gcc Command Cheat Sheet

gcc (GNU Compiler Collection) is the standard compiler for C and C++ on Linux. It transforms source code into executable binaries through preprocessing, compilation, assembly, and linking.

Synopsis

gcc [options] [source_files] [-o output_file]

Basic Compilation

Compile a Simple Program

gcc main.c

Produces a.out (default executable name).

Specify Output Name (-o)

gcc main.c -o myapp

Run with ./myapp.

Compile Multiple Files

gcc main.c utils.c -o myapp

Compilation Stages

You can stop the process at different stages.

Flag	Stage	Output	Description
`-E`	Preprocessing	stdout (or `.i`)	Expands macros, includes headers
`-S`	Compilation	`.s` (Assembly)	Generates assembly code
`-c`	Assembly	`.o` (Object)	Generates machine code (no linking)
(none)	Linking	Binary	Links objects into executable

Create Object File

Useful for Makefiles to compile only changed files.

gcc -c main.c -o main.o

Debugging and Warnings

Enable All Warnings (-Wall)

Always use this during development!

gcc -Wall main.c -o myapp

Enable Extra Warnings (-Wextra)

Even stricter checks.

gcc -Wall -Wextra main.c -o myapp

Treat Warnings as Errors (-Werror)

Forces you to fix warnings.

gcc -Werror main.c -o myapp

Include Debug Symbols (-g)

Required for using gdb or valgrind.

gcc -g main.c -o myapp

Optimization

Levels of optimization affect compilation time and execution speed.

Flag	Description	Trade-off
`-O0`	No optimization (default)	Fast compile, slow run, debuggable
`-O1`	Basic optimization	Balanced
`-O2`	Recommended for deploy	Good speed, safe
`-O3`	Aggressive optimization	Best speed, larger binary, potential bugs
`-Os`	Optimize for size	Smallest binary
`-Ofast`	Disregard standards	Fastest, but may break math accuracy

gcc -O2 main.c -o myapp

Linking Libraries

Link a Standard Library (-l)

To use functions like pow() or sin() from <math.h>, you often need to link libm.

gcc main.c -o myapp -lm

Note: The lib prefix and .so/.a extension are omitted. libm.so becomes -lm.

Specify Library Path (-L)

If the library is in a custom directory (e.g., /opt/lib).

gcc main.c -o myapp -L/opt/lib -lmylib

Specify Header Path (-I)

If headers are in a custom directory.

gcc main.c -o myapp -I/opt/include

Preprocessor Macros

Define a Macro (-D)

Equivalent to #define DEBUG 1 in the code.

gcc -DDEBUG main.c -o myapp

Undefine a Macro (-U)

gcc -UDEBUG main.c

Advanced Options

Static Linking (-static)

Embeds all libraries into the binary. Result is larger but portable (no dependency on system .so files).

gcc -static main.c -o myapp_static

Shared Library Creation (-shared)

Create a .so file. Needs Position Independent Code (-fPIC).

gcc -shared -fPIC -o libutils.so utils.c

32-bit Compilation (-m32)

Compile for 32-bit architecture on a 64-bit system (requires gcc-multilib).

gcc -m32 main.c -o myapp32

Practical Examples

Check Header Search Paths

Where is gcc looking for <stdio.h>?

gcc -xc -E -v /dev/null

Generate Dependency Rules (-MM)

Outputs Makefile-compatible dependency lines.

gcc -MM main.c
# Output: main.o: main.c utils.h config.h

Notes

g++: For C++ code, use g++ instead of gcc. It automatically links the C++ standard library.

std: Specify language standard with -std.

gcc -std=c99 main.c
g++ -std=c++11 main.cpp