GCC
Compiling with GCC[edit]
Example[edit]
Assuming a single source file named main.cpp, the command to compile and link an non-optimized executable is as follows (Compiling without optimization is useful for initial development and debugging, although -Og is officially recommended for newer GCC versions).
g++ -o app -Wall main.cpp -O0
To produce an optimized executable for use in production, use one of the -O options (see: -O1, -O2, -O3, -Os, -Ofast):
g++ -o app -Wall -O2 main.cpp
If the -O option is omitted, -O0, which means no optimizations, is used as default (specifying -O without a number resolves to -O1).
Alternatively, use optimization flags from the O groups (or more experimental optimizations) directly. The following example builds with -O2 optimization, plus one flag from the -O3 optimization level:
g++ -o app -Wall -O2 -ftree-partial-pre main.cpp
To produce a platform-specific optimized executable (for use in production on the machine with the same architecture), use:
g++ -o app -Wall -O2 -march=native main.cpp
Either of the above will produce a binary file that can be run with .\app.exe on Windows and ./app on Linux, Mac OS, etc.
The -o flag can also be skipped. In this case, GCC will create default output executable a.exe on Windows and a.out on Unix-like systems. To compile a file without linking it, use the -c option:
g++ -o file.o -Wall -c file.cpp
This produces an object file named file.o which can later be linked with other files to produce a binary:
g++ -o app file.o otherfile.o
More about optimization options can be found at gcc.gnu.org. Of particular note are -Og (optimization with an emphasis on debugging experience -- recommended for the standard edit-compile-debug cycle) and -Ofast (all optimizations, including ones disregarding strict standards compliance).
The -Wall flag enables warnings for many common errors and should always be used. To improve code quality it is often encouraged also to use -Wextra and other warning flags which are not automatically enabled by -Wall and -Wextra.
If the code expects a specific C++ standard, specify which standard to use by including the -std= flag. Supported values correspond to the year of finalization for each version of the ISO C++ standard. As of GCC 6.1.0, valid values for the std= flag are c++98/c++03, c++11, c++14, and c++17/c++1z. Values separated by a forward slash are equivalent.
g++ -std=c++11 <file>
GCC includes some compiler-specific extensions that are disabled when they conflict with a standard specified by the -std= flag. To compile with all extensions enabled, the value gnu++XX may be used, where XX is any of the years used by the c++ values listed above.
The default standard will be used if none is specified. For versions of GCC prior to 6.1.0, the default is -std=gnu++03; in GCC 6.1.0 and greater, the default is -std=gnu++14.
Note that due to bugs in GCC, the -pthread flag must be present at compilation and linking for GCC to support the C++ standard threading functionality introduced with C++11, such as std::thread and std::wait_for. Omitting it when using threading functions may result in no warnings but invalid results on some platforms. Linking with libraries:
Use the -l option to pass the library name:
g++ main.cpp -lpcre2-8
- pcre2-8 is the PCRE2 library for 8bit code units (UTF-8)
If the library is not in the standard library path, add the path with -L option:
g++ main.cpp -L/my/custom/path/ -lmylib
Multiple libraries can be linked together:
g++ main.cpp -lmylib1 -lmylib2 -lmylib3
If one library depends on another, put the dependent library before the independent library:
g++ main.cpp -lchild-lib -lbase-lib
Or let the linker determine the ordering itself via --start-group and --end-group (note: this has significant performance cost):
g++ main.cpp -Wl,--start-group -lbase-lib -lchild-lib -Wl,--end-group
Compiling C and C++ Programs[edit]
gcc is the GNU C Compiler and g++ is the GNU C++ compiler. Below are several examples that show how to use g++ to compile C++ programs. Example 1: Compiling a simple program Consider the following example: Let "hello.C" be a file that contains the following C++ code.
#include "iostream.h"
int main()
{
cout << "Hello\n";
}
The standard way to compile this program is with the command
g++ hello.C -o hello
This command compiles hello.C into an executable program named "hello" that you run by typing './hello' at the command line. It does nothing more than print the word "hello" on the screen. Alternatively, the above program could be compiled using the following two commands.
g++ -c hello.C g++ hello.o -o hello
The end result is the same, but this two-step method first compiles hello.C into a machine code file named "hello.o" and then links hello.o with some system libraries to produce the final program "hello". In fact the first method also does this two-stage process of compiling and linking, but the stages are done transparently, and the intermediate file "hello.o" is deleted in the process.
Frequently used compilation options
C and C++ compilers allow for many options for how to compile a program, and the examples below demonstrate how to use many of the more commonly used options. In each example, "myprog.C" contains C++ source code for the executable "myprog". In most cases options can be combined, although it is generally not useful to use "debugging" and "optimization" options together.
Compile myprog.C so that myprog contains symbolic information that enables it to be debugged with the gdb debugger.
g++ -g myprog.C -o myprog
Have the compiler generate many warnings about syntactically correct but questionable looking code. It is good practice to always use this option with gcc and g++.
g++ -Wall myprog.C -o myprog
Generate symbolic information for gdb and many warning messages.
g++ -g -Wall myprog.C -o myprog
Generate optimized code on a Linux machine.
g++ -O myprog.C -o myprog
Compile myprog.C when it contains Xlib graphics routines.
g++ myprog.C -o myprog -lX11
If "myprog.c" is a C program, then the above commands will all work by replacing g++ with gcc and "myprog.C" with "myprog.c". Below are a few examples that apply only to C programs.
Compile a C program that uses math functions such as "sqrt".
gcc myprog.C -o myprog -lm
Compile a C program with the "electric fence" library. This library, available on all the Linux machines, causes many incorrectly written programs to crash as soon as an error occurs. It is useful for debugging as the error location can be quickly determined using gdb. However, it should only be used for debugging as the executable myprog will be much slower and use much more memory than usual.
gcc -g myprog.C -o myprog -lefence
Example 2: Compiling a program with multiple source files
If the source code is in several files, say "file1.C" and "file2.C", then they can be compiled into an executable program named "myprog" using the following command:
g++ file1.C file2.C -o myprog
The same result can be achieved using the following three commands:
g++ -c file1.C g++ -c file2.C g++ file1.o file2.o -o myprog
The advantage of the second method is that it compiles each of the source files separately. If, for instance, the above commands were used to create "myprog", and "file1.C" was subsequently modified, then the following commands would correctly update "myprog".
g++ -c file1.C g++ file1.o file2.o -o myprog
Note that file2.C does not need to be recompiled, so the time required to rebuild myprog is shorter than if the first method for compiling myprog were used. When there are numerous source file, and a change is only made to one of them, the time savings can be significant. This process, though somewhat complicated, is generally handled automatically by a makefile.