DLOPEN(3)

NAME
dlopen, dlsym, dlerror, dlclose - programmatic interface to the dynamic linker

SYNOPSIS
#include <dlfcn.h>
void *
dlopen(const char *path, int mode);
void *
dlsym(void * restrict handle, const char * restrict symbol);
const char *
dlerror(void);
int
dlclose(void *handle);

DESCRIPTION
These functions provide a simple programmatic interface to the services of the dynamic linker. Operations are provided to add new shared objects to a program's address space, to obtain the address bindings of symbols defined by such objects, and to remove such objects when their use is no longer required. All these functions use the native dyld(3), NSModule(3), and NSObjectFileImage(3) functions to provide a compatibility library so that common unix source code may be easily compiled.

New code should use the native functions, not this compatibility layer, see the above manual pages for usage.

dlopen() provides access to the shared object in path, returning a descriptor that can be used for later references to the object in calls to dlsym() and dlclose(). If path was not in the address space prior to the call to dlopen(), it is placed in the address space. When an object is first loaded into the address space in this way, its function _init(), if any, is called. If path has already been placed in the address space in a previous call to dlopen(), it is not added a second time, although a reference count of dlopen() operations on path is maintained. A null pointer supplied for path is interpreted as a reference to the main executable of the process. mode controls the way in which external function references from the loaded object are bound to their referents. It must contain one of the following values, possibly ORed with additional flags which will be described subsequently:

RTLD_LAZY
    Each external function reference is resolved when the function is first called.

RTLD_NOW
    All external function references are bound immediately by dlopen().

RTLD_LAZY is normally preferred, for reasons of efficiency. However, RTLD_NOW is useful to ensure that any undefined symbols are discovered during the call to dlopen().

One of the following flags may be ORed into the mode argument:

RTLD_GLOBAL
    Symbols from this shared object and its directed acyclic graph (DAG) of needed objects will be available for resolving undefined references from all other shared objects. This is the only opion available for objects linked with the -dylib flag to ld(1)

RTLD_LOCAL
    Symbols in this shared object and its DAG of needed objects will be available for resolving undefined references only from other objects in the same DAG. This is the default for objects linked with the -bundle flag to ld(1) but it may be set explicitly with this flag.

If dlopen() fails, it returns a null pointer, and sets an error condition which may be interrogated with dlerror().

The dlsym() returns the address binding of the symbol described in the null-terminated character string symbol, as it occurs in the shared object identified by handle. The symbols exported by objects added to the address space by dlopen() can be accessed only through calls to dlsym(). Such symbols do not supersede any definition of those symbols already present in the address space when the object is loaded, nor are they available to satisfy normal dynamic linking references.

See the note about underscores and C symbols, dlsym() may or may not need to be called with an underscore prepended to the C function name, depending upon how this package was configured.

If dlsym() is called with the special handle RTLD_NEXT, then the search for the symbol is limited to the shared objects which were loaded after the one issuing the call to dlsym(). Thus, if the function is called from the main program, all the shared libraries are searched. If it is called from a shared library, all subsequent shared libraries are searched. RTLD_NEXT is useful for implementing wrappers around library functions. For example, a wrapper function getpid() could access the “real” getpid() with dlsym(RTLD_NEXT, "getpid").

If dlsym() is called with the special handle RTLD_SELF, then the search for the symbol is limited to the shared object issuing the call to dlsym() and those shared objects which were loaded after it.

The dlsym() function returns a null pointer if the symbol cannot be found, and sets an error condition which may be queried with dlerror().

The dlerror() returns a null-terminated character string describing the last error that occurred during a call to dlopen(), dlsym(), or dlclose(). If no such error has occurred, dlerror() returns a null pointer. At each call to dlerror(), the error indication is reset. Thus in the case of two calls to dlerror(), where the second call follows the first immediately, the second call will always return a null pointer.

dlclose() deletes a reference to the shared object referenced by handle. If the reference count drops to 0, the object is removed from the address space, and handle is rendered invalid. Just before removing a shared object in this way, the object's _fini() function is called, if such a function is defined by the object. If dlclose() is successful, it returns a value of 0. Otherwise it returns -1, and sets an error condition that can be interrogated with dlerror().

The object-intrinsic functions _init() and _fini() are called with no arguments, and are not expected to return values.

NOTES
In darwin objects, C symbols have an underscore prepended to the symbol name. So, the function getpid() ends up in the object file as _getpid Most code assumes that there is no underscore and calls dlsym() with symbol as "getpid" which would fail to find the symbol.

Depending on how the dlcompat package was built, the dlsym() function may, or may not, automatically prepend an underscore to the passed symbol to compensate for this. In this case calling dlsym() with symbol as "getpid" does %%NOTPREPEND%% find the _getpid symbol.

ERRORS
dlopen() and dlsym() return a null pointer in the event of errors. dlclose() returns 0 on success, or -1 if an error occurred. Whenever an error has been detected, a message detailing it can be retrieved via a call to dlerror().

AUTHORS
The dlcompat package was written by Jorge Acereda <jacereda@users.sourceforge.net> and Peter O'Gorman <ogorman@users.sourceforge.net>

This man page was borrowed from FreeBSD and modified, it originally came from Sun Microsystems, Inc.

BUGS

•   None of these rountines should be expected to work as advertised with MH_DYLIB files. They are always opened globally and may not be closed, RTLD_NOW is ignored for them.

•   The search for symbols in objects which were loaded as a result of loading handle terminates too soon.

•   None of the functions work as advertised for executables linked with the ld(1) flag -force_flat_namespace, or when the dyld(1) DYLD_FORCE_FLAT_NAMESPACE environment variable is set. This is because the NSLookupSymbolInImage() function ignores the passed in mach_header in this case.

•   The underscore or not issue is unnecessarily confusing.

If you find any more, please contact Peter O'Gorman <ogorman@users.sourceforge.net>

SEE ALSO
ld(1) cc(1) NSModule(3) NSObjectFileImage(3) dyld(3)