Linkers and Loaders--page 3 of 4

Linkers and Loaders--page 3 of 4

Nov 26, 2002  By  Sandeep Grover

 in

• Software

Discussing how compilers, links and loaders work and the benefits of shared libraries.

Relocation

Once the linker has resolved all the symbols, each symbol reference has exactly one definition. At this point, linker starts the process of relocation, which involves the following two steps:

• Relocating sections and symbol definitions. Linker merges all the sections of the same type into a new single section. For example, linker merges all the .data sections of all the input relocatable object files into a single .data section for the final executable. A similar process is carried out for the .code section. The linker then assigns runtime memory addresses to new aggregate sections, to each section defined by the input module and also to each symbol. After the completion of this step, every instruction and global variable in the program has a unique loadtime address.

• Relocating symbol reference within sections. In this step, linker modifies every symbol reference in the code and data sections so they point to the correct loadtime addresses.

Whenever assembler encounters an unresolved symbol, it generates a relocation entry for that object and places it in the .relo.text/.relo.data sections. A relocation entry contains information about how to resolve the reference. A typical ELF relocation entry contains the following members:

• Offset, a section offset of the reference that needs to be relocated. For a relocatable file, this value is the byte offset from the beginning of the section to the storage unit affected by relocation.

• Symbol, a symbol the modified reference should point to. It is the symbol table index with respect to which the relocation must be made.

• Type, the relocation type, normally R_386_PC32, that signifies PC-relative addressing. R_386_32 signifies absolute addressing.

The linker iterates over all the relocation entries present in the relocatable object modules and relocates the unresolved symbols depending on the type. For R_386_PC32, the relocating address is calculated as S + A - P; for R_386_32 type, the address is calculated as S + A. In these calculations, S denotes the value of the symbol from the relocation entry, P denotes the section offset or address of the storage unit being relocated (computed using the value of offset from relocation entry) and A is the address needed to compute the value of the relocatable field.

Dynamic Linking: Shared Libraries

Static libraries above have some significant disadvantages; for example, consider standard functions such as printf and scanf. They are used almost by every application. Now, if a system is running 50-100 processes, each process has its own copy of executable code for printf and scanf. This takes up significant space in the memory. Shared libraries, on the other hand, address the disadvantages of static libraries. A shared library is an object module that can be loaded at run time at an arbitrary memory address, and it can be linked to by a program in memory. Shared libraries often are called as shared objects. On most UNIX systems they are denoted with a .so suffix; HP-UX uses a .sl suffix and Microsoft refer to them as DLLs (dynamic link libraries).

To build a shared object, the compiler driver is invoked with a special option:

gcc -shared -fPIC -o libfoo.so a.o b.o

The above command tells the compiler driver to generate a shared library, libfoo.so, comprised of the object modules a.o and b.o. The -fPIC option tells the compiler to generate position independent code (PIC).

Now, suppose the main object module is bar.o, which has dependencies on a.o and b.o. In this case, the linker is invoked with:

gcc bar.o ./libfoo.so

This command creates an executable file, a.out, in a form that can be linked to libfoo.so at load time. Here a.out does not contain the object modules a.o and b.o, which would have been included had we created a static library instead of a shared library. The executable simply contains some relocation and symbol table information that allow references to code and data in libfoo.so to be resolved at run time. Thus, a.out here is a partially executable file that still has its dependency in libfoo.so. The executable also contains a .interp section that contains the name of the dynamic linker, which itself is a shared object on Linux systems (ld-linux.so). So, when the executable is loaded into memory, the loader passes control to the dynamic linker. The dynamic linker contains some start-up code that maps the shared libraries to the program's address space. It then does the following:

• relocates the text and data of libfoo.so into memory segment; and

• relocates any references in a.out to symbols defined by libfoo.so.

Finally, the dynamic linker passes control to the application. From this point on, location of shared object is fixed in the memory.