# GCC Basics

**GCC** is the GNU Compiler Collection—the default C, C++, and Fortran compiler on most Linux systems. Install it via your package manager along with build tools.

```bash
sudo apt install build-essential              # Debian/Ubuntu
sudo dnf groupinstall "Development Tools"     # Fedora/RHEL
```

`build-essential` includes gcc, g++, make, and C library headers. Check the installation:

```bash
gcc --version
gcc -dumpmachine          # target architecture
```

**Compilation pipeline:** GCC runs four stages internally but hides them by default:

```bash
gcc -E file.c -o file.i     # preprocess (expand macros, includes)
gcc -S file.c -o file.s     # compile to assembly
gcc -c file.c -o file.o     # assemble to object code
gcc file.o -o program       # link object files and libraries
```

Or combine them into one command: `gcc -o program file.c`. GCC automatically runs all stages and produces the executable.

**Multi-file projects:**

```bash
gcc -c a.c -o a.o           # compile each source separately
gcc -c b.c -o b.o
gcc a.o b.o -o program      # link objects together
```

This is the basis for makefiles—compile only changed files, relink to save time.

**Common flags:**

```bash
gcc -o output input.c       # output filename
gcc -g input.c              # include debug symbols
gcc -O2 input.c             # optimization level 2
gcc -Wall input.c           # enable common warnings
gcc -I/path/to/headers input.c   # add include path
gcc -L/path/to/libs -lmylib input.c  # link against library
```

`-o` names the output; without it, the default is `a.out`. `-g` includes debugging information (needed for [[gdb]]). `-O` sets optimization level. `-I` adds header search paths; `-L` and `-l` link against libraries.

**Variables used by make:**

```bash
CFLAGS=-O3 -Wall -g        # compiler flags
LDFLAGS=-L/path/to/libs    # linker flags
gcc $CFLAGS -o app app.c
gcc -c $CFLAGS app.c
gcc app.o $LDFLAGS -o app
```

These can be set on the command line or in a Makefile, making build configuration centralized and reproducible.