Static vs dynamic linking

What is static linking?

Static linking links libraries at compile time, copying them to the final binary.

What is dynamic linking?

Dynamic linking loads and links libraries at runtime, loading them to memory.

Only the name of the shared libraries is saved at compile time.

These names are saved in a PLT (Procedure Linkage Table)

Static vs dynamic linking

Static

• Bigger binaries

Dynamic

• Depend on external libraries to be installed and be compatible
• Shared libraries are shared across processes
• Shared library code can be updated/patched without new compilation
• Updates to shared library code can add breaking changes and prevent the program from running

How to create a statically linked binary?

$ ld [options] objfile

ld combines several object and archive files, relocates their data and ties up symbol references. Usually, the last step in compiling a program is to run ld.

$ gcc hello.c -static -o hello

How to create a dynamically linked binary?

$ gcc hello.c -o hello

How to know if a binary is statically or dynamically linked?

Check the type of linking

$ file /usr/bin/gcc

/usr/bin/gcc: ELF 64-bit LSB executable, x86-64, version 1 (SYSV), dynamically linked, interpreter /lib64/ld-linux-x86-64.so.2, BuildID[sha1]=017fc52acbca077c9bc6a4e8f04dd90eb5385243, for GNU/Linux 4.4.0, stripped

Check dynamically linked libraries

$ ldd /bin/gcc

linux-vdso.so.1 (0x00007fff6377e000)
libc.so.6 => /usr/lib/libc.so.6 (0x00007fcd238f2000)
/lib64/ld-linux-x86-64.so.2 => /usr/lib64/ld-linux-x86-64.so.2 (0x00007fcd23b02000)