CODEWITHSUNDEEP
gccld
Vorac
Vorac

Reputation: 9124

What happens when compiling against a shared library?

I understand that when linking against a static library i.e. libname.a, the binary code for the used functions is taken out of the archive and inserted in the application binary. Therefore, the static library MUST be present at compilation time.

However, with shared libraries I am lost. The function definitions are not copied. Then why is it needed that the shared library be provided on the linker command line? Also, are there different ways to link against shared libraries and what are they?

Upvotes: 4

Views: 410

Answers (2)

PetrosB
PetrosB

Reputation: 4154

The shared libraries need to be fed to the linker's command line so that a reference to the specific functions and the file in which these functions reside, is stored into the executable. When the executable is run, the dynamic linker (/lib/ld-linux.so, /libexec/ld-elf.so, etc, depending on your system) is loaded first and checks these references. Once it finds the lib files, it maps them (using the mmap() system call) to your program's adress space.

You can see these references by running

objdump -T a.out

or

nm -D a.out

For ELF executables, the existence of the .interp section implies that the program uses dynamic linking.

Upvotes: 4

Brian Tiffin
Brian Tiffin

Reputation: 4126

See the man pages for dlopen and dlsym for explicit dynamic link loader management.

Upvotes: 0

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