DLOPEN(3) FreeBSD Library Functions Manual DLOPEN(3)
NAME
dlopen, fdlopen, dlsym, dlvsym, dlfunc, dlerror, dlclose - programmatic
interface to the dynamic linker
LIBRARY
Standard C Library (libc, -lc)
SYNOPSIS
#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_t
dlfunc(void * restrict handle, const char * restrict symbol);
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.
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:
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.
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, but it may be specified explicitly with
this flag.
RTLD_TRACE 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.
RTLD_NODELETE Prevents unload of the loaded object on dlclose(). The
same behaviour may be requested by -z nodelete option of
the static linker ld(1).
RTLD_NOLOAD 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.
RTLD_DEEPBIND 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.
NOTES
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.
ERRORS
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().
SEE ALSO
ld(1), rtld(1), dladdr(3), dlinfo(3), link(5)
FreeBSD 13.1-RELEASE-p6 May 14, 2020 FreeBSD 13.1-RELEASE-p6
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