libc/gen/dlopen.3

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.\" @(#) dlopen.3 1.6 90/01/31 SMI
.\" $FreeBSD: head/lib/libc/gen/dlopen.3 211397 2010-08-16 15:18:30Z joel $
.\"
.Dd April 6, 2012
.Dt DLOPEN 3
.Os
.Sh NAME
.Nm dlopen ,
.Nm fdlopen
.Nd returns handle to dynamically loaded shared object
.Sh LIBRARY
This function is not in a library.
It is included in every dynamically linked program automatically.
.Sh SYNOPSIS
.In dlfcn.h
.Ft void *
.Fn dlopen "const char *name" "int mode"
.Ft void *
.Fn fdlopen "int fd" "int mode"
.Sh DESCRIPTION
The
.Fn dlopen
function
provides access to the shared object in
.Fa name ,
returning a descriptor that can be used for later
references to the object in calls to other dl functions.
If
.Fa name
was not in the address space prior to the call to
.Fn dlopen ,
it is placed in the address space.
When an object is first loaded into the address space in this way, its
function
.Fn _init ,
if any, is called by the dynamic linker.
If
.Fa name
has already been placed in the address space in a previous call to
.Fn dlopen ,
it is not added a second time, although a reference count of
.Fn dlopen
operations on
.Fa name
is maintained.
A null pointer supplied for
.Fa name
is interpreted as a reference to the main
executable of the process.
The
.Fa 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:
.Bl -tag -width ".Dv RTLD_NODELETE"
.It Dv RTLD_LAZY
Each external function reference is resolved when the function is first
called.
.It Dv RTLD_NOW
All external function references are bound immediately by
.Fn dlopen .
.El
.Pp
.Dv RTLD_LAZY
is normally preferred, for reasons of efficiency.
However,
.Dv RTLD_NOW
is useful to ensure that any undefined symbols are discovered during the
call to
.Fn dlopen .
.Pp
One of the following flags may be ORed into the
.Fa mode
argument:
.Bl -tag -width ".Dv RTLD_NODELETE"
.It Dv 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.
.It Dv 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.
.It Dv 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
.Fn dlopen
will return to the caller only in the case of error.
.It Dv RTLD_NODELETE
Prevents unload of the loaded object on
.Fn dlclose .
The same behaviour may be requested by
.Fl "z nodelete"
option of the static linker
.Xr ld 1 .
.It Dv RTLD_NOLOAD
Only return valid handle for the object if it is already loaded in
the process address space, otherwise
.Dv NULL
is returned.
Other mode flags may be specified, which will be applied for promotion
for the found object.
.El
.Pp
If
.Fn dlopen
fails, it returns a null pointer, and sets an error condition which may
be interrogated with
.Fn dlerror .
.Pp
The
.Fn fdlopen
function is similar to
.Fn dlopen ,
but it takes the file descriptor argument
.Fa fd ,
which is used for the file operations needed to load an object
into the address space.
The file descriptor
.Fa 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
.Xr lockf 3
lock is held on the passed descriptor.
The
.Fa fd
argument -1 is interpreted as a reference to the main
executable of the process, similar to
.Va NULL
value for the
.Fa name
argument to
.Fn dlopen .
The
.Fn 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.
.Sh RETURN VALUES
The functions return a null pointer in the event of an error.
Whenever an error has been detected, a message detailing it can be
retrieved via a call to
.Fn dlerror .
.Sh EXAMPLES
The following program will open any shared gcc library found
and display the directory in which it was found using the
dfinfo function.
.Bd -literal
#include <dlfcn.h>
#include <stdlib.h>
#include <stdio.h>

int
main (int argc, char *argv[])
{
    void *handle;
    int   result;
    char origin[256];

    /* open shared gcc library  */
    handle = dlopen("libgcc_s.so", RTLD_LAZY);
    if (!handle) {
       fprintf (stderr, "%s\en", dlerror ());
       exit (EXIT_FAILURE);
    }

    /* get information about the library origin */
    result = dlinfo (handle, RTLD_DI_ORIGIN, (void *)&origin);
    if (result < 0) {
       fprintf (stderr, "%s\en", dlerror ());
       dlclose (handle);
       exit (EXIT_FAILURE);
    }

    /* Display the origin */
    printf ("libgcc_s origin is %s\en", &origin[0]);
    dlclose (handle);

    exit(EXIT_SUCCESS);
}
.Ed
.Sh SEE ALSO
.Xr rtld 1 ,
.Xr dlclose 3 ,
.Xr dlerror 3 ,
.Xr dlfcn 3 ,
.Xr dlinfo 3