Autoconf, Automake, and Libtool: Position Independent Code

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10.2.1 Position Independent Code

On most architectures, when you compile source code to object code, you need to specify whether the object code should be position independent or not. There are occasional architectures which don't make the distinction, usually because all object code is position independent by virtue of the ABI(15), or less often because the load address of the object is fixed at compile time (which implies that shared libraries are not supported by such a platform). If an object is compiled as position independent code (PIC), then the operating system can load the object at any address in preparation for execution. This involves a time overhead, in replacing direct address references with relative addresses at compile time, and a space overhead, in maintaining information to help the runtime loader fill in the unresolved addresses at runtime. Consequently, PIC objects are usually slightly larger and slower at runtime than the equivalent non-PIC object. The advantage of sharing library code on disk and in memory outweigh these problems as soon as the PIC object code in shared libraries is reused.

PIC compilation is exactly what is required for objects which will become part of a shared library. Consequently, libtool builds PIC objects for use in shared libraries and non-PIC objects for use in static libraries. Whenever libtool instructs the compiler to generate a PIC object, it also defines the preprocessor symbol, `PIC', so that assembly code can be aware of whether it will reside in a PIC object or not.

Typically, as libtool is compiling sources, it will generate a `.lo' object, as PIC, and a `.o' object, as non-PIC, and then it will use the appropriate one of the pair when linking executables and libraries of various sorts. On architectures where there is no distinction, the `.lo' file is just a soft link to the `.o' file.

In practice, you can link PIC objects into a static archive for a small overhead in execution and load speed, and often you can similarly link non-PIC objects into shared archives. If you find that you need to do this, libtool provides several ways to override the default behavior (see section 10.1 Creating libtool).

This document was generated by Gary V. Vaughan on May, 24 2001 using texi2html