Object files

Base address

The virtual addresses in the program headers might not represent the actual virtual addresses of the program's memory image. Executable files typically contain absolute code. To let the process execute correctly, the segments must reside at the virtual addresses used to build the executable file. On the other hand, shared object segments typically contain position-independent code. This lets a segment's virtual address change from one process to another, without invalidating execution behavior. On some platforms, while the system chooses virtual addresses for individual processes, it maintains the relative position of one segment to another within any one shared object. Because position-independent code on those platforms uses relative addressing between segments, the difference between virtual addresses in memory must match the difference between virtual addresses in the file. The differences between the virtual address of any segment in memory and the corresponding virtual address in the file is thus a single constant value for any one executable or shared object in a given process. This difference is the base address. One use of the base address is to relocate the memory image of the file during dynamic linking.

An executable or shared object file's base address (on platforms that support the concept) is calculated during execution from three values: the virtual memory load address, the maximum page size, and the lowest virtual address of a program's loadable segment. To compute the base address, one determines the memory address associated with the lowest p_vaddr value for a PT_LOAD segment. This address is truncated to the nearest multiple of the maximum page size. The corresponding p_vaddr value itself is also truncated to the nearest multiple of the maximum page size. The base address is the difference between the truncated memory address and the truncated p_vaddr value.

Next topic: Segment permissions
Previous topic: Segment types, p_type table

© 2005 The SCO Group, Inc. All rights reserved.
SCO OpenServer Release 6.0.0 -- 02 June 2005