What Is Relocatable Object Code?

Relocatable object code is machine code that is generated by compilers and assemblers and stored in relocatable object files, or .o files. A relocatable object file contains symbolic references to locations defined within the compilation unit as well as symbolic references to locations defined outside the compilation unit. The object file also contains relocation information. The linker uses this information to replace the symbolic references with actual addresses.

For example, if you write a program that references the external variable errno, the object code created by the compiler contains only a symbolic reference to errno since errno is not defined in your program. Only when the linker links this object code does the reference to errno change (relocate) to an absolute address in virtual memory.

If your program defines a global variable, the compiler assigns a relocatable address to that variable. The compiler also marks all references to that variable as relocatable. The linker replaces the references to the variable with the absolute address of the variable.