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);
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 con‐
tains 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 the environment variable LD_LIBRARY_PATH is defined to con‐
tain 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 programs.)
o (ELF only) If the executable file for the calling program con‐
tains 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 only performed 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 non-empty string, all undefined symbols
in the library are resolved before dlopen() returns. If this
cannot be done, an error is returned.
Zero of 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 reinitialised 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 re-opened 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 han‐
dles, 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 only called 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 non-zero 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 giving gcc the "-nostart‐
files" parameter on the command line.
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.
GNU EXTENSIONS
Glibc adds two functions not described by POSIX, with prototypes
#define _GNU_SOURCE
#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;/* Filename of defining object */
void *dli_fbase; /* Load address of that object */
const char *dli_sname;/* Name of nearest lower symbol */
void *dli_saddr; /* Exact value of nearest symbol */
} Dl_info;
dladdr() returns 0 on error, and non-zero on success.
The function dlvsym() does the same as dlsym() but takes a version
string as an additional argument.
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(1);
}
dlerror(); /* Clear any existing error */
cosine = dlsym(handle, "cos");
if ((error = dlerror()) != NULL) {
fprintf (stderr, "%s\n", error);
exit(1);
}
printf ("%f\n", (*cosine)(2.0));
dlclose(handle);
return 0;
}
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
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().
CONFORMING TO
POSIX.1-2001 describes dlclose(), dlerror(), dlopen(), and dlsym().
SEE ALSOld(1), ldd(1), dl_iterate_phdr(3), ld.so(8), ldconfig(8), ld.so info
pages, gcc info pages, ld info pages
Linux 2003-11-17 DLOPEN(3)
