIT: %p %p\n", start, stop);
  for (uint32_t *x = start; x < stop; x++)
    *x = ++N;  // Guards should start from 1.

// This callback is inserted by the compiler on every edge in the
// control flow (some optimizations apply).
// Typically, the compiler will emit the code like this:
//    if(*guard)
//      __sanitizer_cov_trace_pc_guard(guard);
// But for large functions it will emit a simple call:
//    __sanitizer_cov_trace_pc_guard(guard);
extern "C" void __sanitizer_cov_trace_pc_guard(uint32_t *guard) {
  if (!*guard) return;  // Duplicate the guard check.
  // If you set *guard to 0 this code will not be called again for this edge.
  // Now you can get the PC and do whatever you want:
  //   store it somewhere or symbolize it and print right away.
  // The values of `*guard` are as you set them in
  // __sanitizer_cov_trace_pc_guard_init and so you can make them consecutive
  // and use them to dereference an array or a bit vector.
  void *PC = __builtin_return_address(0);
  char PcDescr[1024];
  // This function is a part of the sanitizer run-time.
  // To use it, link with AddressSanitizer or other sanitizer.
  __sanitizer_symbolize_pc(PC, "%p %F %L", PcDescr, sizeof(PcDescr));
  printf("guard: %p %x PC %s\n", guard, *guard, PcDescr);
void foo() { }
int main(int argc, char **argv) {
  if (argc > 1) foo();
clang++ -g  -fsanitize-coverage=trace-pc-guard -c
clang++ trace-pc-guard-example.o -fsanitize=address
ASAN_OPTIONS=strip_path_prefix=`pwd`/ ./a.out
INIT: 0x71bcd0 0x71bce0
guard: 0x71bcd4 2 PC 0x4ecd5b in main
guard: 0x71bcd8 3 PC 0x4ecd9e in main
ASAN_OPTIONS=strip_path_prefix=`pwd`/ ./a.out with-foo
INIT: 0x71bcd0 0x71bce0
guard: 0x71bcd4 2 PC 0x4ecd5b in main
guard: 0x71bcdc 4 PC 0x4ecdc7 in main
guard: 0x71bcd0 1 PC 0x4ecd20 in foo()

Tracing data flow

Support for data-flow-guided fuzzing. With -fsanitize-coverage=trace-cmp the compiler will insert extra instrumentation around comparison instructions and switch statements. Similarly, with -fsanitize-coverage=trace-div the compiler will instrument integer division instructions (to capture the right argument of division) and with -fsanitize-coverage=trace-gep – the LLVM GEP instructions (to capture array indices).

// Called before a comparison instruction.
// Arg1 and Arg2 are arguments of the comparison.
void __sanitizer_cov_trace_cmp1(uint8_t Arg1, uint8_t Arg2);
void __sanitizer_cov_trace_cmp2(uint16_t Arg1, uint16_t Arg2);
void __sanitizer_cov_trace_cmp4(uint32_t Arg1, uint32_t Arg2);
void __sanitizer_cov_trace_cmp8(uint64_t Arg1, uint64_t Arg2);

// Called before a switch statement.
// Val is the switch operand.
// Cases[0] is the number of case constants.
// Cases[1] is the size of Val in bits.
// Cases[2:] are the case constants.
void __sanitizer_cov_trace_switch(uint64_t Val, uint64_t *Cases);

// Called before a division statement.
// Val is the second argument of division.
void __sanitizer_cov_trace_div4(uint32_t Val);
void __sanitizer_cov_trace_div8(uint64_t Val);

// Called before a GetElemementPtr (GEP) instruction
// for every non-constant array index.
void __sanitizer_cov_trace_gep(uintptr_t Idx);

This interface is a subject to change. The current implementation is not thread-safe and thus can be safely used only for single-threaded targets.

Output directory

By default, .sancov files are created in the current working directory. This can be changed with ASAN_OPTIONS=coverage_dir=/path:

% ASAN_OPTIONS="coverage=1:coverage_dir=/tmp/cov" ./a.out foo
% ls -l /tmp/cov/*sancov
-rw-r----- 1 kcc eng 4 Nov 27 12:21 a.out.22673.sancov
-rw-r----- 1 kcc eng 8 Nov 27 12:21 a.out.22679.sancov

Sudden death

Normally, coverage data is collected in memory and saved to disk when the program exits (with an atexit() handler), when a SIGSEGV is caught, or when __sanitizer_cov_dump() is called.

If the program ends with a signal that ASan does not handle (or can not handle at all, like SIGKILL), coverage data will be lost. This is a big problem on Android, where SIGKILL is a normal way of evicting applications from memory.

With ASAN_OPTIONS=coverage=1:coverage_direct=1 coverage data is written to a memory-mapped file as soon as it collected.

% ASAN_OPTIONS="coverage=1:coverage_direct=1" ./a.out
% ls  7036.sancov.raw  a.out
% rawunpack 7036.sancov.raw reading map unpacking 7036.sancov.raw
writing 1 PCs to a.out.7036.sancov
% print a.out.7036.sancov read 1 PCs from a.out.7036.sancov 1 files merged; 1 PCs total

Note that on 64-bit platforms, this method writes 2x more data than the default, because it stores full PC values instead of 32-bit offsets.

In-process fuzzing

Coverage data could be useful for fuzzers and sometimes it is preferable to run a fuzzer in the same process as the code being fuzzed (in-process fuzzer).

You can use __sanitizer_get_total_unique_coverage() from <sanitizer/coverage_interface.h> which returns the number of currently covered entities in the program. This will tell the fuzzer if the coverage has increased after testing every new input.

If a fuzzer finds a bug in the ASan run, you will need to save the reproducer before exiting the process. Use __asan_set_death_callback from <sanitizer/asan_interface.h> to do that.

An example of such fuzzer can be found in the LLVM tree.


This coverage implementation is fast. With function-level coverage (-fsanitize-coverage=func) the overhead is not measurable. With basic-block-level coverage (-fsanitize-coverage=bb) the overhead varies between 0 and 25%.

benchmark cov0 cov1 diff 0-1 cov2 diff 0-2 diff 1-2
400.perlbench 1296.00 1307.00 1.01 1465.00 1.13 1.12
401.bzip2 858.00 854.00 1.00 1010.00 1.18 1.18
403.gcc 613.00 617.00 1.01 683.00 1.11 1.11
429.mcf 605.00 582.00 0.96 610.00 1.01 1.05
445.gobmk 896.00 880.00 0.98 1050.00 1.17 1.19
456.hmmer 892.00 892.00 1.00 918.00 1.03 1.03
458.sjeng 995.00 1009.00 1.01 1217.00 1.22 1.21
462.libquantum 497.00 492.00 0.99 534.00 1.07 1.09
464.h264ref 1461.00 1467.00 1.00 1543.00 1.06 1.05
471.omnetpp 575.00 590.00 1.03 660.00 1.15 1.12
473.astar 658.00 652.00 0.99 715.00 1.09 1.10
483.xalancbmk 471.00 491.00 1.04 582.00 1.24 1.19
433.milc 616.00 627.00 1.02 627.00 1.02 1.00
444.namd 602.00 601.00 1.00 654.00 1.09 1.09
447.dealII 630.00 634.00 1.01 653.00 1.04 1.03
450.soplex 365.00 368.00 1.01 395.00 1.08 1.07
453.povray 427.00 434.00 1.02 495.00 1.16 1.14
470.lbm 357.00 375.00 1.05 370.00 1.04 0.99
482.sphinx3 927.00 928.00 1.00 1000.00 1.08 1.08

Why another coverage?

Why did we implement yet another code coverage?
  • We needed something that is lightning fast, plays well with AddressSanitizer, and does not significantly increase the binary size.
  • Traditional coverage implementations based in global counters suffer from contention on counters.

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