dlopen(3p) — Linux manual page


DLOPEN(3P)                POSIX Programmer's Manual               DLOPEN(3P)

PROLOG         top

       This manual page is part of the POSIX Programmer's Manual.  The Linux
       implementation of this interface may differ (consult the
       corresponding Linux manual page for details of Linux behavior), or
       the interface may not be implemented on Linux.

NAME         top

       dlopen — open a symbol table handle

SYNOPSIS         top

       #include <dlfcn.h>

       void *dlopen(const char *file, int mode);

DESCRIPTION         top

       The dlopen() function shall make the symbols (function identifiers
       and data object identifiers) in the executable object file specified
       by file available to the calling program.

       The class of executable object files eligible for this operation and
       the manner of their construction are implementation-defined, though
       typically such files are shared libraries or programs.

       Implementations may permit the construction of embedded dependencies
       in executable object files. In such cases, a dlopen() operation shall
       load those dependencies in addition to the executable object file
       specified by file.  Implementations may also impose specific
       constraints on the construction of programs that can employ dlopen()
       and its related services.

       A successful dlopen() shall return a symbol table handle which the
       caller may use on subsequent calls to dlsym() and dlclose().

       The value of this symbol table handle should not be interpreted in
       any way by the caller.

       The file argument is used to construct a pathname to the executable
       object file. If file contains a <slash> character, the file argument
       is used as the pathname for the file. Otherwise, file is used in an
       implementation-defined manner to yield a pathname.

       If file is a null pointer, dlopen() shall return a global symbol
       table handle for the currently running process image. This symbol
       table handle shall provide access to the symbols from an ordered set
       of executable object files consisting of the original program image
       file, any executable object files loaded at program start-up as
       specified by that process file (for example, shared libraries), and
       the set of executable object files loaded using dlopen() operations
       with the RTLD_GLOBAL flag. As the latter set of executable object
       files can change during execution, the set of symbols made available
       by this symbol table handle can also change dynamically.

       Only a single copy of an executable object file shall be brought into
       the address space, even if dlopen() is invoked multiple times in
       reference to the executable object file, and even if different
       pathnames are used to reference the executable object file.

       The mode parameter describes how dlopen() shall operate upon file
       with respect to the processing of relocations and the scope of
       visibility of the symbols provided within file.  When an executable
       object file is brought into the address space of a process, it may
       contain references to symbols whose addresses are not known until the
       executable object file is loaded.

       These references shall be relocated before the symbols can be
       accessed. The mode parameter governs when these relocations take
       place and may have the following values:

       RTLD_LAZY   Relocations shall be performed at an implementation-
                   defined time, ranging from the time of the dlopen() call
                   until the first reference to a given symbol occurs.
                   Specifying RTLD_LAZY should improve performance on
                   implementations supporting dynamic symbol binding since a
                   process might not reference all of the symbols in an
                   executable object file. And, for systems supporting
                   dynamic symbol resolution for normal process execution,
                   this behavior mimics the normal handling of process

       RTLD_NOW    All necessary relocations shall be performed when the
                   executable object file is first loaded. This may waste
                   some processing if relocations are performed for symbols
                   that are never referenced. This behavior may be useful
                   for applications that need to know that all symbols
                   referenced during execution will be available before
                   dlopen() returns.

       Any executable object file loaded by dlopen() that requires
       relocations against global symbols can reference the symbols in the
       original process image file, any executable object files loaded at
       program start-up, from the initial process image itself, from any
       other executable object file included in the same dlopen()
       invocation, and any executable object files that were loaded in any
       dlopen() invocation and which specified the RTLD_GLOBAL flag. To
       determine the scope of visibility for the symbols loaded with a
       dlopen() invocation, the mode parameter should be a bitwise-inclusive
       OR with one of the following values:

       RTLD_GLOBAL The executable object file's symbols shall be made
                   available for relocation processing of any other
                   executable object file. In addition, symbol lookup using
                   dlopen(NULL,mode) and an associated dlsym() allows
                   executable object files loaded with this mode to be

       RTLD_LOCAL  The executable object file's symbols shall not be made
                   available for relocation processing of any other
                   executable object file.

       If neither RTLD_GLOBAL nor RTLD_LOCAL is specified, the default
       behavior is unspecified.

       If an executable object file is specified in multiple dlopen()
       invocations, mode is interpreted at each invocation.

       If RTLD_NOW has been specified, all relocations shall have been
       completed rendering further RTLD_NOW operations redundant and any
       further RTLD_LAZY operations irrelevant.

       If RTLD_GLOBAL has been specified, the executable object file shall
       maintain the RTLD_GLOBAL status regardless of any previous or future
       specification of RTLD_LOCAL, as long as the executable object file
       remains in the address space (see dlclose(3p)).

       Symbols introduced into the process image through calls to dlopen()
       may be used in relocation activities. Symbols so introduced may
       duplicate symbols already defined by the program or previous dlopen()
       operations. To resolve the ambiguities such a situation might
       present, the resolution of a symbol reference to symbol definition is
       based on a symbol resolution order. Two such resolution orders are
       defined: load order and dependency order. Load order establishes an
       ordering among symbol definitions, such that the first definition
       loaded (including definitions from the process image file and any
       dependent executable object files loaded with it) has priority over
       executable object files added later (by dlopen()).  Load ordering is
       used in relocation processing. Dependency ordering uses a breadth-
       first order starting with a given executable object file, then all of
       its dependencies, then any dependents of those, iterating until all
       dependencies are satisfied. With the exception of the global symbol
       table handle obtained via a dlopen() operation with a null pointer as
       the file argument, dependency ordering is used by the dlsym()
       function. Load ordering is used in dlsym() operations upon the global
       symbol table handle.

       When an executable object file is first made accessible via dlopen(),
       it and its dependent executable object files are added in dependency
       order. Once all the executable object files are added, relocations
       are performed using load order. Note that if an executable object
       file or its dependencies had been previously loaded, the load and
       dependency orders may yield different resolutions.

       The symbols introduced by dlopen() operations and available through
       dlsym() are at a minimum those which are exported as identifiers of
       global scope by the executable object file. Typically, such
       identifiers shall be those that were specified in (for example) C
       source code as having extern linkage. The precise manner in which an
       implementation constructs the set of exported symbols for an
       executable object file is implementation-defined.

RETURN VALUE         top

       Upon successful completion, dlopen() shall return a symbol table
       handle. If file cannot be found, cannot be opened for reading, is not
       of an appropriate executable object file format for processing by
       dlopen(), or if an error occurs during the process of loading file or
       relocating its symbolic references, dlopen() shall return a null
       pointer. More detailed diagnostic information shall be available
       through dlerror().

ERRORS         top

       No errors are defined.

       The following sections are informative.

EXAMPLES         top

       Refer to dlsym(3p).



RATIONALE         top




SEE ALSO         top

       dlclose(3p), dlerror(3p), dlsym(3p)

       The Base Definitions volume of POSIX.1‐2008, dlfcn.h(0p)

COPYRIGHT         top

       Portions of this text are reprinted and reproduced in electronic form
       from IEEE Std 1003.1, 2013 Edition, Standard for Information
       Technology -- Portable Operating System Interface (POSIX), The Open
       Group Base Specifications Issue 7, Copyright (C) 2013 by the
       Institute of Electrical and Electronics Engineers, Inc and The Open
       Group.  (This is POSIX.1-2008 with the 2013 Technical Corrigendum 1
       applied.) In the event of any discrepancy between this version and
       the original IEEE and The Open Group Standard, the original IEEE and
       The Open Group Standard is the referee document. The original
       Standard can be obtained online at http://www.unix.org/online.html .

       Any typographical or formatting errors that appear in this page are
       most likely to have been introduced during the conversion of the
       source files to man page format. To report such errors, see
       https://www.kernel.org/doc/man-pages/reporting_bugs.html .

IEEE/The Open Group                 2013                          DLOPEN(3P)

Pages that refer to this page: dlfcn.h(0p)dlclose(3p)dlerror(3p)dlsym(3p)