dlopen.3.in

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.\" Sun Microsystems, Inc.
.\" 2550 Garcia Avenue
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.\" Copyright (c) 1991 Sun Microsystems, Inc.
.\"
.\" @(#) dlopen.3 1.6 90/01/31 SMI
.\" $FreeBSD: src/lib/libc/gen/dlopen.3,v 1.21 2002/05/29 19:35:13 wollman Exp $
.\"
.Dd June 8, 2003
.Os
.Dt DLOPEN 3
.Sh NAME
.Nm dlopen ,
.Nm dlsym ,
.\" .Nm dlfunc ,
.Nm dlerror ,
.Nm dlclose
.Nd programmatic interface to the dynamic linker
.Sh SYNOPSIS
.In dlfcn.h
.Ft void *
.Fn dlopen "const char *path" "int mode"
.Ft void *
.Fn dlsym "void * restrict handle" "const char * restrict symbol"
.\" .Ft dlfunc_t
.\" .Fn dlfunc "void * restrict handle" "const char * restrict symbol"
.Ft const char *
.Fn dlerror "void"
.Ft int
.Fn dlclose "void *handle"
.Sh DESCRIPTION
These functions provide a simple programmatic interface to the services of the
dynamic linker.
Operations are provided to add new shared objects to a
program's address space, to obtain the address bindings of symbols
defined by such
objects, and to remove such objects when their use is no longer required.
All these functions use the native
.Xr dyld 3 , 
.Xr NSModule 3 ,
and
.Xr NSObjectFileImage 3
functions to provide a compatibility library so that common unix source code may be
easily compiled.
.Pp
New code should use the native functions, not this compatibility layer, see the above
manual pages for usage.
.Pp
.Fn dlopen
provides access to the shared object in
.Fa path ,
returning a descriptor that can be used for later
references to the object in calls to
.Fn dlsym
and
.Fn dlclose .
If
.Fa path
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.
If
.Fa path
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 path
is maintained.
A null pointer supplied for
.Fa path
is interpreted as a reference to the main
executable of the process.
.Fa mode
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 RTLD_LAZYX
.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 RTLD_GLOBALX
.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. This is the only opion available for objects
linked with the -dylib flag to
.Xr ld 1
.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 for objects linked with the -bundle
flag to
.Xr ld 1
but it may be set explicitly with this flag.
.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 dlsym
returns the address binding of the symbol described in the null-terminated
character string
.Fa symbol ,
as it occurs in the shared object identified by
.Fa handle .
The symbols exported by objects added to the address space by
.Fn dlopen
can be accessed only through calls to
.Fn dlsym .
Such symbols do not supersede any definition of those symbols already present
in the address space when the object is loaded, nor are they available to
satisfy normal dynamic linking references.
.Pp
See the note about underscores and C symbols,
.Fn dlsym
.\" and
.\" .Fn dlfunc
may or may not need to be called with an underscore prepended to the C function
name, depending upon how this package was configured.
.Pp
If
.Fn dlsym
is called with the special
.Fa handle
.Dv RTLD_NEXT ,
then the search for the symbol is limited to the shared objects
which were loaded after the one issuing the call to
.Fn dlsym .
Thus, if the function is called from the main program, all
the shared libraries are searched.
If it is called from a shared library, all subsequent shared
libraries are searched.
.Dv RTLD_NEXT
is useful for implementing wrappers around library functions.
For example, a wrapper function
.Fn getpid
could access the
.Dq real
.Fn getpid
with
.Li dlsym(RTLD_NEXT, \&"getpid\&") .
.\" (Actually, the
.\" .Fn dlfunc
.\" interface, below, should be used, since
.\" .Fn getpid
.\" is a function and not a data object.)
.Pp
If
.Fn dlsym
is called with the special
.Fa handle
.Dv RTLD_SELF ,
then the search for the symbol is limited to the shared object
issuing the call to
.Fn dlsym
and those shared objects which were loaded after it.
.Pp
The
.Fn dlsym
function
returns a null pointer if the symbol cannot be found, and sets an error
condition which may be queried with
.Fn dlerror .
.Pp
.\" The
.\" .Fn dlfunc
.\" function
.\" implements all of the behavior of
.\" .Fn dlsym ,
.\" but has a return type which can be cast to a function pointer without
.\" triggering compiler diagnostics.
.\" (The
.\" .Fn dlsym
.\" function
.\" returns a data pointer; in the C standard, conversions between
.\" data and function pointer types are undefined.
.\" Some compilers and
.\" .Xr lint 1
.\" utilities warn about such casts.)
.\" The precise return type of
.\" .Fn dlfunc
.\" is unspecified; applications must cast it to an appropriate function pointer
.\" type.
.Pp
The
.Fn dlerror
returns a null-terminated character string describing the last error that
occurred during a call to
.Fn dlopen ,
.Fn dlsym ,
or
.Fn dlclose .
If no such error has occurred,
.Fn dlerror
returns a null pointer.
At each call to
.Fn dlerror ,
the error indication is reset.  Thus in the case of two calls
to
.Fn dlerror ,
where the second call follows the first immediately, the second call
will always return a null pointer.
.Pp
.Fn dlclose
deletes a reference to the shared object referenced by
.Fa handle .
If the reference count drops to 0, the object is removed from the
address space, and
.Fa handle
is rendered invalid.
Just before removing a shared object in this way, the object's
.Fn _fini
function is called, if such a function is defined by the object.
If
.Fn dlclose
is successful, it returns a value of 0.
Otherwise it returns -1, and sets an error condition that can be
interrogated with
.Fn dlerror .
.Pp
The object-intrinsic functions
.Fn _init
and
.Fn _fini
are called with no arguments, and are not expected to return values.
.Sh NOTES
In darwin objects, C symbols have an underscore prepended to the symbol name.
So, the function 
.Fn getpid
ends up in the object file as _getpid
Most code assumes that there is no underscore and calls
.Fn dlsym
with
.Fa symbol
as \&"getpid\&" which would fail to find the symbol.
.Pp
Depending on how the dlcompat package was built, the
.Fn dlsym
function may, or may not, automatically prepend an underscore to the passed
.Fa symbol  
to compensate for this.
In this case calling
.Fn dlsym
with
.Fa symbol
as \&"getpid\&"
does %%NOTPREPEND%% find the _getpid symbol.
.Sh ERRORS
.Fn dlopen
and
.Fn dlsym
return a null pointer in the event of errors.
.Fn dlclose
returns 0 on success, or -1 if an error occurred.
Whenever an error has been detected, a message detailing it can be
retrieved via a call to
.Fn dlerror .
.Sh AUTHORS
The dlcompat package was written by Jorge Acereda 
<[e-mail]> and
Peter O'Gorman <[e-mail]>
.Pp
This man page was borrowed from FreeBSD and modified,
it originally came from Sun Microsystems, Inc.
.Sh BUGS
.Bl -bullet
.It
None of these rountines should be expected to work as advertised with MH_DYLIB files.
They are always opened globally and may not be closed, RTLD_NOW is ignored for them.
.It
The search for symbols in objects which were loaded as a result of loading handle terminates
too soon.
.It
None of the functions work as advertised for executables linked with the
.Xr ld 1
flag -force_flat_namespace, or when the
.Xr dyld 1
DYLD_FORCE_FLAT_NAMESPACE environment variable is set. This is because the
.Fn NSLookupSymbolInImage
function ignores the passed in mach_header in this case.
.It
The underscore or not issue is unnecessarily confusing.
.El
.Pp
If you find any more, please contact Peter O'Gorman <[e-mail]>
.Sh SEE ALSO
.Xr ld 1
.Xr cc 1
.Xr NSModule 3
.Xr NSObjectFileImage 3
.Xr dyld 3