Include files

#include is read by C’s preprocessor (gcc -E) and is substituted by the contents of that file. Preprocessor looks in a regular directory like /usr/include but custom dirs can be passed with -I (e.g. -I/home/user/.local/include).

If a symbol or function isn’t declared anywhere (neither in the local .c file or in any of the included .h files), the compiler errors (e.g. error: implicit declaration).

Libraries

After preprocessing, an object file is compiled (gcc -c) with undefined symbols which come from libraries. To tell the compiler which libraries to use, use the -l flag. If I have a file named libfoo-1.a, then i need to pass a flag like this: -lfoo-1. Same with .so libs. The runtime loader has a name→location cache (ld.so.cache) for system-wide .so files, for fast lookup at runtime. It covers /lib, /usr/lib, and dirs in /etc/ld.so.conf. (Build-time: the linker finds libs via -L + defaults, analogous to -I for headers.) Truly undefined symbols after linking (neither in the local .o files or any of the passed libraries) surfaces as undefined reference.

Static linking

Build-time linker (ld) finds libs via -L + defaults. Linker takes the used code from the lib and combines it with your .o file into a final executable. Undefined symbols are resolved since the addresses for them are now known.

Dynamic linking

-rpath embeds the path where the runtime linker should look for custom libraries, the -L flag only tells the linker this information during build time. Runtime loader (ld.so) finds .so via ld.so.cache + -rpath. Linker leaves the undefined symbols pointing at stubs or nothing and resolves them at runtime from the ELF’s NEEDED library entries.

Shared library naming and versioning

libfoo.so (unversioned linker name, used at build time via -l) -> libfoo.so. (the soname baked into binaries and resolved at runtime; major changes only on ABI breaks) -> libfoo.so.. (the real file, whose full version lets multiple versions coexist on disk).

CLI tools

ldd - run linker .so resolver and print deps + their paths on your system. nm - output symbol information (undefined, text, data). readelf - inspect ELF file contents, the header, dynamic section etc. patchelf - modify ELF file. objdump - can parse many object format, disassemble them. ldconfig - update linker’s runtime cache (/etc/ld.so.cache).