CODEWITHSUNDEEP
clinker
codey modey
codey modey

Reputation: 983

Regarding Static and Shared Libraries Linking in C

  #include <stdio.h>

  int main()
  {
          printf("%s","Hello world");
          return 0;      
  }

  $gcc -o hello hello.c

Question:

1 - I believe the printf function's object file is statically linked. Is that correct?

2 - How should I configure/write this code so that the library files are dynamically linked or I mean it uses shared libraries at runtime?

Note: I am beginner in this concept, so feel free to correct me wherever it doesn't makes sense

Upvotes: 0

Views: 640

Answers (1)

Olaf Dietsche
Olaf Dietsche

Reputation: 74088

The linker takes whatever it finds. This is usually the shared library.

On a Linux system, you can use 

file hello

to find out, whether it is linked statically or dynamically.

E.g. 

file /bin/bash

gives

/bin/bash: ELF 64-bit LSB executable, x86-64, version 1 (SYSV), dynamically linked (uses shared libs), for GNU/Linux 2.6.24, BuildID[sha1]=0x6dafe33f9353cbb054b1b1f7b079545992575757, stripped

whereas 

file /bin/busybox

gives

/bin/busybox: ELF 64-bit LSB executable, x86-64, version 1 (SYSV), statically linked, for GNU/Linux 2.6.24, BuildID[sha1]=0xac4943b7daf7c3c204a2866ea5398f2337ff93c9, stripped

You can force a static link, by adding the -static option to gcc 

gcc -static -o hello hello.c

file hello

/tmp/hello: ELF 64-bit LSB executable, x86-64, version 1 (GNU/Linux), statically linked, for GNU/Linux 2.6.24, BuildID[sha1]=0x790ec9b287fd2a276162560e5e6669ba6b73e68f, not stripped

Update:

Linking is the process of putting object files, dynamic and static libraries and some boilerplate objects together, to form a binary executable file.

You can use both dynamic and static libraries in one executable. The needed object files of a static library are copied into the executable. On the other side, dynamic libraries (dynamic objects actually) are not copied, but rather referenced by the resulting binary.

Update:

There are two kinds of libraries, static libraries (ar archives, see man ar) 

file /usr/lib/libnet.a

/usr/lib/libnet.a: current ar archive

and dynamic libraries (dynamic objects) 

file /usr/lib/libnet.so.1.5.0 

/usr/lib/libnet.so.1.5.0: ELF 64-bit LSB shared object, x86-64, version 1 (SYSV), dynamically linked, BuildID[sha1]=0x0c596357947e79001025b3c57be933690085dffb, stripped

You can have both types of library installed at the same time, e.g. 

ls -l /usr/lib/libnet.*

-rw-r--r-- 1 root root 207780 Okt 28  2011 /usr/lib/libnet.a
-rw-r--r-- 1 root root    802 Okt 28  2011 /usr/lib/libnet.la
lrwxrwxrwx 1 root root     15 Okt 28  2011 /usr/lib/libnet.so -> libnet.so.1.5.0
lrwxrwxrwx 1 root root     15 Okt 28  2011 /usr/lib/libnet.so.1 -> libnet.so.1.5.0
-rw-r--r-- 1 root root  92712 Okt 28  2011 /usr/lib/libnet.so.1.5.0

An ar archive contains one or more object files, which are selected by the linker if needed by the executable file. A shared object is an object with subroutines, which allows to be called by other shared objects or executables at runtime.

If you're interested in this subject, you can also look at this Wikipedia - Library (computing) article.

Upvotes: 3

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