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Invocation

Useful GCC command line options.

Most important flags:

  • -std
  • -o
  • -c
  • -Wall
  • -pedantic and -pedantic-errors
  • -L, -l and -I
  • -O
  • -g

What the gcc utility does

Nothing. Almost.

gcc is a driver: it orchestrates the build process, by calling other programs with the right options, notably:

  • cc1: preprocessor + compiler. Generates .s file. Not in PATH by default. The external cpp may be called if -save-temps is used.
  • as: assembler . Generates .o files.
  • collect2: linker. Generates executable files. Not in PATH, but almost identical to ld.

Options you pass to GCC determine how it will call the other programs.

Therefore, to understand GCC you should first understand the other programs separately, and then just see exactly how exactly GCC is calling them.

The exact way in which GCC calls the other programs is determined by spec files.

GCC does not contain bin-utils: they are maintained in separate git repositories. Binutils has not knowledge about programming languages, only assembly, and can be used separately from GCC. Binutils is a dependency of GCC.

Recommended compilation flags

Good discussion: http://stackoverflow.com/questions/154630/recommended-gcc-warning-options-for-c

Always use this for production output code if possible:

gcc -std=c99 -pedantic-errors -Wall -Wextra -03 -march=native -o a a.c

This will make for portable, efficient code.

For test code, omit the -03, since that will make compilation faster.

o

Output destination.

Default: a.out.

For stdout: -o -.

Input from stdin

x

Specify the language explicitly.

Required when input is from stdin with -.

Compile from stdin:

echo 'int main(){}' | gcc -xc -

For a C header use c-header. Note that this generates a -.gch file:

echo 'int i;' | gcc -xc-header -

W options

Most of them are for warnings, but a few are not.

Wall

Enables many useful warnings:

gcc -Wall

Understanding each warning and being able to write warning free code is a good way to improve language skills.

Enable all warnings

Wextra

Enables even more useful warnings than wall.

`gcc -Wextra`

Not warnings

Wp

Pass options to the preprocessor.

Wa

Pass options to the assembler, cc1 by default TODO check

Example:

gcc -c -fverbose-asm -Wa,-adhln a.c

Wl

Pass options to the linker. Multiple options can be separated by commas.

Xlinker

Pass options to the linker. Multiple options require multiple -Xlinker arguments.

std

Specifies the language to be used.

Disables GCC extensions that conflict with ANSI C standard.

Sample usage:

gcc -std=c90
gcc -std=c99
gcc -std=1x

c11 will be the next version and is still being developed at the time of writing.

To allow GNU extensions:

gcc -std=gnu90

This is necessary on projects that rely on the extensions, such as the Linux kernel.

ansi

Don't use this, use std instead:

gcc -ansi

Implies the most recent -std which gnu considers is stable manner (not necessarily the latest).

Changes with time, currently equals:

gcc -std=c90

It is a bit confusing not to have a fixed version of the standard to comply to, so just use std instead.

pedantic

Give warnings for code that does not comply with c1x standard:

gcc -std=c1x -pedantic

This does not mean FULL compliance, but greatly increases compliance.

There is currently no full compliance check in gcc.

Give errors instead of warnings:

gcc -std=c1x -pedantic-errors

m

Machine options.

m32

m64

-m32 to compile IA-32 output from a 64 bit host. This does two things mainly:

  • use IA-32 instructions
  • link to IA-32 libraries, in particular libc. You may need to install them.

mfpmath

Chose between FPU and SSE for the floating point operations.

gcc -mfpmath=387
gcc -mfpmath=sse

march

Optimizes code to given CPU (arch is for architecture).

May use instructions only available to given CPU.

Optimize for current compiling machine:

gcc -march=native

Strict 80386 instruction set. old, compatible, used on almost all desktops and laptops:

gcc -march=i386

Arm v.7, used on mobiles today:

gcc -march=armv7

mtune

TODO: finer than -march? E.g.: http://www.valvers.com/open-software/raspberry-pi/step01-bare-metal-programming-in-cpt1 proposes:

-march=armv7-a -mtune=cortex-a7

where armv7-a is an ISA, and cortex-a7 an specific processor.

Optimization

List possible optimizations for -O:

gcc -Q -O --help=optimizers

The options:

  • O0: no speed optimization. This is the default
  • O : basic speed optimization. Same as -O1.
  • O2: more than O1
  • O3: more than O2
  • Og: optimize for debugging
  • Os: optimize for size
  • Ofast: optimize for speed more than O3, even if it breaks standards. In 5.1, the only thing it does for C if adding -ffast-math.

Best general code optimization method:

gcc -O3 a.c -o a.out

Always use this for production code.

ffast-math

Drop strict IEEE to further optimize.

http://stackoverflow.com/questions/6430448/why-doesnt-gcc-optimize-aaaaaa-to-aaaaaa

fexpensive-optimizations

Even more than -O3.

debugging

GCC has options that allow you to add debugging information to binary outputs.

This leads to larger output files, but the information can then be used by programs such as GDB or objdump to help debug programs, or for educational purposes.

Add debug information to executable on compilation:

gcc -ggdb3 a.c

Options:

  • g: generate debug info for GDB
  • ggdb: adds more info
  • ggdb3: adds max info. Default if 2 when ggdb is used.

fsanitize

Enable different types of static analysis to detect potential errors like undefined behaviour, memory leaks and illegal accesses.

Implemented by the in-source libsanitize.

M

Don't compile, but generate a list of dependencies for the given source code in a format suitable for a makefile rule, and output it to stdout.

Dependencies are basically the file itself and the required headers.

For example, in a.c:

#include <stdio.h>
#include "a.h"

then:

gcc -M a.c

Output to stdout:

a.c : /usr/include/stdio.h \
    a.h

To make this even more suitable for a makefile, you should suppress the standard system files with -MM:

gcc -MM a.c

giving:

a.c : a.h

You can then use those on a makefile as:

$(shell gcc -MM a.c)
    gcc a.c

S

Generate assembly code:

gcc -S a.c

Generates gas assembly code to a new file called a.S.

syntax-only

Only check if the syntax is correct, don't compile anything.

$ echo 'int i;' | gcc -fsyntax-only -xc -
$ echo 'int i' | gcc -fsyntax-only -xc -
<stdin>:1:1: error: expected ‘=’, ‘,’, ‘;’, ‘asm’ or ‘__attribute__’ at end of input

Note how the above code would not compile because there is no main function:

$ echo 'int i;' | gcc -xc -
/usr/lib/gcc/x86_64-linux-gnu/4.8/../../../x86_64-linux-gnu/crt1.o: In function `_start':
(.text+0x20): undefined reference to `main'
collect2: error: ld returned 1 exit status

As of GCC 4.8 fails on c-header files with "output filename specified twice" http://stackoverflow.com/questions/12513665/how-can-i-use-g-to-get-assembly-code-for-a-c-header-hpp-file The only solution seems to be to set the input type to -xc.

Xassembler

Pass options directly to the as assembler.

Example:

gcc -c -fverbose-asm -Xassembler -adhln a.c

f

Flags.

These are regular options, most of which have value either true or false.

For true:

gcc -ffoo

and false:

gcc -fno-foo

Does not have any other meaning.

Not all -f flags are booleans however:

gcc -frandom-seed=1234

inline

Allow the compile to inline functions. Implied by -O3.

freestanding

Compile in freestanding mode as defined by ANSI C.

verbose-asm

Outputs comments on the generated assembly which say variable names are being operated on each statement:

gcc -fverbose-asm -S a.c

Sample C input:

i = 1;
j = i;

Output without -fverbose-asm:

movl $1, -64(%ebp)
movl -64(%ebp), %eax
movl %eax, -68(%ebp)

With -fverbose-asm:

movl $1, -64(%ebp)   # ,i
movl -64(%ebp), %eax #i, tmp123
movl %eax, -68(%ebp) #tmp123, j

To get original line number / code from C code into asm comments, use either:

gcc -c -fverbose-asm -Wa,-adhln a.c

or:

gcc -ggdb3 -o a.o a.c
objdump -S a.o

profile-generate

Profile guided optimization

https://en.wikipedia.org/wiki/Profile-guided_optimization

http://stackoverflow.com/questions/4365980/how-to-use-profile-guided-optimizations-in-g

Preprocessor options

I

Add directory to the include search path:

gcc -I/some/dir main.c

CPATH

C_INCLUDE_PATH

Add to the search path after -I:

  • CPATH: all languages
  • C_INCLUDE_PATH: only C
  • CPLUS_INCLUDE_PATH: only C++

Trailing or leading : implies current directory:

:/some/dir
/some/dir:
/some/dir::/another/dir

nostdinc

Don't use the standard include path.

Allows you to set up a custom path with:

gcc -nostdinc -I/my/custom/path/

http://stackoverflow.com/questions/2988779/gcc-how-to-ignore-standard-include-paths

l

Linker options

nostdlib

Don't link to the stdlib.

nostartfiles

Don't pass crt1.o, crti.o, and other crt files to the linker, as normally done by default.

If you use this, you must provide your own, or else the linker will not find the _start symbol which is present in those files.

nodefaultlibs

TODO: vs nostdlib?

static

Link statically to all libraries, not just glibc.

gcc -static main.c

The resulting build has no dependencies:

ldd a.out

The effect is that after -lxyz, only libxyz.a is searched in the search path, not libxyz.so.

Without -static TODO: -lxyz seems to use libxyz.a, but that may lead to link errors.

TODO links some libraries statically and others dynamically:

static-libgcc

Link statically to libgcc. This is not libc, but an internal GCC library.

Informational commands

verbose

Get basic GCC version information:

gcc --version

Sample output:

gcc (Ubuntu 4.8.2-19ubuntu1) 4.8.2
Copyright (C) 2013 Free Software Foundation, Inc.
This is free software; see the source for copying conditions.  There is NO
warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.

This is only a general indication of version, since there are many configuration options. To see that, use -v.

v

Get verbose build information.

For shorter version information use --version.

Sample output:

Using built-in specs.
COLLECT_GCC=gcc
COLLECT_LTO_WRAPPER=/usr/local/libexec/gcc/x86_64-unknown-linux-gnu/5.1.0/lto-wrapper
Target: x86_64-unknown-linux-gnu
Configured with: ../gcc/configure --enable-languages=c,c++
Thread model: posix
gcc version 5.1.0 (GCC)

Non-obvious lines:

  • Using built-in specs.. Means that the default specs files are being used, that is, -specs was not passed as an argument.
  • Configured with: how configure was called before building

If you also compile a program, you will see many more lines explaining how exactly gcc called its subprograms:

Using built-in specs.
COLLECT_GCC=gcc
COLLECT_LTO_WRAPPER=/usr/lib/gcc/x86_64-linux-gnu/4.8/lto-wrapper
Target: x86_64-linux-gnu
Configured with: ../src/configure -v --with-pkgversion='Ubuntu 4.8.2-19ubuntu1' --with-bugurl=file:///usr/share/doc/gcc-4.8/README.Bugs --enable-languages=c,c++,java,go,d,fortran,objc,obj-c++ --prefix=/usr --program-suffix=-4.8 --enable-shared --enable-linker-build-id --libexecdir=/usr/lib --without-included-gettext --enable-threads=posix --with-gxx-include-dir=/usr/include/c++/4.8 --libdir=/usr/lib --enable-nls --with-sysroot=/ --enable-clocale=gnu --enable-libstdcxx-debug --enable-libstdcxx-time=yes --enable-gnu-unique-object --disable-libmudflap --enable-plugin --with-system-zlib --disable-browser-plugin --enable-java-awt=gtk --enable-gtk-cairo --with-java-home=/usr/lib/jvm/java-1.5.0-gcj-4.8-amd64/jre --enable-java-home --with-jvm-root-dir=/usr/lib/jvm/java-1.5.0-gcj-4.8-amd64 --with-jvm-jar-dir=/usr/lib/jvm-exports/java-1.5.0-gcj-4.8-amd64 --with-arch-directory=amd64 --with-ecj-jar=/usr/share/java/eclipse-ecj.jar --enable-objc-gc --enable-multiarch --disable-werror --with-arch-32=i686 --with-abi=m64 --with-multilib-list=m32,m64,mx32 --with-tune=generic --enable-checking=release --build=x86_64-linux-gnu --host=x86_64-linux-gnu --target=x86_64-linux-gnu
Thread model: posix
gcc version 4.8.2 (Ubuntu 4.8.2-19ubuntu1)
COLLECT_GCC_OPTIONS='-v' '-mtune=generic' '-march=x86-64'
 /usr/lib/gcc/x86_64-linux-gnu/4.8/cc1 -quiet -v -imultiarch x86_64-linux-gnu main.c -quiet -dumpbase main.c -mtune=generic -march=x86-64 -auxbase main -version -fstack-protector -Wformat -Wformat-security -o /tmp/ccsZiMpp.s
GNU C (Ubuntu 4.8.2-19ubuntu1) version 4.8.2 (x86_64-linux-gnu)
    compiled by GNU C version 4.8.2, GMP version 5.1.3, MPFR version 3.1.2-p3, MPC version 1.0.1
GGC heuristics: --param ggc-min-expand=100 --param ggc-min-heapsize=131072
ignoring nonexistent directory "/usr/local/include/x86_64-linux-gnu"
ignoring nonexistent directory "/usr/lib/gcc/x86_64-linux-gnu/4.8/../../../../x86_64-linux-gnu/include"
#include "..." search starts here:
#include <...> search starts here:
 /usr/lib/gcc/x86_64-linux-gnu/4.8/include
 /usr/local/include
 /usr/lib/gcc/x86_64-linux-gnu/4.8/include-fixed
 /usr/include/x86_64-linux-gnu
 /usr/include
End of search list.
GNU C (Ubuntu 4.8.2-19ubuntu1) version 4.8.2 (x86_64-linux-gnu)
    compiled by GNU C version 4.8.2, GMP version 5.1.3, MPFR version 3.1.2-p3, MPC version 1.0.1
GGC heuristics: --param ggc-min-expand=100 --param ggc-min-heapsize=131072
Compiler executable checksum: dc75e0628c9356affcec059d0c81cc01
COLLECT_GCC_OPTIONS='-v' '-mtune=generic' '-march=x86-64'
 as -v --64 -o /tmp/ccaVhizy.o /tmp/ccsZiMpp.s
GNU assembler version 2.25 (x86_64-unknown-linux-gnu) using BFD version (GNU Binutils) 2.25
COMPILER_PATH=/usr/lib/gcc/x86_64-linux-gnu/4.8/:/usr/lib/gcc/x86_64-linux-gnu/4.8/:/usr/lib/gcc/x86_64-linux-gnu/:/usr/lib/gcc/x86_64-linux-gnu/4.8/:/usr/lib/gcc/x86_64-linux-gnu/
LIBRARY_PATH=/usr/lib/gcc/x86_64-linux-gnu/4.8/:/usr/lib/gcc/x86_64-linux-gnu/4.8/../../../x86_64-linux-gnu/:/usr/lib/gcc/x86_64-linux-gnu/4.8/../../../../lib/:/lib/x86_64-linux-gnu/:/lib/../lib/:/usr/lib/x86_64-linux-gnu/:/usr/lib/../lib/:/usr/lib/gcc/x86_64-linux-gnu/4.8/../../../:/lib/:/usr/lib/
COLLECT_GCC_OPTIONS='-v' '-mtune=generic' '-march=x86-64'
 /usr/lib/gcc/x86_64-linux-gnu/4.8/collect2 --sysroot=/ --build-id --eh-frame-hdr -m elf_x86_64 --hash-style=gnu --as-needed -dynamic-linker /lib64/ld-linux-x86-64.so.2 -z relro /usr/lib/gcc/x86_64-linux-gnu/4.8/../../../x86_64-linux-gnu/crt1.o /usr/lib/gcc/x86_64-linux-gnu/4.8/../../../x86_64-linux-gnu/crti.o /usr/lib/gcc/x86_64-linux-gnu/4.8/crtbegin.o -L/usr/lib/gcc/x86_64-linux-gnu/4.8 -L/usr/lib/gcc/x86_64-linux-gnu/4.8/../../../x86_64-linux-gnu -L/usr/lib/gcc/x86_64-linux-gnu/4.8/../../../../lib -L/lib/x86_64-linux-gnu -L/lib/../lib -L/usr/lib/x86_64-linux-gnu -L/usr/lib/../lib -L/usr/lib/gcc/x86_64-linux-gnu/4.8/../../.. /tmp/ccaVhizy.o -lgcc --as-needed -lgcc_s --no-as-needed -lc -lgcc --as-needed -lgcc_s --no-as-needed /usr/lib/gcc/x86_64-linux-gnu/4.8/crtend.o /usr/lib/gcc/x86_64-linux-gnu/4.8/../../../x86_64-linux-gnu/crtn.o

print-search-dirs

gcc -print-search-dirs | tr ':' '\n'

Sample output:

install
 /usr/local/lib/gcc/x86_64-unknown-linux-gnu/5.1.0/
programs
 =/usr/local/libexec/gcc/x86_64-unknown-linux-gnu/5.1.0/
/usr/local/libexec/gcc/x86_64-unknown-linux-gnu/5.1.0/
/usr/local/libexec/gcc/x86_64-unknown-linux-gnu/
[...]
libraries
 =/usr/local/lib/gcc/x86_64-unknown-linux-gnu/5.1.0/
/usr/local/lib/gcc/x86_64-unknown-linux-gnu/5.1.0/../../../../x86_64-unknown-linux-gnu/lib/x86_64-unknown-linux-gnu/5.1.0/
/usr/local/lib/gcc/x86_64-unknown-linux-gnu/5.1.0/../../../../x86_64-unknown-linux-gnu/lib/x86_64-linux-gnu/
[...]

The search paths are:

  • install TODO
  • programs: where GCC will look for its internal programs like cc1 and collect2. GCC also uses the PATH for external programs, which are called even though they are not there.

print-file-name

Find in the libraries section of -print-search-dirs.

This horribly named option searches in the library path for a given library:

gcc -print-file-name=libc.so

If the library is found it returns the full path:

/usr/lib/x86_64-linux-gnu/libc.so

otherwise, it returns the input string:

libc.so

which indicates that the program only checks if directories exist, not the actual file searched for.

print-prog-name

Same as -print-file-name, but for the internal called programs instead.