DLOPEN(3) Linux Programmer’s Manual DLOPEN(3)
NAME
dladdr, dlclose, dlerror, dlopen, dlsym, dlvsym - programming inter-
face 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 any of the dl routines (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-UID and set-GID 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.)
The value of flag can be either RTLD_LAZY or RTLD_NOW. When RTLD_NOW
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.
Otherwise binding is lazy: symbol values are first resolved when
needed.
Optionally, RTLD_GLOBAL may be or’ed into flag, in which case the
external symbols defined in the library will be made available for
symbol resolution of subsequently loaded libraries. (The converse of
RTLD_GLOBAL is RTLD_LOCAL. This is the default.)
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 symbol 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 han-
dle 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 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 NUL-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 automatically
loaded by dlopen() when that library was loaded, dlsym() returns NULL.
(The search performed by dlsym() is breadth first through the depen-
dency 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 "-nos-
tartfiles" parameter on the command line.
Using these routines, or the gcc -nostartupfiles or -nostdlib options,
is not recommended. Their use may result in undesired behavior, since
the constructor/destructor routines will not be executed (unless spe-
cial measures are taken).
Instead, libraries should export routines using the
__attribute__((constructor)) and __attribute__((destructor)) function
attributes. See the gcc info pages for information on these. Con-
structor 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;/* File name 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 additional argument.
EXAMPLE
Load the math library, and print the cosine of 2.0:
#include <stdio.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 */
*(void **) (&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.
HISTORY
The dlopen interface standard comes from SunOS. That system also has
dladdr, but not dlvsym.
CONFORMING TO
POSIX 1003.1-2003 describes dlclose, dlerror, dlopen, dlsym.
SEE ALSO
ld(1), ldd(1), ld.so(8), ldconfig(8), ld.so info pages, gcc info
pages, ld info pages
Linux 2003-11-17 DLOPEN(3)