.Dd Aug 28, 2008
.Os
.Dt DLOPEN 3
.Sh NAME
.Nm dlopen 
.Nd load and link a dynamic library or bundle
.Sh SYNOPSIS
.In dlfcn.h
.Ft void*
.Fn dlopen "const char* path" "int mode"
.Sh DESCRIPTION
.Fn dlopen
examines the mach-o file specified by 
.Fa path .
If the file is compatible with the current process and has not already been 
loaded into the current process, it is loaded and linked.  After being linked,
if it contains any initializer functions, they are called, before
.Fn dlopen
returns.  
.Fn dlopen
can load dynamic libraries and bundles.  It returns a handle that can
be used with 
.Fn dlsym
and
.Fn dlclose .
A second call to 
.Fn dlopen
with the same path will return the same handle, but the internal reference
count for the handle will be incremented.  Therefore all 
.Fn dlopen
calls should be balanced with a 
.Fn dlclose
call.
.Pp
If a null pointer is passed in 
.Fa path ,
.Fn dlopen
returns a handle equivalent to RTLD_DEFAULT.
.Pp
.Fa mode
contains options to 
.Fn dlopen .
It must contain one or more of the following values, possibly ORed together:
.Pp
.Bl -tag -width RTLD_LAZYX
.It Dv RTLD_LAZY
Each external function reference is bound the first time the function is called.
.It Dv RTLD_NOW
All external function references are bound immediately during the call to
.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 .
If neither 
RTLD_LAZY nor RTLD_NOW is specified, the default is RTLD_LAZY.
.Pp
One of the following flags may be ORed into the
.Fa mode
argument:
.Bl -tag -width RTLD_LOCALX
.It Dv RTLD_GLOBAL
Symbols exported from this image (dynamic library or bundle) will be available to any 
images build with -flat_namespace option to  
.Xr ld 1
or to calls to
.Fn dlsym
when using a special handle.
.It Dv RTLD_LOCAL
Symbols exported from this image (dynamic library or bundle) are generally hidden
and only availble to
.Fn dlsym
when directly using the handle returned by this call to 
.Fn dlopen .
.Pp
.El
If neither 
RTLD_GLOBAL nor RTLD_LOCAL is specified, the default is RTLD_GLOBAL.
.Pp
One of the following may be ORed into the
.Fa mode
argument:
.Bl -tag -width RTLD_NODELETEX
.It Dv RTLD_NOLOAD
The specified image is not loaded.  However, a valid  
.Fa handle
is returned if the image already exists in the process. This provides a way
to query if an image is already loaded.  The 
.Fa handle
returned is ref-counted, so you eventually need a corresponding call to  
.Fn dlclose
.It Dv RTLD_NODELETE
The specified image is tagged so that will never be removed from the address space,
even after all clients have released it via 
.Fn dlclose
.El
.Pp
Additionally, the following may be ORed into the
.Fa mode
argument:
.Bl -tag -width RTLD_FIRSTX
.It Dv RTLD_FIRST
The retuned    
.Fa handle
is tagged so that any 
.Fn dlsym
calls on the 
.Fa handle
will only search the image specified, and not subsequent images.  If 
.Fa path
is NULL and the option RTLD_FIRST is used, the 
.Fa handle 
returned will only search the main executable.
.El
.Sh SEARCHING
.Fn dlopen
searches for a compatible Mach-O file in the directories specified by a set of environment variables and 
the process's current working directory.
When set, the environment variables must contain a colon-separated list of directory paths, 
which can be absolute or relative to the current working directory. The environment variables 
are LD_LIBRARY_PATH, DYLD_LIBRARY_PATH, and DYLD_FALLBACK_LIBRARY_PATH. 
The first two variables have no default value. The default value of DYLD_FALLBACK_LIBRARY_PATH
is $HOME/lib;/usr/local/lib;/usr/lib. 
.Fn dlopen 
searches the directories specified in the environment variables in the order they are listed. 
.Pp
When 
.Fa path
doesn't contain a slash character (i.e. it is just a leaf name), 
.Fn dlopen
searches the following the following until it finds a compatible Mach-O file: $LD_LIBRARY_PATH, 
$DYLD_LIBRARY_PATH, current working directory, $DYLD_FALLBACK_LIBRARY_PATH.
.Pp
When 
.Fa path 
contains a slash (i.e. a full path or a partial path)
.Fn dlopen
searches the following the following until it finds a compatible Mach-O file: 
$DYLD_LIBRARY_PATH (with leaf name from 
.Fa path
), then the supplied 
.Fa path 
(using current working directory for partial paths).
.Pp
Note: There are no configuration files to control dlopen searching.  
.Pp
Note: If the main executable is a set[ug]id binary, then all environment variables are ignored, and only a full path can be used. 
.Pp
Note: Mac OS X uses "universal" files to combine 32-bit and 64-bit libraries.  This means there are no separate 32-bit and 64-bit search paths.
.Pp
.Sh RETURN VALUES
If 
.Fn dlopen
fails, it returns a null pointer, and sets an error condition which may be interrogated with 
.Fn dlerror .
.Sh AUTHORS
Mac OS X 10.3 incorporated the dlcompat package written by Jorge Acereda <jacereda@users.sourceforge.net>
and Peter O'Gorman <ogorman@users.sourceforge.net>.
.Pp
In Mac OS X 10.4, dlopen was rewritten to be a native part of dyld.
.Pp
.Sh SEE ALSO
.Xr dlopen_preflight 3
.Xr dlclose 3
.Xr dlsym 3
.Xr dlerror 3
.Xr dyld 3
.Xr ld 1