+By default (``-reload=1``) the fuzzer processes will periodically scan the CORPUS directory
+and reload any new tests. This way the test inputs found by one process will be picked up
+by all others.
+
+If ``-workers=$M`` is not supplied, ``min($N,NumberOfCpuCore/2)`` will be used.
+
+Heartbleed
+----------
+Remember Heartbleed_?
+As it was recently `shown <https://blog.hboeck.de/archives/868-How-Heartbleed-couldve-been-found.html>`_,
+fuzzing with AddressSanitizer can find Heartbleed. Indeed, here are the step-by-step instructions
+to find Heartbleed with LibFuzzer::
+
+ wget https://www.openssl.org/source/openssl-1.0.1f.tar.gz
+ tar xf openssl-1.0.1f.tar.gz
+ COV_FLAGS="-fsanitize-coverage=edge,indirect-calls" # -fsanitize-coverage=8bit-counters
+ (cd openssl-1.0.1f/ && ./config &&
+ make -j 32 CC="clang -g -fsanitize=address $COV_FLAGS")
+ # Get and build LibFuzzer
+ svn co http://llvm.org/svn/llvm-project/llvm/trunk/lib/Fuzzer
+ clang -c -g -O2 -std=c++11 Fuzzer/*.cpp -IFuzzer
+ # Get examples of key/pem files.
+ git clone https://github.com/hannob/selftls
+ cp selftls/server* . -v
+ cat << EOF > handshake-fuzz.cc
+ #include <openssl/ssl.h>
+ #include <openssl/err.h>
+ #include <assert.h>
+ #include <stdint.h>
+ #include <stddef.h>
+
+ SSL_CTX *sctx;
+ int Init() {
+ SSL_library_init();
+ SSL_load_error_strings();
+ ERR_load_BIO_strings();
+ OpenSSL_add_all_algorithms();
+ assert (sctx = SSL_CTX_new(TLSv1_method()));
+ assert (SSL_CTX_use_certificate_file(sctx, "server.pem", SSL_FILETYPE_PEM));
+ assert (SSL_CTX_use_PrivateKey_file(sctx, "server.key", SSL_FILETYPE_PEM));
+ return 0;
+ }
+ extern "C" int LLVMFuzzerTestOneInput(const uint8_t *Data, size_t Size) {
+ static int unused = Init();
+ SSL *server = SSL_new(sctx);
+ BIO *sinbio = BIO_new(BIO_s_mem());
+ BIO *soutbio = BIO_new(BIO_s_mem());
+ SSL_set_bio(server, sinbio, soutbio);
+ SSL_set_accept_state(server);
+ BIO_write(sinbio, Data, Size);
+ SSL_do_handshake(server);
+ SSL_free(server);
+ return 0;
+ }
+ EOF
+ # Build the fuzzer.
+ clang++ -g handshake-fuzz.cc -fsanitize=address \
+ openssl-1.0.1f/libssl.a openssl-1.0.1f/libcrypto.a Fuzzer*.o
+ # Run 20 independent fuzzer jobs.
+ ./a.out -jobs=20 -workers=20
+
+Voila::
+
+ #1048576 pulse cov 3424 bits 0 units 9 exec/s 24385
+ =================================================================
+ ==17488==ERROR: AddressSanitizer: heap-buffer-overflow on address 0x629000004748 at pc 0x00000048c979 bp 0x7fffe3e864f0 sp 0x7fffe3e85ca8
+ READ of size 60731 at 0x629000004748 thread T0
+ #0 0x48c978 in __asan_memcpy
+ #1 0x4db504 in tls1_process_heartbeat openssl-1.0.1f/ssl/t1_lib.c:2586:3
+ #2 0x580be3 in ssl3_read_bytes openssl-1.0.1f/ssl/s3_pkt.c:1092:4
+
+Note: a `similar fuzzer <https://boringssl.googlesource.com/boringssl/+/HEAD/FUZZING.md>`_
+is now a part of the boringssl source tree.
+
+Advanced features
+=================
+
+Dictionaries
+------------
+*EXPERIMENTAL*.
+LibFuzzer supports user-supplied dictionaries with input language keywords
+or other interesting byte sequences (e.g. multi-byte magic values).
+Use ``-dict=DICTIONARY_FILE``. For some input languages using a dictionary
+may significantly improve the search speed.
+The dictionary syntax is similar to that used by AFL_ for its ``-x`` option::
+
+ # Lines starting with '#' and empty lines are ignored.
+
+ # Adds "blah" (w/o quotes) to the dictionary.
+ kw1="blah"
+ # Use \\ for backslash and \" for quotes.
+ kw2="\"ac\\dc\""
+ # Use \xAB for hex values
+ kw3="\xF7\xF8"
+ # the name of the keyword followed by '=' may be omitted:
+ "foo\x0Abar"
+
+Data-flow-guided fuzzing
+------------------------
+
+*EXPERIMENTAL*.
+With an additional compiler flag ``-fsanitize-coverage=trace-cmp`` (see SanitizerCoverageTraceDataFlow_)
+and extra run-time flag ``-use_traces=1`` the fuzzer will try to apply *data-flow-guided fuzzing*.
+That is, the fuzzer will record the inputs to comparison instructions, switch statements,
+and several libc functions (``memcmp``, ``strcmp``, ``strncmp``, etc).
+It will later use those recorded inputs during mutations.
+
+This mode can be combined with DataFlowSanitizer_ to achieve better sensitivity.
+
+AFL compatibility
+-----------------
+LibFuzzer can be used in parallel with AFL_ on the same test corpus.
+Both fuzzers expect the test corpus to reside in a directory, one file per input.
+You can run both fuzzers on the same corpus in parallel::
+
+ ./afl-fuzz -i testcase_dir -o findings_dir /path/to/program -r @@
+ ./llvm-fuzz testcase_dir findings_dir # Will write new tests to testcase_dir