Index for Section 3

Alphabetical listing for D

dlopen(3)
NAME
  dlopen, dlsym, dlclose, dlerror - interface to dynamic library loader

SYNOPSIS
  #include <stdio.h>
  #include <dlfcn.h>

  void *dlopen(pathname, mode)
  char *pathname;
  int mode;

  void *dlsym(handle, name)
  void *handle;
  char *name;

  void dlclose(handle)
  void *handle;

  char *dlerror(void)

DESCRIPTION
  The dlopen function provides an interface to the dynamic library loader to
  allow shared libraries to be loaded and called at run time.  The dlopen
  function attempts to load pathname in the address space of the process,
  resolving symbols as appropriate.  Any libraries that pathname depends upon
  are also loaded.

  If pathname includes a /, dlopen will attempt to open it as specified.
  Otherwise, dlopen will attempt to locate pathname using shared library
  search directories in the order specified below (see loader(5) for more
  details on shared library search directories):

   1.  The current directory

   2.  The program's rpath directories

   3.  LD_LIBRARY_PATH directories

   4.  Default shared library directories

  If mode is RTLD_LAZY, then the run-time loader does symbol resolution only
  as needed.  Typically, this means that the first call to a function in the
  newly loaded library will cause the resolution of the address of that
  function to occur.  If mode is RTLD_NOW, then the run-time loader must do
  all symbol binding during the dlopen call.  The dlopen function returns a
  handle that is used by dlsym or dlclose call.	 If an error occurs, a NULL
  pointer is returned.

  If a NULL pathname is specified, dlopen returns a handle for the main
  executable, which allows access to dynamic symbols in the running program.

  The dlsym function returns the address of the symbol name found in the
  shared library corresponding to handle.  If the symbol is not found, a NULL
  pointer is returned.


  The dlclose function deallocates the address space for the library
  corresponding to handle.  The results are undefined if any user function
  continues to call a symbol resolved in the address space of a library that
  has since been deallocated by dlclose.

  The dlerror function returns a string describing the last error that
  occurred from a call to dlopen, dlclose, or dlsym.

NOTES
  The dlopen and dlclose routines might dynamically change the resolution of
  certain symbols referenced by a program or its shared library dependencies.
  The dlopen routine might resolve symbols that were previously unresolved,
  and dlclose might cause resolved symbols to become unresolved or to be
  reresolved to a different symbol definition.

  Use of the dlsym routine is the preferred mechanism for retrieving symbol
  addresses.  This routine reliably returns the current address of a symbol
  at any point in the program, while the dynamic symbol resolution described
  previously might not function as expected due to compiler optimizations.
  For example, the address of a symbol might be saved in a register prior to
  a dlopen call.  The saved address might then be used after the dlopen call,
  even if the dlopen call changed the resolution of the symbol.

  Dynamic symbol resolution functions reliably for programs compiled with the
  -O0 flag.  Also, routines that do not call dlopen or dlclose, either
  directly or indirectly, can safely depend on dynamic symbol resolution.

  The maximum number of shared libraries that can be loaded simultaneously by
  a single process is approximately 60.	 This limit can be raised by
  reconfiguring the kernel's vm-mapentries parameter.  This parameter should
  be set to at least three times the desired maximum number of shared
  libraries that can be loaded by a process.  See the manual System
  Administration for instructions on reconfiguring the vm-mapentries
  parameter.

RELATED INFORMATION
  ld(1), loader(5).