GCC Study

Compile a C Program

gcc -std=c99 main.c -o main

Compiler steps

# Step 1: My C to Preprocessed C
# We'll dump the output into its own file. The preprocessor extends 
# macros, header files, etc.
gcc -std=c99 -E main.c > main.i

# Step 2: Preprocessed to Assembly Language
# This take output.i and create output.s
gcc -std=c99 -S main.i 

# Step 3: Assembly to Machine Code
# This will take output.s and create output.o
gcc --std=c99 -c main.s

# Step 4: Run the linker
# Inputs output.o and create an executable. If executable name is 
# not specified, it'll be named a.out, otherwise, it will be named 
# the specified output
gcc --std=c99 -o main main.o

Frequently used flags

-g         # Compile with debugging info for GDB
-c         # Compiles only, without linking
-S         # Generates assembly code
-O3        # Optimizes as much as possible
-o         # Specifies the name of the output file
-Wall      # Turns on all warnings
-llibrary  # Adds support for library when linking 
-std=c99   # Use C99 standard
-std=gnu99 # Use C99 standard with GNU extensions

Linking Files Together

# My Directory
.
├── main.c
├── source1.c
├── source2.c
└── sources.h

// main.c
#include <stdio.h>
#include "sources.h"

int main(void) {
  hello_from_source1();
  hello_from_source2();
  return 0;
}

// source1.c
#include <stdio.h>

void hello_from_source1(void)
{
    printf("Hello from source1.c\n");
}

// source2.c
#include <stdio.h>

void hello_from_source2(void)
{
    printf("Hello from source2.c\n");
}

// sources.h
void hello_from_source1(void);
void hello_from_source2(void);

# Compile all files and link them together
gcc --std=c99 -o main main.c source1.c source2.c

# Run the executable
./main
# stdout: Hello from source1.c
# stdout: hello from source2.c

Compiling and Linking Separately

Separate compilation is useful when one object file source code is not changing frequently. Suppose source1.c and source2.c are not changing frequently, but main.c is. We can compile source1.c and source2.c once then instead of compiling them every time we make a change to main.c, we link them in the final step of the compilation process.

# Compile source1.c and source2.c
gcc --std=c99 -c source1.c # > source1.o
gcc --std=c99 -c source2.c # > source2.o

# When we want to compile main.c..
gcc --std=c99 -c main.c # > main.o
gcc --std=c99 -o main main.o source1.o source2.o # > main (executable)

Creating a Library File

# STATIC library from two sources, creates libUtil.a
ar -r libSources.a source1.o source2.o

# DYNAMIC library from two sources, creates libUtil.so
gcc -shared -o libSources.so source1.o source2.o

# DYNAMIC library for mac
gcc -shared -o libSources.dylib source1.o source2.o