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DLOPEN(3) FreeBSD Library Functions Manual DLOPEN(3)

dlopen, fdlopen, dlsym, dlvsym, dlfunc, dlerror, dlclose
programmatic interface to the dynamic linker

Standard C Library (libc, -lc)

#include <dlfcn.h>

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

void *
fdlopen(int fd, int mode);

void *
dlsym(void * restrict handle, const char * restrict symbol);

void *
dlvsym(void * restrict handle, const char * restrict symbol, const char * restrict version);

dlfunc(void * restrict handle, const char * restrict symbol);

char *

dlclose(void *handle);

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.

The dlopen() function 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(), dlvsym() 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 by the dynamic linker. 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. The mode argument 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:

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

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.
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, but it may be specified explicitly with this flag.
When set, causes dynamic linker to exit after loading all objects needed by this shared object and printing a summary which includes the absolute pathnames of all objects, to standard output. With this flag dlopen() will return to the caller only in the case of error.
Prevents unload of the loaded object on dlclose(). The same behaviour may be requested by -z nodelete option of the static linker ld(1).
Only return valid handle for the object if it is already loaded in the process address space, otherwise NULL is returned. Other mode flags may be specified, which will be applied for promotion for the found object.
Symbols from the loaded library are put before global symbols when resolving symbolic references originated from the library.

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

The fdlopen() function is similar to dlopen(), but it takes the file descriptor argument fd, which is used for the file operations needed to load an object into the address space. The file descriptor fd is not closed by the function regardless a result of execution, but a duplicate of the file descriptor is. This may be important if a lockf(3) lock is held on the passed descriptor. The fd argument -1 is interpreted as a reference to the main executable of the process, similar to NULL value for the name argument to dlopen(). The fdlopen() function can be used by the code that needs to perform additional checks on the loaded objects, to prevent races with symlinking or renames.

The dlsym() function 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.

If dlsym() is called with the special handle NULL, it is interpreted as a reference to the executable or shared object from which the call is being made. Thus a shared object can reference its own symbols.

If dlsym() is called with the special handle RTLD_DEFAULT, the search for the symbol follows the algorithm used for resolving undefined symbols when objects are loaded. The objects searched are as follows, in the given order:

  1. The referencing object itself (or the object from which the call to dlsym() is made), if that object was linked using the -Bsymbolic option to ld(1).
  2. All objects loaded at program start-up.
  3. All objects loaded via dlopen() with the RTLD_GLOBAL flag set in the mode argument.
  4. All objects loaded via dlopen() which are in needed-object DAGs that also contain the referencing object.

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"). (Actually, the dlfunc() interface, below, should be used, since getpid() is a function and not a data object.)

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 dlvsym() function behaves like dlsym(), but takes an extra argument version: a null-terminated character string which is used to request a specific version of symbol.

The dlfunc() function implements all of the behavior of dlsym(), but has a return type which can be cast to a function pointer without triggering compiler diagnostics. (The dlsym() function returns a data pointer; in the C standard, conversions between data and function pointer types are undefined. Some compilers and lint(1) utilities warn about such casts.) The precise return type of dlfunc() is unspecified; applications must cast it to an appropriate function pointer type.

The dlerror() function returns a null-terminated character string describing the last error that occurred during a call to dlopen(), dladdr(), dlinfo(), dlsym(), dlvsym(), dlfunc(), 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.

The dlclose() function 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 dynamic linker calls the object's _fini() function, 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.

ELF executables need to be linked using the -export-dynamic option to ld(1) for symbols defined in the executable to become visible to dlsym(), dlvsym() or dlfunc()

Other ELF platforms require linking with Dynamic Linker Services Filter (libdl, -ldl) to provide dlopen() and other functions. FreeBSD does not require linking with the library, but supports it for compatibility.

In previous implementations, it was necessary to prepend an underscore to all external symbols in order to gain symbol compatibility with object code compiled from the C language. This is still the case when using the (obsolete) -aout option to the C language compiler.

The dlopen(), fdlopen(), dlsym(), dlvsym(), and dlfunc() functions return a null pointer in the event of errors. The dlclose() function 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().

ld(1), rtld(1), dladdr(3), dlinfo(3), link(5)
May 14, 2020 FreeBSD 13.1-RELEASE

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