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Diffstat (limited to 'lib/libc/gen/dlopen.3')
| -rw-r--r-- | lib/libc/gen/dlopen.3 | 230 |
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diff --git a/lib/libc/gen/dlopen.3 b/lib/libc/gen/dlopen.3 new file mode 100644 index 0000000..6485e75 --- /dev/null +++ b/lib/libc/gen/dlopen.3 @@ -0,0 +1,230 @@ +.\" This source code is a product of Sun Microsystems, Inc. and is provided +.\" for unrestricted use provided that this legend is included on all tape +.\" media and as a part of the software program in whole or part. Users +.\" may copy or modify this source code without charge, but are not authorized +.\" to license or distribute it to anyone else except as part of a product or +.\" program developed by the user. +.\" +.\" THIS PROGRAM CONTAINS SOURCE CODE COPYRIGHTED BY SUN MICROSYSTEMS, INC. +.\" SUN MICROSYSTEMS, INC., MAKES NO REPRESENTATIONS ABOUT THE SUITABLITY +.\" OF SUCH SOURCE CODE FOR ANY PURPOSE. IT IS PROVIDED "AS IS" WITHOUT +.\" EXPRESS OR IMPLIED WARRANTY OF ANY KIND. SUN MICROSYSTEMS, INC. DISCLAIMS +.\" ALL WARRANTIES WITH REGARD TO SUCH SOURCE CODE, INCLUDING ALL IMPLIED +.\" WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE. IN +.\" NO EVENT SHALL SUN MICROSYSTEMS, INC. BE LIABLE FOR ANY SPECIAL, INDIRECT, +.\" INCIDENTAL, OR CONSEQUENTIAL DAMAGES OR ANY DAMAGES WHATSOEVER RESULTING +.\" FROM USE OF SUCH SOURCE CODE, REGARDLESS OF THE THEORY OF LIABILITY. +.\" +.\" This source code is provided with no support and without any obligation on +.\" the part of Sun Microsystems, Inc. to assist in its use, correction, +.\" modification or enhancement. +.\" +.\" SUN MICROSYSTEMS, INC. SHALL HAVE NO LIABILITY WITH RESPECT TO THE +.\" INFRINGEMENT OF COPYRIGHTS, TRADE SECRETS OR ANY PATENTS BY THIS +.\" SOURCE CODE OR ANY PART THEREOF. +.\" +.\" Sun Microsystems, Inc. +.\" 2550 Garcia Avenue +.\" Mountain View, California 94043 +.\" +.\" Copyright (c) 1991 Sun Microsystems, Inc. +.\" +.\" @(#) dlopen.3 1.6 90/01/31 SMI +.\" $FreeBSD$ +.\" +.Dd September 24, 1989 +.Os FreeBSD +.Dt DLOPEN 3 +.Sh NAME +.Nm dlopen, dlsym, dlerror, dlclose +.Nd programmatic interface to the dynamic linker +.Sh SYNOPSIS +.Fd #include <dlfcn.h> +.Ft void * +.Fn dlopen "const char *path" "int mode" +.Ft void * +.Fn dlsym "void *handle" "const char *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. +.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 by the dynamic linker. +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: +.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 . +If +.Fn dlopen +fails, it returns a null pointer, and sets an error condition which may +be interrogated with +.Fn dlerror . +.Pp +.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. +A null pointer supplied as the value of +.Fa handle +is interpreted as a reference to the executable from which the call to +.Fn dlsym +is being made. Thus a shared object can reference its own symbols. +.Fn dlsym +returns a null pointer if the symbol cannot be found, and sets an error +condition which may be queried with +.Fn dlerror . +.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\&") . +.Pp +.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 dynamic linker +calls the object's +.Fn _fini +function, 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 +ELF executables need to be linked +using the +.Fl export-dynamic +option to +.Xr ld 1 +for symbols defined in the executable to become visible to +.Fn dlsym . +.Pp +In previous implementations, it was necessary to prepend an underscore +to all external symbols in order to gain symbol +compatibility with object code compiled from the C language. This is +still the case when using the (obsolete) +.Fl aout +option to the C language compiler. +.Sh ERRORS +.Fn dlopen +and +.Fn dlsym +return the 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 SEE ALSO +.Xr ld 1 , +.Xr rtld 1 , +.Xr link 5 + |
