Build Automation
Tools
COP-3402

Building software
- Downsides of building all at once
Separate compilation
Build automation
hello project

Building software

How do you build a C program?

gcc -o hello hello.c

What if I have two C files?

gcc -o main main.c hello.c

What if I have 30,000+ C files?

gcc -o vmlinux kernel/locking/mutex-debug.c \
  kernel/locking/rwsem.c kernel/locking/rtmutex.c \
  kernel/locking/qrwlock.c kernel/locking/irqflag-debug.c \
  kernel/locking/test-ww_mutex.c kernel/locking/mutex.c \
  # 30,000+ more

Why not build the whole program at once?

git clone https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git
find kernel/ arch/ block/ crypto/ drivers/ fs/ init/ ipc/ lib/ mm net/ security/ sound/ virt/ -name "*.c" | wc -l

Downsides of building all at once

Long compilation time
Small changes require complete recompile

Intentionally organize your code into separate parts
Learn this in software engineering
Projects won't be that large in this class
I'll give you the organization for the compiler project

Separate compilation

Divide program into separate C files
Compile individually
Link compiled files

This is how C/C++ do separate compilation, but other languages, like Java, further incorporate language abstractions with source code organization, such as Java's use of one-class per file.

Diagram

main.c     -> gcc -> main.o
square.c   -> gcc -> square.o
exponent.c -> gcc -> exponent.o
                        v
                       ld
                        v
                       main

Example

main.c

#include <stdio.h>

int square(int);

int main() {
  printf("%d\n", square(3));
}

square.c

int exponent(int x, int y);

int square(int x) {
  return exponent(x, 2);
}

exponent.c

int exponent(int x, int y) {
  int result = 1;
  for (; y > 0; result *= x, y--);
  return result;
}

How do we separately compile this program?

gcc -c main.c

-c compiles the .c file to an object file .o

Object files are translations of C to machine code.

Without -c, gcc will also link the C runtime and standard libraries, which is why trying to compile without main will throw a linker error.

We will cover how source code becomes a program and and running process later in the semester.

Compiling our example

Compile each .c file (without linking) to a .o object file

gcc -c main.c  # -c flag produces .o files
gcc -c square.c
gcc -c exponent.c

Then link all object files into a single program binary

gcc -o main main.o square.o exponent.o  # pass .o files

Notice gcc takes the .o files instead of

gcc looks at file extensions to distinguish source code from object files.

gcc will run linking for you, but you can also run ld the linker yourself. We will cover this more later in the semester.

Incremental builds

Only rebuild source files that changed.

emacs main.c   # modify one source file
gcc -c main.c  # recompile it

Re-link with existing object files

gcc -o main main.o square.o exponent.o