Program compile and link in linux.

Program compile and link in linux.

Most of system software provides the source code and we can use compile to build it into the executable binaries. This blog will introduce what is inside the compilation process.

Compile tools

On linux platform, gcc is the most wide used tool for c, c++ and even java compiler.

You will use

#which gcc
to determine if gcc has been installed

Single file compilation

here is a single c program:

#include <stdio.h> 
void main(void) 
{ 
  printf("hello world!"); 
} 

now we can compile the program and run it

[root@X001 cprogram]# ls
hello.c
[root@X001 cprogram]# gcc hello.c
[root@X001 cprogram]# ls
a.out  hello.c
[root@X001 cprogram]# ./a.out
hello world!

or you may use 'gcc -o outputfile sourcefile' to assign an output executable file name

multiple files compilation

if we have more than one single source file, which is the most case in application development. source code A will refer to other files, when one of the files is changed,  do we need to re-compile all of the related files?
the answer is no.
we have two files

File:a.c 
#include <stdio.h> 
int main () 
{ 
    printf("this is from first file\n");
    method(); 
} 

File:b.c 
#include <stdio.h> 
void method(void) 
{ 
    printf("this is from second file:\n"); 
}

now we will do
1. compile the individual source files into object files
2. link the object files into binary

[root@X001 cprogram]# ll
total 8
-rw-r--r--. 1 root root 95 Apr 14 14:11 a.c
-rw-r--r--. 1 root root 87 Apr 14 14:05 b.c
======compile the files here ===========
[root@X001 cprogram]# gcc -c a.c b.c
[root@X001 cprogram]# ll
total 16
-rw-r--r--. 1 root root   95 Apr 14 14:11 a.c
-rw-r--r--. 1 root root 1568 Apr 14 14:11 a.o
-rw-r--r--. 1 root root   87 Apr 14 14:05 b.c
-rw-r--r--. 1 root root 1504 Apr 14 14:11 b.o
=======link the files here ==============
[root@X001 cprogram]# gcc -o result a.o b.o

[root@X001 cprogram]# ./result
this is from first file
this is from second file

if we change the print statement of the b.c to "this is the updated file" .
we only need re-compile the b.o file and then regenerate the executable binary. 

link library

none of the commercial software is developed from scratch. We will use some of common software component called library.  for example, you may use mathmatics library to get the sin value for PI.