Position-Independent Code

To create programs that require the smallest amount of page modification at run time, the compiler will generate position-independent code under the -K pic option. Whereas the code within a dynamic executable is usually tied to a fixed address in memory, position-independent code can be loaded anywhere in the address space of a process. Because the code is not tied to a specific address, it will execute correctly without page modification at a different address in each process that uses it.

When you use position-independent code, relocatable references are generated in the form of an indirection which will use data in the shared object's data segment. The result is that the text segment code will remain read-only, and all relocation updates will be applied to corresponding entries within the data segment. See "Global Offset Table (Processor-Specific)" and "Procedure Linkage Table (Processor-Specific)" for more details on the use of these two sections.

If a shared object is built from code that is not position-independent, the text segment will usually require a large number of relocations to be performed at runtime. Although the runtime linker is equipped to handle this, the system overhead this creates can cause serious performance degradation.

A shared object that requires relocations against its text segment can be identified by using dump(1) and inspecting the output for any TEXTREL entry. For example:

$ cc -o libfoo.so.1 -G -R. foo.c
$ dump -Lv libfoo.so.1 | grep TEXTREL
[9]     TEXTREL  0

The value of the TEXTREL entry is irrelevant, its presence in a shared object indicates that text relocations exist.

A recommended practice to prevent the creation of a shared object that contains text relocations is to use the link-editor's -z text flag. This flag causes the link-editor to generate diagnostics indicating the source of any non position-independent code used as input, and results in a failure to generate the intended shared object. For example:

$ cc -o libfoo.so.1 -z text -G -R. foo.c
Text relocation remains                       referenced
    against symbol                  offset      in file
foo                                 0x0         foo.o
bar                                 0x8         foo.o
ld: fatal: relocations remain against allocatable but \
non-writable sections

Here, two relocations are generated against the text segment because of the non-position-independent code generated from the file foo.o. Where possible, these diagnostics will indicate any symbolic references that are required to carry out the relocations. In this case the relocations are against the symbols foo and bar.

Besides not using the -K pic option, the most common cause of creating text relocations when generating a shared object is by including hand written assembler code that has not been coded with the appropriate position-independent prototypes.

By using the compiler's ability to generate an intermediate assembler file, the coding techniques used to enable position-independence can usually be revealed by experimenting with some simple test case source files.

A second form of the position-independence flag, -K PIC, is also available on some processors, and provides for a larger number of relocations to be processed at the cost of some additional code overhead (see cc(1) for more details).