Static Libraries

• inside a static library, you are likely to find that it contains many modules with only one function per module. – This gives the linker maximum opportunities to

remove unnecessary code.– The linker reads the object files contained in the

archive one by one when it tries to resolve external symbols. The linker is free to discard any object files that are not required.

Dynamic Libraries1. Unlike a static library, which is just an archive of object files, a dynamic library is

itself an object file. 2. Dynamic libraries are created by the linker, unlike static libraries, which are

created by the compiler and the ar command. 3. When you link with a dynamic library, the compiler does not copy any object code

into your executable file. – Instead, it records information about the dynamic library in your executable (stub) so

that it can be dynamically linked at runtime. This means that part of the link process is deferred until runtime. When a dynamically linked program executes, the program performs the final linking

stages before it enters main.

4. When the linker sees a static library, it is free to pull in only the object files that your program actually needs, which keeps the executable as small as possible. – When your code links with a shared object, however, it’s all or nothing. linker cannot

discard pieces of the shared object, so your process’s memory will map all the code contained in the shared object, whether it calls it or not.

– 這是為了在有 VM 時， OS 方便管理記憶體映射。若在沒有 VM 下，使用動態函式庫就可能過於昂貴了 !!

Creating a Dynamic Library

• hello.c for the main program and empty.c, be an empty shared object.

$ cat <<EOF > hello.c#include <stdio.h>int main() { printf("Hello World\n"); }EOF$ cat /dev/null > empty.c

Creating shared object$ gcc -shared -fpic -o empty.so empty.c

To link hello with this shared object,$ gcc -o hello hello.c empty.so

To execute, you need to set LD_LIBRARY_PATH 環境變數$ LD_LIBRARY_PATH=. ./helloHello World

Example 展示靜態與動態鏈結的差異

• we will create a new shared object that does nothing but consume memory. Let’s call it piggy.c:

$ cat <<EOF > piggy.c>char bank[0x100000];>EOF$

• hello.c program that just waits for us to press Ctrl+C so we can examine it while it runs:

$ cat <<EOF > hello.c#include <unistd.h>int main(){ pause(); }EOF

靜態鏈結$ gcc -o hello hello.c piggy.c$ size hellotext data bss dec hex filename1117 264 1048608 1049989 100585 hello$ nm -S hello | grep bank080496c0 00100000 B bank• When the modules are linked like this, the memory

consumption is visible with the size command. The nm command shows that the array bank is present and consuming 1MB of space.

動態鏈結函式庫$ gcc -c piggy.c$ ar clq libpig.a piggy.o$ gcc -o hello hello.c -L ./ -lpig$ size hellotext data bss dec hex filename1117 264 4 1385 569 hello$ nm -S hello | grep bank• This time, the size of the executable is noticeably smaller because

the bank array is not included, which tells us that the piggy.o module was excluded. – The linker is able to determine that there are no references to piggy.o,

so it does not link it with the executable.

查看執行動態鏈結 process 的 memory map

• We’ll run it using the pmap command to get a look at the process’s memory map:$ ./hello &$ jobs -x pmap -q %17382: ./hello08048000 4K r-x-- /hello08049000 4K rw--- /hellob7dab000 4K rw--- [ anon ]b7dac000 1160K r-x-- /libc-2.3.2.sob7ece000 36K rw--- /libc-2.3.2.sob7ed7000 8K rw--- [ anon ]b7ee7000 4K r-x-- /piggy.sob7ee8000 4K rw--- /piggy.sob7ee9000 1032K rw--- [ anon ] //consumed by bank array in virtual address space

b7feb000 84K r-x-- /ld-2.3.2.sob8000000 4K rw--- /ld-2.3.2.sobffeb000 84K rw--- [ stack ]ffffe000 4K ----- [ anon ]

• So linking with a shared object can force you to consume virtual memory that you otherwise would not use

• By default, the compiler uses dynamic linking, which means that if it can choose between a static library and a dynamic library, it will choose the dynamic library. If you want to link exclusively with static objects, you must specify the –static option to gcc/g++.

Build Process

• A typical source distribution created with the GNU build tools includes a configure script, which is used by the end user to create a Makefile.

$ ./configure$ make$ make install

Configure Stage