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First of all, what's library - in real world,library is collection of books.In computer terms,library has collection of functions. For example,printf() is a library function.That means,this printf functions definition is available in gcc libraries - libc. Let's create a new directory called c-lib, [[email protected] ~]$ mkdir c-lib [[email protected] ~]$ cd c-lib/ create a new file called "hi.c" with a function named fun1. [[email protected] c-lib]$ cat > hi.c void fun1(){ printf("\n Fun1"); } ^C #press ctrl+C to signal end of input from keyboard. now create a new file called "hi2.c" with a function named fun2. [[email protected] c-lib]$ cat > hi2.c void fun2(){ printf("\n Fun2"); } ^C #press ctrl+C to signal end of input from keyboard. So here is the two files we have created. [[email protected] c-lib]$ cat hi.c void fun1(){ printf("\n Fun1"); } [[email protected] c-lib]$ cat hi2.c void fun2(){ printf("\n Fun2"); } Now create a header file called lib.h [[email protected] c-lib]$ cat > lib.h void fun1(); void fun2(); Now are going to use above two functions fun1 and fun2 in final C program named hi_main.c [[email protected] c-lib]$ cat > hi_main.c #include "lib.h" main(){ fun1(); fun2(); } ^CYou can see both functions are called inside main(). Now compile the c program to create intermediate object file named hi_main.o [[email protected] c-lib]$ gcc -c hi_main.c [[email protected] c-lib]$ ls -l total 20 -rw-r--r-- 1 lg lg 34 2009-07-16 10:49 hi2.c -rw-r--r-- 1 lg lg 34 2009-07-16 10:49 hi.c -rw-r--r-- 1 lg lg 43 2009-07-16 10:50 hi_main.c -rw-r--r-- 1 lg lg 784 2009-07-16 10:51 hi_main.o -rw-r--r-- 1 lg lg 26 2009-07-16 10:50 lib.h Now compile our function files to create .o files. [[email protected] c-lib]$ gcc -c hi.c hi2.c hi.c: In function â??fun1â??: hi.c:2: warning: incompatible implicit declaration of built-in function â??printfâ?? hi2.c: In function â??fun2â??: hi2.c:2: warning: incompatible implicit declaration of built-in function â??printfâ?? [[email protected] c-lib]$ ls -l total 28 -rw-r--r-- 1 lg lg 34 2009-07-16 10:49 hi2.c -rw-r--r-- 1 lg lg 816 2009-07-16 10:51 hi2.o -rw-r--r-- 1 lg lg 34 2009-07-16 10:49 hi.c -rw-r--r-- 1 lg lg 43 2009-07-16 10:50 hi_main.c -rw-r--r-- 1 lg lg 784 2009-07-16 10:51 hi_main.o -rw-r--r-- 1 lg lg 816 2009-07-16 10:51 hi.o -rw-r--r-- 1 lg lg 26 2009-07-16 10:50 lib.hAs you can see two new files hi.o and hi2.o are created. We finally create an executable binary using these three object files. [[email protected] c-lib]$ gcc -o hello hi_main.o hi.o hi2.o [[email protected] c-lib]$ ls -l total 36 -rwxr-xr-x 1 lg lg 6482 2009-07-16 10:51 hello* -rw-r--r-- 1 lg lg 34 2009-07-16 10:49 hi2.c -rw-r--r-- 1 lg lg 816 2009-07-16 10:51 hi2.o -rw-r--r-- 1 lg lg 34 2009-07-16 10:49 hi.c -rw-r--r-- 1 lg lg 43 2009-07-16 10:50 hi_main.c -rw-r--r-- 1 lg lg 784 2009-07-16 10:51 hi_main.o -rw-r--r-- 1 lg lg 816 2009-07-16 10:51 hi.o -rw-r--r-- 1 lg lg 26 2009-07-16 10:50 lib.h time to run the binary. [[email protected] c-lib]$ ./hello Fun1 Fun2Yeah,we got the desired output :) You can use nm command to view list of functions on a specific object file. [[email protected] c-lib]$ nm hi.o 00000000 T fun1 U printf got it ? hi.o has function called fun1. "U" refers those functions are unresolved. [[email protected] c-lib]$ nm hi2.o 00000000 T fun2 U printf [[email protected] c-lib]$ nm hi_main.o U fun1 U fun2 00000000 T main [[email protected] c-lib]$ nm hello 080494a8 d _DYNAMIC 08049574 d _GLOBAL_OFFSET_TABLE_ 0804847c R _IO_stdin_used w _Jv_RegisterClasses 08049498 d __CTOR_END__ 08049494 d __CTOR_LIST__ 080494a0 D __DTOR_END__ 0804949c d __DTOR_LIST__ 08048490 r __FRAME_END__ 080494a4 d __JCR_END__ 080494a4 d __JCR_LIST__ 08049594 A __bss_start 0804958c D __data_start 08048430 t __do_global_ctors_aux 080482f0 t __do_global_dtors_aux 08049590 D __dso_handle w __gmon_start__ 0804842a T __i686.get_pc_thunk.bx 08049494 d __init_array_end 08049494 d __init_array_start 080483c0 T __libc_csu_fini 080483d0 T __libc_csu_init U __libc_start_main@@GLIBC_2.0 08049594 A _edata 0804959c A _end 0804845c T _fini 08048478 R _fp_hw 08048250 T _init 080482c0 T _start 08049594 b completed.6610 0804958c W data_start 08049598 b dtor_idx.6612 08048350 t frame_dummy 08048398 T fun1 080483ac T fun2 08048374 T main U printf@@GLIBC_2.0Now we could use ar command archive our object files (hi.o and hi2.o) into a library file named libhi.a [[email protected] c-lib]$ ar crv libhi.a hi.o hi2.o a - hi.o a - hi2.o here is the list of files : [[email protected] c-lib]$ ls -ltr total 40 -rw-r--r-- 1 lg lg 34 2009-07-16 10:49 hi.c -rw-r--r-- 1 lg lg 34 2009-07-16 10:49 hi2.c -rw-r--r-- 1 lg lg 26 2009-07-16 10:50 lib.h -rw-r--r-- 1 lg lg 43 2009-07-16 10:50 hi_main.c -rw-r--r-- 1 lg lg 784 2009-07-16 10:51 hi_main.o -rw-r--r-- 1 lg lg 816 2009-07-16 10:51 hi.o -rw-r--r-- 1 lg lg 816 2009-07-16 10:51 hi2.o -rwxr-xr-x 1 lg lg 6482 2009-07-16 10:51 hello* -rw-r--r-- 1 lg lg 1842 2009-07-16 10:53 libhi.aNow we use the -L option to mention our own library file,while creating binary file named hello1. -lhi is the short form for -libhi. [[email protected] c-lib]$ gcc -o hello1 hi_main.o -L . -lhi [[email protected] c-lib]$ ls -ltr total 48 -rw-r--r-- 1 lg lg 34 2009-07-16 10:49 hi.c -rw-r--r-- 1 lg lg 34 2009-07-16 10:49 hi2.c -rw-r--r-- 1 lg lg 26 2009-07-16 10:50 lib.h -rw-r--r-- 1 lg lg 43 2009-07-16 10:50 hi_main.c -rw-r--r-- 1 lg lg 784 2009-07-16 10:51 hi_main.o -rw-r--r-- 1 lg lg 816 2009-07-16 10:51 hi.o -rw-r--r-- 1 lg lg 816 2009-07-16 10:51 hi2.o -rwxr-xr-x 1 lg lg 6482 2009-07-16 10:51 hello* -rw-r--r-- 1 lg lg 1842 2009-07-16 10:53 libhi.a -rwxr-xr-x 1 lg lg 6482 2009-07-16 10:54 hello1* you can see binary file is created successfully [[email protected] c-lib]$ ./hello1 Fun1 Fun2 Cool,we got the output too. So we have used our own library to create a binary file. |
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