.\" Copyright (c) 2001-2003 The Open Group, All Rights Reserved .TH "DLOPEN" P 2003 "IEEE/The Open Group" "POSIX Programmer's Manual" .\" dlopen .SH NAME dlopen \- gain access to an executable object file .SH SYNOPSIS .LP \fB#include .br .sp void *dlopen(const char *\fP\fIfile\fP\fB, int\fP \fImode\fP\fB); \fP \fB .br \fP .SH DESCRIPTION .LP The \fIdlopen\fP() function shall make an executable object file specified by \fIfile\fP available to the calling program. The class of files eligible for this operation and the manner of their construction are implementation-defined, though typically such files are executable objects such as shared libraries, relocatable files, or programs. Note that some implementations permit the construction of dependencies between such objects that are embedded within files. In such cases, a \fIdlopen\fP() operation shall load such dependencies in addition to the object referenced by \fIfile\fP. Implementations may also impose specific constraints on the construction of programs that can employ \fIdlopen\fP() and its related services. .LP A successful \fIdlopen\fP() shall return a \fIhandle\fP which the caller may use on subsequent calls to \fIdlsym\fP() and \fIdlclose\fP(). The value of this \fIhandle\fP should not be interpreted in any way by the caller. .LP The \fIfile\fP argument is used to construct a pathname to the object file. If \fIfile\fP contains a slash character, the \fIfile\fP argument is used as the pathname for the file. Otherwise, \fIfile\fP is used in an implementation-defined manner to yield a pathname. .LP If the value of \fIfile\fP is 0, \fIdlopen\fP() shall provide a \fIhandle\fP on a global symbol object. This object shall provide access to the symbols from an ordered set of objects consisting of the original program image file, together with any objects loaded at program start-up as specified by that process image file (for example, shared libraries), and the set of objects loaded using a \fIdlopen\fP() operation together with the RTLD_GLOBAL flag. As the latter set of objects can change during execution, the set identified by \fIhandle\fP can also change dynamically. .LP Only a single copy of an object file is brought into the address space, even if \fIdlopen\fP() is invoked multiple times in reference to the file, and even if different pathnames are used to reference the file. .LP The \fImode\fP parameter describes how \fIdlopen\fP() shall operate upon \fIfile\fP with respect to the processing of relocations and the scope of visibility of the symbols provided within \fIfile\fP. When an object is brought into the address space of a process, it may contain references to symbols whose addresses are not known until the object is loaded. These references shall be relocated before the symbols can be accessed. The \fImode\fP parameter governs when these relocations take place and may have the following values: .TP 7 RTLD_LAZY Relocations shall be performed at an implementation-defined time, ranging from the time of the \fIdlopen\fP() call until the first reference to a given symbol occurs. Specifying RTLD_LAZY should improve performance on implementations supporting dynamic symbol binding as a process may not reference all of the functions in any given object. And, for systems supporting dynamic symbol resolution for normal process execution, this behavior mimics the normal handling of process execution. .TP 7 RTLD_NOW All necessary relocations shall be performed when the object is first loaded. This may waste some processing if relocations are performed for functions that are never referenced. This behavior may be useful for applications that need to know as soon as an object is loaded that all symbols referenced during execution are available. .sp .LP Any object loaded by \fIdlopen\fP() that requires relocations against global symbols can reference the symbols in the original process image file, any objects loaded at program start-up, from the object itself as well as any other object included in the same \fIdlopen\fP() invocation, and any objects that were loaded in any \fIdlopen\fP() invocation and which specified the RTLD_GLOBAL flag. To determine the scope of visibility for the symbols loaded with a \fIdlopen\fP() invocation, the \fImode\fP parameter should be a bitwise-inclusive OR with one of the following values: .TP 7 RTLD_GLOBAL The object's symbols shall be made available for the relocation processing of any other object. In addition, symbol lookup using \fIdlopen\fP(\fI0, mode\fP) and an associated \fIdlsym\fP() allows objects loaded with this \fImode\fP to be searched. .TP 7 RTLD_LOCAL The object's symbols shall not be made available for the relocation processing of any other object. .sp .LP If neither RTLD_GLOBAL nor RTLD_LOCAL are specified, then an implementation-defined default behavior shall be applied. .LP If a \fIfile\fP is specified in multiple \fIdlopen\fP() invocations, \fImode\fP is interpreted at each invocation. Note, however, that once RTLD_NOW has been specified all relocations shall have been completed rendering further RTLD_NOW operations redundant and any further RTLD_LAZY operations irrelevant. Similarly, note that once RTLD_GLOBAL has been specified the object shall maintain the RTLD_GLOBAL status regardless of any previous or future specification of RTLD_LOCAL, as long as the object remains in the address space (see \fIdlclose\fP() ). .LP Symbols introduced into a program through calls to \fIdlopen\fP() may be used in relocation activities. Symbols so introduced may duplicate symbols already defined by the program or previous \fIdlopen\fP() 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: \fIload\fP or \fIdependency\fP ordering. Load order establishes an ordering among symbol definitions, such that the definition first loaded (including definitions from the image file and any dependent objects loaded with it) has priority over objects added later (via \fIdlopen\fP()). Load ordering is used in relocation processing. Dependency ordering uses a breadth-first order starting with a given object, then all of its dependencies, then any dependents of those, iterating until all dependencies are satisfied. With the exception of the global symbol object obtained via a \fIdlopen\fP() operation on a \fIfile\fP of 0, dependency ordering is used by the \fIdlsym\fP() function. Load ordering is used in \fIdlsym\fP() operations upon the global symbol object. .LP When an object is first made accessible via \fIdlopen\fP() it and its dependent objects are added in dependency order. Once all the objects are added, relocations are performed using load order. Note that if an object or its dependencies had been previously loaded, the load and dependency orders may yield different resolutions. .LP The symbols introduced by \fIdlopen\fP() operations and available through \fIdlsym\fP() are at a minimum those which are exported as symbols of global scope by the object. Typically such symbols shall be those that were specified in (for example) C source code as having \fIextern\fP linkage. The precise manner in which an implementation constructs the set of exported symbols for a \fIdlopen\fP() object is specified by that implementation. .SH RETURN VALUE .LP If \fIfile\fP cannot be found, cannot be opened for reading, is not of an appropriate object format for processing by \fIdlopen\fP(), or if an error occurs during the process of loading \fIfile\fP or relocating its symbolic references, \fIdlopen\fP() shall return NULL. More detailed diagnostic information shall be available through \fIdlerror\fP() . .SH ERRORS .LP No errors are defined. .LP \fIThe following sections are informative.\fP .SH EXAMPLES .LP None. .SH APPLICATION USAGE .LP None. .SH RATIONALE .LP None. .SH FUTURE DIRECTIONS .LP None. .SH SEE ALSO .LP \fIdlclose\fP() , \fIdlerror\fP() , \fIdlsym\fP() , the Base Definitions volume of IEEE\ Std\ 1003.1-2001, \fI\fP .SH COPYRIGHT Portions of this text are reprinted and reproduced in electronic form from IEEE Std 1003.1, 2003 Edition, Standard for Information Technology -- Portable Operating System Interface (POSIX), The Open Group Base Specifications Issue 6, Copyright (C) 2001-2003 by the Institute of Electrical and Electronics Engineers, Inc and The Open Group. 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.opengroup.org/unix/online.html .