Building Programs from Source Code
From CLFS-HINTS
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Basics of Building Programs/Libraries
To build a program you need to convert source code into machine readable code, called an object file, and link it with operating system specific executable code.
From Code to a Bin file
We start with the source code and the header files (if used)
Step 1. The compilers preprocessor takes the source code and puts the header file into the source code, wich is stored in ram.
Step 2. The Compiler (proper) takes the full source code from Step 1, and translates it into assembly language code.
Step 3. The assembler takes the assembly code and translates it to machine instructions of the the target computer where it will run. The machine instructions are stored in object files corresponding to the source files.
Step 4. The link editor makes links between different object files, if multiple object files are made, as well checks any shared/dynamic libs to make sure that the libs will work with the new object file. At this time, all static libs are included at this step.
Step 5. The output of the linker is a file called a.out. This file is the compiled, ready to run version of the source code. We can rename the a.out file to a better name.
The CLFS System
On the CLFS System, we use:
C and C++ compilation -> GCC (GNU C Compiler) Linker, Assembler -> Binutils Main C library -> GLibC
Critical libraries
Glibc (aka GNU Glibc) is an implementation of the C library. It is the core of all programs running on your computer. This library also contains all the C++ library.
Definitions
Linking
Linking is when object code and OS specific executable code is combined to create an executable. There are two types of executable files. Static and dynamic. Static programs contain ALL the library code needed to run the program. A dynamic program is a program which loads it's libraries on the fly.
Linker
Programs that are designed to combine the compiled sourcecode, with executable headers.
Example Program: ld
Assembler
The assembler takes sourcecode written for the processor, called assembly language, and turns it into machine readable code (called an object file).
Example Program: as
Compiler
The compiler is the set of programs which translate sourcecode written in english into machine object code. It does NOT create a binary which is executable. That is the job of the linker. The most common languages being compiled are C, C++ and Java. Other languages include the archaic COBOLT, BASIC and FORTRAN.
Example Program: (g)cc, (g)(c)++
Static Lib
Libraries which contain ONLY the APIs. Also can be called object ARchives. These libs are loaded into the program at linking. After the linking is done the lib file is not needed to run the binary.
Example: /lib/libc.a
Dynamic (Shared) Lib
Libraries which contain an ARchive of object files. The only difference is that it also has special executable code that allows it to be loaded/unloaded by a program when it is needed. These libs are only checked at the linking stage, and require to be present at runtime.
Example: /lib/libc.so (a symlink to /lib/libc.so.6)
Headers
Header files serve as the interface between your program and the libraries supplied by the C compilation system. Because the the functions that perform standard I/O; for example, very often use the same definitions and declarations, the system supplies a common interface to the functions in the header file stdio.h.
You can create a header file with any editor, store it in a convenient directory, and include it in your program.
Header files are traditionally designated by the suffix .h, and are brought into a program at compile time.
The preprocessor component of the compiler does this because it interprets the #include statement in your program
as a directive. The two most commonly used directives are #include and #define.
The #include directive is used to call in and process the contents of the named file.
#include <stdio.h> #include "headers/myheader.h"
The #define directive is used to define the replacement token string for an identifier:
#define NULL 0
Commonly used headers:
assert.h assertion checking
ctype.h character handling
errno.h errno handling
float.h floating point limits
limits.h other data type limits
locale.h program's locale
math.h mathematics
setjmp.h nonlocal jumps
signal.h signal handling
stdarg.h variable arguments
stddef.h common definitions
stdio.h standard input/output
stdlib.h general utiltiies
string.h string handling
time.h date and time
unistd.h system calls
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