открыть дескриптор таблицы символов (open a symbol table handle)
Пролог (Prolog)
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.
Имя (Name)
dlopen — open a symbol table handle
Синопсис (Synopsis)
#include <dlfcn.h>
void *dlopen(const char *file, int mode);
Описание (Description)
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 execution.
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 searched.
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.
Возвращаемое значение (Return value)
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().
Ошибки (Error)
No errors are defined.
The following sections are informative.
Примеры (Examples)
Refer to dlsym(3p).
Использование в приложениях (Application usage)
None.
Обоснование (Rationale)
None.
Будущие направления (Future directions)
None.
Смотри также (See also)
dlclose(3p), dlerror(3p), dlsym(3p)
The Base Definitions volume of POSIX.1‐2017, dlfcn.h(0p)
