dlopen - open a symbol table handle
void *dlopen(const char *file, int mode);
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:
- 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.
- 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:
- 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.
- 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).
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.
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().
No errors are defined.
Refer to dlsym.
dlclose, dlerror, dlsym
First released in Issue 5.
IEEE Std 1003.1-2001/Cor 2-2004, item XSH/TC2/D6/21 is applied, changing the default behavior in the DESCRIPTION when neither RTLD_GLOBAL nor RTLD_LOCAL are specified from implementation-defined to unspecified.
The dlopen() function is moved from the XSI option to the Base.
The EXAMPLES section is updated to refer to dlsym.
POSIX.1-2008, Technical Corrigendum 1, XSH/TC1-2008/0073  is applied.
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