GCC supports a number of command-line options that control adding run-time instrumentation to the code it normally generates. For example, one purpose of instrumentation is collect profiling statistics for use in finding program hot spots, code coverage analysis, or profile-guided optimizations. Another class of program instrumentation is adding run-time checking to detect programming errors like invalid pointer dereferences or out-of-bounds array accesses, as well as deliberately hostile attacks such as stack smashing or C++ vtable hijacking. There is also a general hook which can be used to implement other forms of tracing or function-level instrumentation for debug or program analysis purposes.
-p ¶Generate extra code to write profile information suitable for the
analysis program prof. You must use this option when compiling
the source files you want data about, and you must also use it when
linking.
-pg ¶Generate extra code to write profile information suitable for the
analysis program gprof. You must use this option when compiling
the source files you want data about, and you must also use it when
linking.
-fprofile-arcs ¶Add code so that program flow arcs are instrumented. During execution the program records how many times each branch and call is executed and how many times it is taken or returns. When the compiled program exits it saves this data to a file called auxname.gcda for each source file. The data may be used for profile-directed optimizations (-fbranch-probabilities), or for test coverage analysis (-ftest-coverage). Each object file’s auxname is generated from the name of the output file, if explicitly specified and it is not the final executable, otherwise it is the basename of the source file. In both cases any suffix is removed (e.g. foo.gcda for input file dir/foo.c, or dir/foo.gcda for output file specified as -o dir/foo.o). See Data File Relocation to Support Cross-Profiling.
--coverage ¶This option is used to compile and link code instrumented for coverage analysis. The option is a synonym for -fprofile-arcs -ftest-coverage (when compiling) and -lgcov (when linking). See the documentation for those options for more details.
fork calls are detected and correctly handled (double counting
will not happen).
gcov to produce human readable
information from the .gcno and .gcda files. Refer to the
gcov documentation for further information.
With -fprofile-arcs, for each function of your program GCC creates a program flow graph, then finds a spanning tree for the graph. Only arcs that are not on the spanning tree have to be instrumented: the compiler adds code to count the number of times that these arcs are executed. When an arc is the only exit or only entrance to a block, the instrumentation code can be added to the block; otherwise, a new basic block must be created to hold the instrumentation code.
-ftest-coverage ¶Produce a notes file that the gcov code-coverage utility
(see gcov—a Test Coverage Program) can use to
show program coverage. Each source file’s note file is called
auxname.gcno. Refer to the -fprofile-arcs option
above for a description of auxname and instructions on how to
generate test coverage data. Coverage data matches the source files
more closely if you do not optimize.
-fprofile-dir=path ¶Set the directory to search for the profile data files in to path. This option affects only the profile data generated by -fprofile-generate, -ftest-coverage, -fprofile-arcs and used by -fprofile-use and -fbranch-probabilities and its related options. Both absolute and relative paths can be used. By default, GCC uses the current directory as path, thus the profile data file appears in the same directory as the object file.
-fprofile-generate ¶-fprofile-generate=pathEnable options usually used for instrumenting application to produce profile useful for later recompilation with profile feedback based optimization. You must use -fprofile-generate both when compiling and when linking your program.
The following options are enabled: -fprofile-arcs, -fprofile-values, -fvpt.
If path is specified, GCC looks at the path to find the profile feedback data files. See -fprofile-dir.
To optimize the program based on the collected profile information, use -fprofile-use. See Options That Control Optimization, for more information.
-fsanitize=address ¶Enable AddressSanitizer, a fast memory error detector.
Memory access instructions are instrumented to detect
out-of-bounds and use-after-free bugs.
See https://github.com/google/sanitizers/wiki/AddressSanitizer for
more details. The run-time behavior can be influenced using the
ASAN_OPTIONS environment variable. When set to help=1,
the available options are shown at startup of the instrumented program. See
https://github.com/google/sanitizers/wiki/AddressSanitizerFlags#run-time-flags
for a list of supported options.
-fsanitize=kernel-address ¶Enable AddressSanitizer for Linux kernel. See https://github.com/google/kasan/wiki for more details.
-fsanitize=thread ¶Enable ThreadSanitizer, a fast data race detector.
Memory access instructions are instrumented to detect
data race bugs. See https://github.com/google/sanitizers/wiki#threadsanitizer for more
details. The run-time behavior can be influenced using the TSAN_OPTIONS
environment variable; see
https://github.com/google/sanitizers/wiki/ThreadSanitizerFlags for a list of
supported options.
-fsanitize=leak ¶Enable LeakSanitizer, a memory leak detector.
This option only matters for linking of executables and if neither
-fsanitize=address nor -fsanitize=thread is used. In that
case the executable is linked against a library that overrides malloc
and other allocator functions. See
https://github.com/google/sanitizers/wiki/AddressSanitizerLeakSanitizer for more
details. The run-time behavior can be influenced using the
LSAN_OPTIONS environment variable.
-fsanitize=undefined ¶Enable UndefinedBehaviorSanitizer, a fast undefined behavior detector. Various computations are instrumented to detect undefined behavior at runtime. Current suboptions are:
-fsanitize=shift ¶This option enables checking that the result of a shift operation is not undefined. Note that what exactly is considered undefined differs slightly between C and C++, as well as between ISO C90 and C99, etc.
-fsanitize=integer-divide-by-zero ¶Detect integer division by zero as well as INT_MIN / -1 division.
-fsanitize=unreachable ¶With this option, the compiler turns the __builtin_unreachable
call into a diagnostics message call instead. When reaching the
__builtin_unreachable call, the behavior is undefined.
-fsanitize=vla-bound ¶This option instructs the compiler to check that the size of a variable length array is positive.
-fsanitize=null ¶This option enables pointer checking. Particularly, the application built with this option turned on will issue an error message when it tries to dereference a NULL pointer, or if a reference (possibly an rvalue reference) is bound to a NULL pointer, or if a method is invoked on an object pointed by a NULL pointer.
-fsanitize=return ¶This option enables return statement checking. Programs built with this option turned on will issue an error message when the end of a non-void function is reached without actually returning a value. This option works in C++ only.
-fsanitize=signed-integer-overflow ¶This option enables signed integer overflow checking. We check that
the result of +, *, and both unary and binary -
does not overflow in the signed arithmetics. Note, integer promotion
rules must be taken into account. That is, the following is not an
overflow:
signed char a = SCHAR_MAX; a++;
-fsanitize=bounds ¶This option enables instrumentation of array bounds. Various out of bounds accesses are detected. Flexible array members, flexible array member-like arrays, and initializers of variables with static storage are not instrumented.
-fsanitize=bounds-strict ¶This option enables strict instrumentation of array bounds. Most out of bounds accesses are detected, including flexible array members and flexible array member-like arrays. Initializers of variables with static storage are not instrumented.
-fsanitize=alignment ¶This option enables checking of alignment of pointers when they are dereferenced, or when a reference is bound to insufficiently aligned target, or when a method or constructor is invoked on insufficiently aligned object.
-fsanitize=object-size ¶This option enables instrumentation of memory references using the
__builtin_object_size function. Various out of bounds pointer
accesses are detected.
-fsanitize=float-divide-by-zero ¶Detect floating-point division by zero. Unlike other similar options, -fsanitize=float-divide-by-zero is not enabled by -fsanitize=undefined, since floating-point division by zero can be a legitimate way of obtaining infinities and NaNs.
-fsanitize=float-cast-overflow ¶This option enables floating-point type to integer conversion checking.
We check that the result of the conversion does not overflow.
Unlike other similar options, -fsanitize=float-cast-overflow is
not enabled by -fsanitize=undefined.
This option does not work well with FE_INVALID exceptions enabled.
-fsanitize=nonnull-attribute ¶This option enables instrumentation of calls, checking whether null values
are not passed to arguments marked as requiring a non-null value by the
nonnull function attribute.
-fsanitize=returns-nonnull-attribute ¶This option enables instrumentation of return statements in functions
marked with returns_nonnull function attribute, to detect returning
of null values from such functions.
-fsanitize=bool ¶This option enables instrumentation of loads from bool. If a value other than 0/1 is loaded, a run-time error is issued.
-fsanitize=enum ¶This option enables instrumentation of loads from an enum type. If a value outside the range of values for the enum type is loaded, a run-time error is issued.
-fsanitize=vptr ¶This option enables instrumentation of C++ member function calls, member accesses and some conversions between pointers to base and derived classes, to verify the referenced object has the correct dynamic type.
While -ftrapv causes traps for signed overflows to be emitted, -fsanitize=undefined gives a diagnostic message. This currently works only for the C family of languages.
-fno-sanitize=all ¶This option disables all previously enabled sanitizers. -fsanitize=all is not allowed, as some sanitizers cannot be used together.
-fasan-shadow-offset=number ¶This option forces GCC to use custom shadow offset in AddressSanitizer checks. It is useful for experimenting with different shadow memory layouts in Kernel AddressSanitizer.
-fsanitize-sections=s1,s2,... ¶Sanitize global variables in selected user-defined sections. si may contain wildcards.
-fsanitize-recover[=opts] ¶-fsanitize-recover= controls error recovery mode for sanitizers mentioned in comma-separated list of opts. Enabling this option for a sanitizer component causes it to attempt to continue running the program as if no error happened. This means multiple runtime errors can be reported in a single program run, and the exit code of the program may indicate success even when errors have been reported. The -fno-sanitize-recover= option can be used to alter this behavior: only the first detected error is reported and program then exits with a non-zero exit code.
Currently this feature only works for -fsanitize=undefined (and its suboptions except for -fsanitize=unreachable and -fsanitize=return), -fsanitize=float-cast-overflow, -fsanitize=float-divide-by-zero, -fsanitize=kernel-address and -fsanitize=address. For these sanitizers error recovery is turned on by default, except -fsanitize=address, for which this feature is experimental. -fsanitize-recover=all and -fno-sanitize-recover=all is also accepted, the former enables recovery for all sanitizers that support it, the latter disables recovery for all sanitizers that support it.
Syntax without explicit opts parameter is deprecated. It is equivalent to
-fsanitize-recover=undefined,float-cast-overflow,float-divide-by-zero
Similarly -fno-sanitize-recover is equivalent to
-fno-sanitize-recover=undefined,float-cast-overflow,float-divide-by-zero
-fsanitize-undefined-trap-on-error ¶The -fsanitize-undefined-trap-on-error option instructs the compiler to
report undefined behavior using __builtin_trap rather than
a libubsan library routine. The advantage of this is that the
libubsan library is not needed and is not linked in, so this
is usable even in freestanding environments.
-fsanitize-coverage=trace-pc ¶Enable coverage-guided fuzzing code instrumentation.
Inserts a call to __sanitizer_cov_trace_pc into every basic block.
-fbounds-check ¶For front ends that support it, generate additional code to check that indices used to access arrays are within the declared range. This is currently only supported by the Java and Fortran front ends, where this option defaults to true and false respectively.
-fcheck-pointer-bounds ¶Enable Pointer Bounds Checker instrumentation. Each memory reference is instrumented with checks of the pointer used for memory access against bounds associated with that pointer.
Currently there is only an implementation for Intel MPX available, thus x86 GNU/Linux target and -mmpx are required to enable this feature. MPX-based instrumentation requires a runtime library to enable MPX in hardware and handle bounds violation signals. By default when -fcheck-pointer-bounds and -mmpx options are used to link a program, the GCC driver links against the libmpx and libmpxwrappers libraries. Bounds checking on calls to dynamic libraries requires a linker with -z bndplt support; if GCC was configured with a linker without support for this option (including the Gold linker and older versions of ld), a warning is given if you link with -mmpx without also specifying -static, since the overall effectiveness of the bounds checking protection is reduced. See also -static-libmpxwrappers.
MPX-based instrumentation
may be used for debugging and also may be included in production code
to increase program security. Depending on usage, you may
have different requirements for the runtime library. The current version
of the MPX runtime library is more oriented for use as a debugging
tool. MPX runtime library usage implies -lpthread. See
also -static-libmpx. The runtime library behavior can be
influenced using various CHKP_RT_* environment variables. See
https://gcc.gnu.org/wiki/Intel%20MPX%20support%20in%20the%20GCC%20compiler
for more details.
Generated instrumentation may be controlled by various
-fchkp-* options and by the bnd_variable_size
structure field attribute (see Specifying Attributes of Types) and
bnd_legacy, and bnd_instrument function attributes
(see Declaring Attributes of Functions). GCC also provides a number of built-in
functions for controlling the Pointer Bounds Checker. See Pointer Bounds Checker Built-in Functions, for more information.
-fchkp-check-incomplete-type ¶Generate pointer bounds checks for variables with incomplete type. Enabled by default.
-fchkp-narrow-bounds ¶Controls bounds used by Pointer Bounds Checker for pointers to object fields. If narrowing is enabled then field bounds are used. Otherwise object bounds are used. See also -fchkp-narrow-to-innermost-array and -fchkp-first-field-has-own-bounds. Enabled by default.
-fchkp-first-field-has-own-bounds ¶Forces Pointer Bounds Checker to use narrowed bounds for the address of the first field in the structure. By default a pointer to the first field has the same bounds as a pointer to the whole structure.
-fchkp-narrow-to-innermost-array ¶Forces Pointer Bounds Checker to use bounds of the innermost arrays in case of nested static array access. By default this option is disabled and bounds of the outermost array are used.
-fchkp-optimize ¶Enables Pointer Bounds Checker optimizations. Enabled by default at optimization levels -O, -O2, -O3.
-fchkp-use-fast-string-functions ¶Enables use of *_nobnd versions of string functions (not copying bounds)
by Pointer Bounds Checker. Disabled by default.
-fchkp-use-nochk-string-functions ¶Enables use of *_nochk versions of string functions (not checking bounds)
by Pointer Bounds Checker. Disabled by default.
-fchkp-use-static-bounds ¶Allow Pointer Bounds Checker to generate static bounds holding bounds of static variables. Enabled by default.
-fchkp-use-static-const-bounds ¶Use statically-initialized bounds for constant bounds instead of generating them each time they are required. By default enabled when -fchkp-use-static-bounds is enabled.
-fchkp-treat-zero-dynamic-size-as-infinite ¶With this option, objects with incomplete type whose dynamically-obtained size is zero are treated as having infinite size instead by Pointer Bounds Checker. This option may be helpful if a program is linked with a library missing size information for some symbols. Disabled by default.
-fchkp-check-read ¶Instructs Pointer Bounds Checker to generate checks for all read accesses to memory. Enabled by default.
-fchkp-check-write ¶Instructs Pointer Bounds Checker to generate checks for all write accesses to memory. Enabled by default.
-fchkp-store-bounds ¶Instructs Pointer Bounds Checker to generate bounds stores for pointer writes. Enabled by default.
-fchkp-instrument-calls ¶Instructs Pointer Bounds Checker to pass pointer bounds to calls. Enabled by default.
-fchkp-instrument-marked-only ¶Instructs Pointer Bounds Checker to instrument only functions
marked with the bnd_instrument attribute
(see Declaring Attributes of Functions). Disabled by default.
-fchkp-use-wrappers ¶Allows Pointer Bounds Checker to replace calls to built-in functions with calls to wrapper functions. When -fchkp-use-wrappers is used to link a program, the GCC driver automatically links against libmpxwrappers. See also -static-libmpxwrappers. Enabled by default.
-fstack-protector ¶Emit extra code to check for buffer overflows, such as stack smashing
attacks. This is done by adding a guard variable to functions with
vulnerable objects. This includes functions that call alloca, and
functions with buffers larger than 8 bytes. The guards are initialized
when a function is entered and then checked when the function exits.
If a guard check fails, an error message is printed and the program exits.
-fstack-protector-all ¶Like -fstack-protector except that all functions are protected.
-fstack-protector-strong ¶Like -fstack-protector but includes additional functions to be protected — those that have local array definitions, or have references to local frame addresses.
-fstack-protector-explicit ¶Like -fstack-protector but only protects those functions which
have the stack_protect attribute.
-fstack-check ¶Generate code to verify that you do not go beyond the boundary of the stack. You should specify this flag if you are running in an environment with multiple threads, but you only rarely need to specify it in a single-threaded environment since stack overflow is automatically detected on nearly all systems if there is only one stack.
Note that this switch does not actually cause checking to be done; the operating system or the language runtime must do that. The switch causes generation of code to ensure that they see the stack being extended.
You can additionally specify a string parameter: ‘no’ means no checking, ‘generic’ means force the use of old-style checking, ‘specific’ means use the best checking method and is equivalent to bare -fstack-check.
Old-style checking is a generic mechanism that requires no specific target support in the compiler but comes with the following drawbacks:
Note that old-style stack checking is also the fallback method for ‘specific’ if no target support has been added in the compiler.
-fstack-limit-register=reg ¶-fstack-limit-symbol=sym-fno-stack-limitGenerate code to ensure that the stack does not grow beyond a certain value, either the value of a register or the address of a symbol. If a larger stack is required, a signal is raised at run time. For most targets, the signal is raised before the stack overruns the boundary, so it is possible to catch the signal without taking special precautions.
For instance, if the stack starts at absolute address ‘0x80000000’ and grows downwards, you can use the flags -fstack-limit-symbol=__stack_limit and -Wl,--defsym,__stack_limit=0x7ffe0000 to enforce a stack limit of 128KB. Note that this may only work with the GNU linker.
You can locally override stack limit checking by using the
no_stack_limit function attribute (see Declaring Attributes of Functions).
-fsplit-stack ¶Generate code to automatically split the stack before it overflows. The resulting program has a discontiguous stack which can only overflow if the program is unable to allocate any more memory. This is most useful when running threaded programs, as it is no longer necessary to calculate a good stack size to use for each thread. This is currently only implemented for the x86 targets running GNU/Linux.
When code compiled with -fsplit-stack calls code compiled without -fsplit-stack, there may not be much stack space available for the latter code to run. If compiling all code, including library code, with -fsplit-stack is not an option, then the linker can fix up these calls so that the code compiled without -fsplit-stack always has a large stack. Support for this is implemented in the gold linker in GNU binutils release 2.21 and later.
-fvtable-verify=[std|preinit|none] ¶This option is only available when compiling C++ code. It turns on (or off, if using -fvtable-verify=none) the security feature that verifies at run time, for every virtual call, that the vtable pointer through which the call is made is valid for the type of the object, and has not been corrupted or overwritten. If an invalid vtable pointer is detected at run time, an error is reported and execution of the program is immediately halted.
This option causes run-time data structures to be built at program startup,
which are used for verifying the vtable pointers.
The options ‘std’ and ‘preinit’
control the timing of when these data structures are built. In both cases the
data structures are built before execution reaches main. Using
-fvtable-verify=std causes the data structures to be built after
shared libraries have been loaded and initialized.
-fvtable-verify=preinit causes them to be built before shared
libraries have been loaded and initialized.
If this option appears multiple times in the command line with different values specified, ‘none’ takes highest priority over both ‘std’ and ‘preinit’; ‘preinit’ takes priority over ‘std’.
-fvtv-debug ¶When used in conjunction with -fvtable-verify=std or
-fvtable-verify=preinit, causes debug versions of the
runtime functions for the vtable verification feature to be called.
This flag also causes the compiler to log information about which
vtable pointers it finds for each class.
This information is written to a file named vtv_set_ptr_data.log
in the directory named by the environment variable VTV_LOGS_DIR
if that is defined or the current working directory otherwise.
Note: This feature appends data to the log file. If you want a fresh log file, be sure to delete any existing one.
-fvtv-counts ¶This is a debugging flag. When used in conjunction with
-fvtable-verify=std or -fvtable-verify=preinit, this
causes the compiler to keep track of the total number of virtual calls
it encounters and the number of verifications it inserts. It also
counts the number of calls to certain run-time library functions
that it inserts and logs this information for each compilation unit.
The compiler writes this information to a file named
vtv_count_data.log in the directory named by the environment
variable VTV_LOGS_DIR if that is defined or the current working
directory otherwise. It also counts the size of the vtable pointer sets
for each class, and writes this information to vtv_class_set_sizes.log
in the same directory.
Note: This feature appends data to the log files. To get fresh log files, be sure to delete any existing ones.
-finstrument-functions ¶Generate instrumentation calls for entry and exit to functions. Just
after function entry and just before function exit, the following
profiling functions are called with the address of the current
function and its call site. (On some platforms,
__builtin_return_address does not work beyond the current
function, so the call site information may not be available to the
profiling functions otherwise.)
void __cyg_profile_func_enter (void *this_fn,
void *call_site);
void __cyg_profile_func_exit (void *this_fn,
void *call_site);
The first argument is the address of the start of the current function, which may be looked up exactly in the symbol table.
This instrumentation is also done for functions expanded inline in other
functions. The profiling calls indicate where, conceptually, the
inline function is entered and exited. This means that addressable
versions of such functions must be available. If all your uses of a
function are expanded inline, this may mean an additional expansion of
code size. If you use extern inline in your C code, an
addressable version of such functions must be provided. (This is
normally the case anyway, but if you get lucky and the optimizer always
expands the functions inline, you might have gotten away without
providing static copies.)
A function may be given the attribute no_instrument_function, in
which case this instrumentation is not done. This can be used, for
example, for the profiling functions listed above, high-priority
interrupt routines, and any functions from which the profiling functions
cannot safely be called (perhaps signal handlers, if the profiling
routines generate output or allocate memory).
-finstrument-functions-exclude-file-list=file,file,… ¶Set the list of functions that are excluded from instrumentation (see the description of -finstrument-functions). If the file that contains a function definition matches with one of file, then that function is not instrumented. The match is done on substrings: if the file parameter is a substring of the file name, it is considered to be a match.
For example:
-finstrument-functions-exclude-file-list=/bits/stl,include/sys
excludes any inline function defined in files whose pathnames contain /bits/stl or include/sys.
If, for some reason, you want to include letter ‘,’ in one of sym, write ‘\,’. For example, -finstrument-functions-exclude-file-list='\,\,tmp' (note the single quote surrounding the option).
-finstrument-functions-exclude-function-list=sym,sym,… ¶This is similar to -finstrument-functions-exclude-file-list,
but this option sets the list of function names to be excluded from
instrumentation. The function name to be matched is its user-visible
name, such as vector<int> blah(const vector<int> &), not the
internal mangled name (e.g., _Z4blahRSt6vectorIiSaIiEE). The
match is done on substrings: if the sym parameter is a substring
of the function name, it is considered to be a match. For C99 and C++
extended identifiers, the function name must be given in UTF-8, not
using universal character names.