man-pages/man3/dlopen.3

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.\" Modified by David A. Wheeler <dwheeler@dwheeler.com> 2000-11-28.
.\" Applied patch by Terran Melconian, aeb, 2001-12-14.
.\" Modified by Hacksaw <hacksaw@hacksaw.org> 2003-03-13.
.\" Modified by Matt Domsch, 2003-04-09: _init and _fini obsolete
.\" Modified by Michael Kerrisk <mtk.manpages@gmail.com> 2003-05-16.
.\" Modified by Walter Harms: dladdr, dlvsym
.\" Modified by Petr Baudis <pasky@suse.cz>, 2008-12-04: dladdr caveat
.\"
.TH DLOPEN 3 2008-12-06 "Linux" "Linux Programmer's Manual"
.SH NAME
dladdr, dlclose, dlerror, dlopen, dlsym, dlvsym \- programming interface to
dynamic linking loader
.SH SYNOPSIS
.B #include <dlfcn.h>
.sp
.BI "void *dlopen(const char *" filename ", int " flag );
.sp
.B "char *dlerror(void);"
.sp
.BI "void *dlsym(void *" handle ", const char *" symbol );
.sp
.BI "int dlclose(void *" handle );
.sp
Link with \fI\-ldl\fP.
.SH DESCRIPTION
The four functions
.BR dlopen (),
.BR dlsym (),
.BR dlclose (),
.BR dlerror ()
implement the interface to the dynamic linking loader.
.SS dlerror()
The function
.BR dlerror ()
returns a human readable string describing the most recent error
that occurred from
.BR dlopen (),
.BR dlsym ()
or
.BR dlclose ()
since the last call to
.BR dlerror ().
It returns NULL if no errors have occurred since initialization or since
it was last called.
.SS dlopen()
The function
.BR dlopen ()
loads the dynamic library file named by the null-terminated
string
.I filename
and returns an opaque "handle" for the dynamic library.
If
.I filename
is NULL, then the returned handle is for the main program.
If
.I 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
.BR ld.so (8)
for further details):
.IP o 4
(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.
.IP o
If, at the time that the program was started, the environment variable
.B 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 programs.)
.IP 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.
.IP o
The cache file
.I /etc/ld.so.cache
(maintained by
.BR ldconfig (8))
is checked to see whether it contains an entry for
.IR filename .
.IP o
The directories
.I /lib
and
.I /usr/lib
are searched (in that order).
.PP
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.)
.PP
One of the following two values must be included in
.IR flag :
.TP
.B 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 immediately bound when
the library is loaded.)
.TP
.B RTLD_NOW
If this value is specified, or the environment variable
.B LD_BIND_NOW
is set to a nonempty string,
all undefined symbols in the library are resolved before
.BR dlopen ()
returns.
If this cannot be done, an error is returned.
.PP
Zero or more of the following values may also be ORed in
.IR flag :
.TP
.B RTLD_GLOBAL
The symbols defined by this library will be
made available for symbol resolution of subsequently loaded libraries.
.TP
.B RTLD_LOCAL
This is the converse of
.BR 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.
.TP
.BR RTLD_NODELETE " (since glibc 2.2)"
Do not unload the library during
.BR dlclose ().
Consequently, the library's static variables are not reinitialized
if the library is reloaded with
.BR dlopen ()
at a later time.
This flag is not specified in POSIX.1-2001.
.\" (But it is present on Solaris.)
.TP
.BR RTLD_NOLOAD " (since glibc 2.2)"
Don't load the library.
This can be used to test if the library is already resident
.RB ( 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
.B RTLD_LOCAL
can be reopened with
.BR RTLD_NOLOAD\ |\ RTLD_GLOBAL .
This flag is not specified in POSIX.1-2001.
.\" (But it is present on Solaris.)
.\"
.TP
.BR RTLD_DEEPBIND " (since glibc 2.3.4)"
.\" Inimitably described by UD in
.\" http://sources.redhat.com/ml/libc-hacker/2004-09/msg00083.html.
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.
.PP
If
.I filename
is a NULL pointer, then the returned handle is for the main program.
When given to
.BR 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
.BR dlopen ()
with the flag
.BR RTLD_GLOBAL .
.PP
External references in the library are resolved using the libraries
in that library's dependency list and any other libraries previously
opened with the
.B 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 references in a dynamically loaded library.
.PP
If the same library is loaded again with
.BR dlopen (),
the same file handle is returned.
The dl library maintains reference
counts for library handles, so a dynamic library is not
deallocated until
.BR dlclose ()
has been called on it as many times as
.BR dlopen ()
has succeeded on it.
The
.BR _init ()
routine, if present, is only called once.
But a subsequent call with
.B RTLD_NOW
may force symbol resolution for a library earlier loaded with
.BR RTLD_LAZY .
.PP
If
.BR dlopen ()
fails for any reason, it returns NULL.
.SS dlsym()
The function
.BR dlsym ()
takes a "handle" of a dynamic library returned by
.BR 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 automatically loaded by
.BR dlopen ()
when that library was loaded,
.BR dlsym ()
returns NULL.
(The search performed by
.BR 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
.BR dlsym ()
need not indicate an error), the correct way to test for an error
is to call
.BR dlerror ()
to clear any old error conditions, then call
.BR dlsym (),
and then call
.BR dlerror ()
again, saving its return value into a variable, and check whether
this saved value is not NULL.
.PP
There are two special pseudo-handles,
.B RTLD_DEFAULT
and
.BR 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.
.SS dlclose()
The function
.BR dlclose ()
decrements the reference count on the dynamic library handle
.IR handle .
If the reference count drops to zero and no other loaded libraries use
symbols in it, then the dynamic library is unloaded.
.LP
The function
.BR dlclose ()
returns 0 on success, and nonzero on error.
.SS The obsolete symbols _init() and _fini()
The linker recognizes special symbols
.B _init
and
.BR _fini .
If a dynamic library exports a routine named
.BR _init (),
then that code is executed after the loading, before
.BR dlopen ()
returns.
If the dynamic library exports a routine named
.BR _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
.BR gcc (1)
.I \-nostartfiles
command-line option.
.LP
Using these routines, or the gcc
.B \-nostartfiles
or
.B \-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).
.\" void _init(void) __attribute__((constructor));
.\" void _fini(void) __attribute__((destructor));
.LP
Instead, libraries should export routines using the
.B __attribute__((constructor))
and
.B __attribute__((destructor))
function attributes.
See the gcc info pages for information on these.
Constructor routines are executed before
.BR dlopen ()
returns, and destructor routines are executed before
.BR dlclose ()
returns.
.SS Glibc extensions: dladdr() and dlvsym()
Glibc adds two functions not described by POSIX, with prototypes
.sp
.nf
.BR "#define _GNU_SOURCE" "         /* See feature_test_macros(7) */"
.B #include <dlfcn.h>
.sp
.BI "int dladdr(void *" addr ", Dl_info *" info );
.sp
.BI "void *dlvsym(void *" handle ", char *" symbol ", char *" version );
.fi
.PP
The function
.BR dladdr ()
takes a function pointer and tries to resolve name
and file where it is located.
Information is stored in the
.I Dl_info
structure:
.sp
.in +4n
.nf
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 \fIaddr\fP */
    void       *dli_saddr;  /* Exact address of symbol named
                               in \fIdli_sname\fP */
} Dl_info;
.fi
.in
.PP
If no symbol matching
.I addr
could be found, then
.I dli_sname
and
.I dli_saddr
are set to NULL.
.PP
.BR dladdr ()
returns 0 on error, and nonzero on success.
.PP
The function
.BR dlvsym (),
provided by glibc since version 2.1,
does the same as
.BR dlsym ()
but takes a version string as an additional argument.
.SH CONFORMING TO
POSIX.1-2001 describes
.BR dlclose (),
.BR dlerror (),
.BR dlopen (),
and
.BR dlsym ().
.SH NOTES
The symbols
.B RTLD_DEFAULT
and
.B RTLD_NEXT
are defined by
.I <dlfcn.h>
only when
.B _GNU_SOURCE
was defined before including it.
.\" .LP
.\" The string returned by
.\" .BR dlerror ()
.\" should not be modified.
.\" Some systems give the prototype as
.\" .sp
.\" .in +5
.\" .B "const char *dlerror(void);"
.\" .in

Since glibc 2.2.3,
.BR atexit (3)
can be used to register an exit handler that is automatically
called when a library is unloaded.
.SS History
The dlopen interface standard comes from SunOS.
That system also has
.BR dladdr (),
but not
.BR dlvsym ().
.SH BUGS
Sometimes, the function pointers you pass to
.BR dladdr ()
may surprise you.
On some architectures (notably i386 and x86_64),
.I dli_fname
and
.I dli_fbase
may end up pointing back at the object from which you called
.BR dladdr (),
even if the function used as an argument should come from
a dynamically linked library.
.PP
The problem is that the function pointer will still be resolved
at compile time, but merely point to the
.I plt
(Procedure Linkage Table)
section 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
.BR gcc (1)
will generate code that just loads the final symbol address from the
.I got
(Global Offset Table) at run time before passing it to
.BR dladdr ().
.SH EXAMPLE
Load the math library, and print the cosine of 2.0:
.nf

#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\en", 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");
.\" But in fact "gcc -O2 -Wall" will complain about the preceding cast.

    if ((error = dlerror()) != NULL)  {
        fprintf(stderr, "%s\en", error);
        exit(EXIT_FAILURE);
    }

    printf("%f\en", (*cosine)(2.0));
    dlclose(handle);
    exit(EXIT_SUCCESS);
}
.fi
.PP
If this program were in a file named "foo.c", you would build the program
with the following command:
.in +4n
.LP
    gcc \-rdynamic \-o foo foo.c \-ldl
.in
.PP
Libraries exporting
.BR _init ()
and
.BR _fini ()
will want to be compiled as
follows, using \fIbar.c\fP as the example name:
.in +4n
.LP
    gcc \-shared \-nostartfiles \-o bar bar.c
.in
.SH SEE ALSO
.BR ld (1),
.BR ldd (1),
.BR dl_iterate_phdr (3),
.BR rtld-audit (7),
.BR ld.so (8),
.BR ldconfig (8)

ld.so info pages, gcc info pages, ld info pages