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Diffstat (limited to 'lib/libc/gen/dlopen.3')
| -rw-r--r-- | lib/libc/gen/dlopen.3 | 360 |
1 files changed, 360 insertions, 0 deletions
diff --git a/lib/libc/gen/dlopen.3 b/lib/libc/gen/dlopen.3 new file mode 100644 index 0000000..118e165 --- /dev/null +++ b/lib/libc/gen/dlopen.3 @@ -0,0 +1,360 @@ +.\" 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 10, 2002 +.Os +.Dt DLOPEN 3 +.Sh NAME +.Nm dlopen , +.Nm dlsym , +.Nm dlfunc , +.Nm dlerror , +.Nm dlclose +.Nd programmatic interface to the dynamic linker +.Sh LIBRARY +.Lb libc +.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. +.Pp +The +.Fn dlopen +function +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. +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 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. +.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. +.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 +function +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 +If +.Fn dlsym +is called with the special +.Fa handle +.Dv NULL , +it is interpreted as a reference to the executable or shared object +from which the call +is being made. +Thus a shared object can reference its own symbols. +.Pp +If +.Fn dlsym +is called with the special +.Fa handle +.Dv RTLD_DEFAULT , +the search for the symbol follows the algorithm used for resolving +undefined symbols when objects are loaded. +The objects searched are +as follows, in the given order: +.Bl -enum +.It +The referencing object itself (or the object from which the call to +.Fn dlsym +is made), if that object was linked using the +.Fl Wsymbolic +option to +.Xr ld 1 . +.It +All objects loaded at program start-up. +.It +All objects loaded via +.Fn dlopen +with the +.Dv RTLD_GLOBAL +flag set in the +.Fa mode +argument. +.It +All objects loaded via +.Fn dlopen +which are in needed-object DAGs that also contain the referencing object. +.El +.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 +function +returns a null-terminated character string describing the last error that +occurred during a call to +.Fn dlopen , +.Fn dladdr , +.Fn dlinfo , +.Fn dlsym , +.Fn dlfunc , +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 +The +.Fn dlclose +function +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 +The +.Fn dlopen , +.Fn dlsym , +and +.Fn dlfunc +functions +return a null pointer in the event of errors. +The +.Fn dlclose +function +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 dladdr 3 , +.Xr dlinfo 3 , +.Xr link 5 |
