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HP-UX Linker and Libraries User's Guide: HP 9000 Computers > Chapter 2 What Happens When You Compile
and Link a Program
Running the Program |
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An executable file is created after the program has been compiled and linked. The next step is to run or load the program. When you run an executable file created by ld, the program is loaded into memory by the HP-UX program loader, exec. This routine is actually a system call and can be called by other programs to load a new program into the current process space. The exec function performs many tasks; some of the more important ones are:
For details on exec, see the exec(2) page in the HP-UX Reference. Since shared library routines and data are not actually contained in the a.out file, the dynamic loader must attach the routines and data to the program at run time. Attaching a shared library entails mapping the shared library code and data into the process's address space, relocating any pointers in the shared library data that depend on actual virtual addresses, allocating the bss segment, and binding routines and data in the shared library to the program. The dynamic loader binds only those symbols that are reachable during the execution of the program. This is similar to how archive libraries are treated by the linker; namely, ld pulls in an object file from an archive library only if the object file is needed for program execution. To accelerate program startup time, routines in a shared library are not bound until referenced. (Data items are always bound at program startup.) This deferred binding of shared library routines distributes the overhead of binding across the execution time of the program and is especially expedient for programs that contain many references that are not likely to be executed. In essence, deferred binding is similar to demand-loading. |
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