DLOPEN(3) Linux Programmer's Manual DLOPEN(3)NAME
dladdr, dlclose, dlerror, dlopen, dlsym, dlvsym - programming interface
to dynamic linking loader
SYNOPSIS
#include <dlfcn.h>
void *dlopen(const char *filename, int flag);
char *dlerror(void);
void *dlsym(void *handle, const char *symbol);
int dlclose(void *handle);
Link with -ldl.
DESCRIPTION
The four functions dlopen(), dlsym(), dlclose(), dlerror() implement
the interface to the dynamic linking loader.
dlerror()
The function dlerror() returns a human readable string describing the
most recent error that occurred from dlopen(), dlsym() or dlclose()
since the last call to dlerror(). It returns NULL if no errors have
occurred since initialization or since it was last called.
dlopen()
The function dlopen() loads the dynamic library file named by the null-
terminated string filename and returns an opaque "handle" for the
dynamic library. If filename is NULL, then the returned handle is for
the main program. If filename contains a slash ("/"), then it is
interpreted as a (relative or absolute) pathname. Otherwise, the
dynamic linker searches for the library as follows (see ld.so(8) for
further details):
o (ELF only) If the executable file for the calling program contains
a DT_RPATH tag, and does not contain a DT_RUNPATH tag, then the
directories listed in the DT_RPATH tag are searched.
o If, at the time that the program was started, the environment vari‐
able LD_LIBRARY_PATH was defined to contain a colon-separated list
of directories, then these are searched. (As a security measure
this variable is ignored for set-user-ID and set-group-ID pro‐
grams.)
o (ELF only) If the executable file for the calling program contains
a DT_RUNPATH tag, then the directories listed in that tag are
searched.
o The cache file /etc/ld.so.cache (maintained by ldconfig(8)) is
checked to see whether it contains an entry for filename.
o The directories /lib and /usr/lib are searched (in that order).
If the library has dependencies on other shared libraries, then these
are also automatically loaded by the dynamic linker using the same
rules. (This process may occur recursively, if those libraries in turn
have dependencies, and so on.)
One of the following two values must be included in flag:
RTLD_LAZY
Perform lazy binding. Only resolve symbols as the code that
references them is executed. If the symbol is never referenced,
then it is never resolved. (Lazy binding is performed only for
function references; references to variables are always immedi‐
ately bound when the library is loaded.)
RTLD_NOW
If this value is specified, or the environment variable
LD_BIND_NOW is set to a nonempty string, all undefined symbols
in the library are resolved before dlopen() returns. If this
cannot be done, an error is returned.
Zero or more of the following values may also be ORed in flag:
RTLD_GLOBAL
The symbols defined by this library will be made available for
symbol resolution of subsequently loaded libraries.
RTLD_LOCAL
This is the converse of RTLD_GLOBAL, and the default if neither
flag is specified. Symbols defined in this library are not made
available to resolve references in subsequently loaded
libraries.
RTLD_NODELETE (since glibc 2.2)
Do not unload the library during dlclose(). Consequently, the
library's static variables are not reinitialized if the library
is reloaded with dlopen() at a later time. This flag is not
specified in POSIX.1-2001.
RTLD_NOLOAD (since glibc 2.2)
Don't load the library. This can be used to test if the library
is already resident (dlopen() returns NULL if it is not, or the
library's handle if it is resident). This flag can also be used
to promote the flags on a library that is already loaded. For
example, a library that was previously loaded with RTLD_LOCAL
can be reopened with RTLD_NOLOAD | RTLD_GLOBAL. This flag is
not specified in POSIX.1-2001.
RTLD_DEEPBIND (since glibc 2.3.4)
Place the lookup scope of the symbols in this library ahead of
the global scope. This means that a self-contained library will
use its own symbols in preference to global symbols with the
same name contained in libraries that have already been loaded.
This flag is not specified in POSIX.1-2001.
If filename is a NULL pointer, then the returned handle is for the main
program. When given to dlsym(), this handle causes a search for a sym‐
bol in the main program, followed by all shared libraries loaded at
program startup, and then all shared libraries loaded by dlopen() with
the flag RTLD_GLOBAL.
External references in the library are resolved using the libraries in
that library's dependency list and any other libraries previously
opened with the RTLD_GLOBAL flag. If the executable was linked with
the flag "-rdynamic" (or, synonymously, "--export-dynamic"), then the
global symbols in the executable will also be used to resolve refer‐
ences in a dynamically loaded library.
If the same library is loaded again with dlopen(), the same file handle
is returned. The dl library maintains reference counts for library
handles, so a dynamic library is not deallocated until dlclose() has
been called on it as many times as dlopen() has succeeded on it. The
_init() routine, if present, is called only once. But a subsequent
call with RTLD_NOW may force symbol resolution for a library earlier
loaded with RTLD_LAZY.
If dlopen() fails for any reason, it returns NULL.
dlsym()
The function dlsym() takes a "handle" of a dynamic library returned by
dlopen() and the null-terminated symbol name, returning the address
where that symbol is loaded into memory. If the symbol is not found,
in the specified library or any of the libraries that were automati‐
cally loaded by dlopen() when that library was loaded, dlsym() returns
NULL. (The search performed by dlsym() is breadth first through the
dependency tree of these libraries.) Since the value of the symbol
could actually be NULL (so that a NULL return from dlsym() need not
indicate an error), the correct way to test for an error is to call
dlerror() to clear any old error conditions, then call dlsym(), and
then call dlerror() again, saving its return value into a variable, and
check whether this saved value is not NULL.
There are two special pseudo-handles, RTLD_DEFAULT and RTLD_NEXT. The
former will find the first occurrence of the desired symbol using the
default library search order. The latter will find the next occurrence
of a function in the search order after the current library. This
allows one to provide a wrapper around a function in another shared
library.
dlclose()
The function dlclose() decrements the reference count on the dynamic
library handle handle. If the reference count drops to zero and no
other loaded libraries use symbols in it, then the dynamic library is
unloaded.
The function dlclose() returns 0 on success, and nonzero on error.
The obsolete symbols _init() and _fini()
The linker recognizes special symbols _init and _fini. If a dynamic
library exports a routine named _init(), then that code is executed
after the loading, before dlopen() returns. If the dynamic library
exports a routine named _fini(), then that routine is called just
before the library is unloaded. In case you need to avoid linking
against the system startup files, this can be done by using the gcc(1)-nostartfiles command-line option.
Using these routines, or the gcc -nostartfiles or -nostdlib options, is
not recommended. Their use may result in undesired behavior, since the
constructor/destructor routines will not be executed (unless special
measures are taken).
Instead, libraries should export routines using the __attribute__((con‐
structor)) and __attribute__((destructor)) function attributes. See
the gcc info pages for information on these. Constructor routines are
executed before dlopen() returns, and destructor routines are executed
before dlclose() returns.
Glibc extensions: dladdr() and dlvsym()
Glibc adds two functions not described by POSIX, with prototypes
#define _GNU_SOURCE /* See feature_test_macros(7) */
#include <dlfcn.h>
int dladdr(void *addr, Dl_info *info);
void *dlvsym(void *handle, char *symbol, char *version);
The function dladdr() takes a function pointer and tries to resolve
name and file where it is located. Information is stored in the
Dl_info structure:
typedef struct {
const char *dli_fname; /* Pathname of shared object that
contains address */
void *dli_fbase; /* Address at which shared object
is loaded */
const char *dli_sname; /* Name of nearest symbol with address
lower than addr */
void *dli_saddr; /* Exact address of symbol named
in dli_sname */
} Dl_info;
If no symbol matching addr could be found, then dli_sname and dli_saddr
are set to NULL.
dladdr() returns 0 on error, and nonzero on success.
The function dlvsym(), provided by glibc since version 2.1, does the
same as dlsym() but takes a version string as an additional argument.
CONFORMING TO
POSIX.1-2001 describes dlclose(), dlerror(), dlopen(), and dlsym().
NOTES
The symbols RTLD_DEFAULT and RTLD_NEXT are defined by <dlfcn.h> only
when _GNU_SOURCE was defined before including it.
Since glibc 2.2.3, atexit(3) can be used to register an exit handler
that is automatically called when a library is unloaded.
History
The dlopen interface standard comes from SunOS. That system also has
dladdr(), but not dlvsym().
BUGS
Sometimes, the function pointers you pass to dladdr() may surprise you.
On some architectures (notably i386 and x86_64), dli_fname and
dli_fbase may end up pointing back at the object from which you called
dladdr(), even if the function used as an argument should come from a
dynamically linked library.
The problem is that the function pointer will still be resolved at com‐
pile time, but merely point to the plt (Procedure Linkage Table) sec‐
tion of the original object (which dispatches the call after asking the
dynamic linker to resolve the symbol). To work around this, you can
try to compile the code to be position-independent: then, the compiler
cannot prepare the pointer at compile time anymore and today's gcc(1)
will generate code that just loads the final symbol address from the
got (Global Offset Table) at run time before passing it to dladdr().
EXAMPLE
Load the math library, and print the cosine of 2.0:
#include <stdio.h>
#include <stdlib.h>
#include <dlfcn.h>
int
main(int argc, char **argv)
{
void *handle;
double (*cosine)(double);
char *error;
handle = dlopen("libm.so", RTLD_LAZY);
if (!handle) {
fprintf(stderr, "%s\n", dlerror());
exit(EXIT_FAILURE);
}
dlerror(); /* Clear any existing error */
/* Writing: cosine = (double (*)(double)) dlsym(handle, "cos");
would seem more natural, but the C99 standard leaves
casting from "void *" to a function pointer undefined.
The assignment used below is the POSIX.1-2003 (Technical
Corrigendum 1) workaround; see the Rationale for the
POSIX specification of dlsym(). */
*(void **) (&cosine) = dlsym(handle, "cos");
if ((error = dlerror()) != NULL) {
fprintf(stderr, "%s\n", error);
exit(EXIT_FAILURE);
}
printf("%f\n", (*cosine)(2.0));
dlclose(handle);
exit(EXIT_SUCCESS);
}
If this program were in a file named "foo.c", you would build the pro‐
gram with the following command:
gcc -rdynamic -o foo foo.c -ldl
Libraries exporting _init() and _fini() will want to be compiled as
follows, using bar.c as the example name:
gcc -shared -nostartfiles -o bar bar.c
SEE ALSOld(1), ldd(1), dl_iterate_phdr(3), rtld-audit(7), ld.so(8), ldconfig(8)
ld.so info pages, gcc info pages, ld info pages
COLOPHON
This page is part of release 3.54 of the Linux man-pages project. A
description of the project, and information about reporting bugs, can
be found at http://www.kernel.org/doc/man-pages/.
Linux 2008-12-06 DLOPEN(3)
