Objects, Symbols, and Archives

Inside an object file

A .o holds sections: .text (code), .data (initialized globals), .bss (zero-initialized globals, no storage until load), plus a symbol table. nm prints the symbols with a type letter: T (defined in .text), U (undefined — the linker must find it), D/B (data/bss), t (static — file-local).

int counter = 5;          /* .data, symbol D */
static int hidden = 9;    /* still .data but symbol d: not exported */
int bump(void);           /* no definition: callers emit U */

Archives are objects with an index

ar rcs libmath.a add.o mul.o bundles objects; gcc main.c -L. -lmath links against it. The archive member is pulled in only if it defines a symbol that is currently undefined — pull-in semantics that explain both why link order matters and why an unused member costs nothing.

The three classic link errors

• undefined reference: you used it, nobody defined it (or the archive defining it was not linked).
• multiple definition: two objects export the same external symbol.
• layout mismatch: no error at all — an ABI mismatch links cleanly and breaks at runtime, which is why header discipline (module 18 of Beginner, sharpened here) is not bureaucracy.