The names a program needs to find

The road from the words you type to the bytes the chip runs. This lesson zooms in on its last two stops: joining the pieces, then loading them.

Before you start. A compiler turns the code you write into the chip’s own instructions, one file at a time, and a running program keeps its short-lived work on the stack and its longer-lived things on the heap (How Code Becomes Execution (compilers, stack & heap)). The kernel starts a program by loading it into a fresh process, the walled-off space a running program lives in (Processes, Threads & the Kernel). Memory is handed out in fixed-size blocks called pages. A file can be mapped into a process’s memory, page by page. When two processes map the same file, they share one copy of those pages (The address book every process carries).

The idea in 30 seconds

A compiler turns each source file into an object file. Each object file lists the names, called symbols, that it provides and the ones it uses but does not contain.
The linker joins object files into one program and matches every used symbol to the file that provides it. “Undefined symbols” means one was never matched.
A library is a file of ready-made functions many programs use. Static linking copies a library’s code into the program. The program needs nothing at run time, and it never sees a fix to that library until it is rebuilt.
A shared library is a library found and loaded by the dynamic loader, a small program that runs before yours and fills in its missing pieces. One updated file fixes every program that uses it, but a program already running keeps the old copy until it is restarted.
The same sharing breaks every program at once when a shared library goes missing, or renames or changes something they rely on. The program then refuses to start.

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