Direct binding
Encyclopedia
Direct binding is a feature of the linker and dynamic linker
on Solaris
and OpenSolaris
. It provides a method to allow libraries to directly bind symbols to other libraries, rather than weakly bind to them and leave the dynamic linker to figure out which library contains the symbol.
for that library with all required symbols. It adds symbols in the current object with their offset, and leaves undefined symbols in the object unresolved. Unresolved symbols must still be known at link time, but the linker is passed shared libraries to link with and, when it finds a symbol in another library, it places a note in the output library indicating that it depends on these other shared libraries.
This kind of weak linking forces the dynamic linker to resolve which library contains which symbol when the executable is run. The dynamic linker on Linux, for example, reads through the DT_NEEDED section of an ELF
object and loads needed libraries; but isn't told where required symbols are. For this, it must iterate through each unresolved symbol in the object, and for each of these, iterate through each loaded library, checking until it finds a matching symbol. This can be time consuming.
Direct linking works around this problem by storing an equivalent list of pointers to DT_NEEDED entries in a separate ELF section. Each pointer corresponds to a symbol in the object; thus, these pointers create a relation between a symbol and a DT_NEEDED entry. This allows the linker to locate the library specified by a particular DT_NEEDED entry and check only its symbol table for each symbol, rather than iterate through potentially every library for every symbol.
Dynamic linker
In computing, a dynamic linker is the part of an operating system that loads and links the shared libraries for an executable when it is executed. The specific operating system and executable format determine how the dynamic linker functions and how it is implemented...
on Solaris
Solaris Operating System
Solaris is a Unix operating system originally developed by Sun Microsystems. It superseded their earlier SunOS in 1993. Oracle Solaris, as it is now known, has been owned by Oracle Corporation since Oracle's acquisition of Sun in January 2010....
and OpenSolaris
OpenSolaris
OpenSolaris was an open source computer operating system based on Solaris created by Sun Microsystems. It was also the name of the project initiated by Sun to build a developer and user community around the software...
. It provides a method to allow libraries to directly bind symbols to other libraries, rather than weakly bind to them and leave the dynamic linker to figure out which library contains the symbol.
Theory
When linking a shared library or dynamic linked executable, the linker normally populates the symbol tableSymbol table
In computer science, a symbol table is a data structure used by a language translator such as a compiler or interpreter, where each identifier in a program's source code is associated with information relating to its declaration or appearance in the source, such as its type, scope level and...
for that library with all required symbols. It adds symbols in the current object with their offset, and leaves undefined symbols in the object unresolved. Unresolved symbols must still be known at link time, but the linker is passed shared libraries to link with and, when it finds a symbol in another library, it places a note in the output library indicating that it depends on these other shared libraries.
This kind of weak linking forces the dynamic linker to resolve which library contains which symbol when the executable is run. The dynamic linker on Linux, for example, reads through the DT_NEEDED section of an ELF
Executable and Linkable Format
In computing, the Executable and Linkable Format is a common standard file format for executables, object code, shared libraries, and core dumps. First published in the System V Application Binary Interface specification, and later in the Tool Interface Standard, it was quickly accepted among...
object and loads needed libraries; but isn't told where required symbols are. For this, it must iterate through each unresolved symbol in the object, and for each of these, iterate through each loaded library, checking until it finds a matching symbol. This can be time consuming.
Direct linking works around this problem by storing an equivalent list of pointers to DT_NEEDED entries in a separate ELF section. Each pointer corresponds to a symbol in the object; thus, these pointers create a relation between a symbol and a DT_NEEDED entry. This allows the linker to locate the library specified by a particular DT_NEEDED entry and check only its symbol table for each symbol, rather than iterate through potentially every library for every symbol.