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BE LIABLE FOR ANY SPECIAL, INDIRECT, 15.\" INCIDENTAL, OR CONSEQUENTIAL DAMAGES OR ANY DAMAGES WHATSOEVER RESULTING 16.\" FROM USE OF SUCH SOURCE CODE, REGARDLESS OF THE THEORY OF LIABILITY. 17.\" 18.\" This source code is provided with no support and without any obligation on 19.\" the part of Sun Microsystems, Inc. to assist in its use, correction, 20.\" modification or enhancement. 21.\" 22.\" SUN MICROSYSTEMS, INC. SHALL HAVE NO LIABILITY WITH RESPECT TO THE 23.\" INFRINGEMENT OF COPYRIGHTS, TRADE SECRETS OR ANY PATENTS BY THIS 24.\" SOURCE CODE OR ANY PART THEREOF. 25.\" 26.\" Sun Microsystems, Inc. 27.\" 2550 Garcia Avenue 28.\" Mountain View, California 94043 29.\" 30.\" Copyright (c) 1991 Sun Microsystems, Inc. 31.\" 32.\" @(#) dlopen.3 1.6 90/01/31 SMI 33.\" $FreeBSD$ 34.\" 35.Dd September 24, 1989 36.Os FreeBSD 37.Dt DLOPEN 3 38.Sh NAME 39.Nm dlopen, dlsym, dlerror, dlclose 40.Nd programmatic interface to the dynamic linker 41.Sh LIBRARY 42.Lb libc 43.Sh SYNOPSIS 44.Fd #include <dlfcn.h> 45.Ft void * 46.Fn dlopen "const char *path" "int mode" 47.Ft void * 48.Fn dlsym "void *handle" "const char *symbol" 49.Ft const char * 50.Fn dlerror "void" 51.Ft int 52.Fn dlclose "void *handle" 53.Sh DESCRIPTION 54These functions provide a simple programmatic interface to the services of the 55dynamic linker. 56Operations are provided to add new shared objects to a 57program's address space, to obtain the address bindings of symbols 58defined by such 59objects, and to remove such objects when their use is no longer required. 60.Pp 61.Fn dlopen 62provides access to the shared object in 63.Fa path , 64returning a descriptor that can be used for later 65references to the object in calls to 66.Fn dlsym 67and 68.Fn dlclose . 69If 70.Fa path 71was not in the address space prior to the call to 72.Fn dlopen , 73it is placed in the address space. 74When an object is first loaded into the address space in this way, its 75function 76.Fn _init , 77if any, is called by the dynamic linker. 78If 79.Fa path 80has already been placed in the address space in a previous call to 81.Fn dlopen , 82it is not added a second time, although a reference count of 83.Fn dlopen 84operations on 85.Fa path 86is maintained. 87A null pointer supplied for 88.Fa path 89is interpreted as a reference to the main 90executable of the process. 91.Fa mode 92controls the way in which external function references from the 93loaded object are bound to their referents. 94It must contain one of the following values: 95.Bl -tag -width RTLD_LAZYX 96.It Dv RTLD_LAZY 97Each external function reference is resolved when the function is first 98called. 99.It Dv RTLD_NOW 100All external function references are bound immediately by 101.Fn dlopen . 102.El 103.Pp 104.Dv RTLD_LAZY 105is normally preferred, for reasons of efficiency. 106However, 107.Dv RTLD_NOW 108is useful to ensure that any undefined symbols are discovered during the 109call to 110.Fn dlopen . 111If 112.Fn dlopen 113fails, it returns a null pointer, and sets an error condition which may 114be interrogated with 115.Fn dlerror . 116.Pp 117.Fn dlsym 118returns the address binding of the symbol described in the null-terminated 119character string 120.Fa symbol , 121as it occurs in the shared object identified by 122.Fa handle . 123The symbols exported by objects added to the address space by 124.Fn dlopen 125can be accessed only through calls to 126.Fn dlsym . 127Such symbols do not supersede any definition of those symbols already present 128in the address space when the object is loaded, nor are they available to 129satisfy normal dynamic linking references. 130A null pointer supplied as the value of 131.Fa handle 132is interpreted as a reference to the executable from which the call to 133.Fn dlsym 134is being made. Thus a shared object can reference its own symbols. 135.Fn dlsym 136returns a null pointer if the symbol cannot be found, and sets an error 137condition which may be queried with 138.Fn dlerror . 139.Pp 140If 141.Fn dlsym 142is called with the special 143.Fa handle 144.Dv RTLD_NEXT , 145then the search for the symbol is limited to the shared objects 146which were loaded after the one issuing the call to 147.Fn dlsym . 148Thus, if the function is called from the main program, all 149the shared libraries are searched. 150If it is called from a shared library, all subsequent shared 151libraries are searched. 152.Dv RTLD_NEXT 153is useful for implementing wrappers around library functions. 154For example, a wrapper function 155.Fn getpid 156could access the 157.Dq real 158.Fn getpid 159with 160.Li dlsym(RTLD_NEXT, \&"getpid\&") . 161.Pp 162.Fn dlerror 163returns a null-terminated character string describing the last error that 164occurred during a call to 165.Fn dlopen , 166.Fn dlsym , 167or 168.Fn dlclose . 169If no such error has occurred, 170.Fn dlerror 171returns a null pointer. 172At each call to 173.Fn dlerror , 174the error indication is reset. Thus in the case of two calls 175to 176.Fn dlerror , 177where the second call follows the first immediately, the second call 178will always return a null pointer. 179.Pp 180.Fn dlclose 181deletes a reference to the shared object referenced by 182.Fa handle . 183If the reference count drops to 0, the object is removed from the 184address space, and 185.Fa handle 186is rendered invalid. 187Just before removing a shared object in this way, the dynamic linker 188calls the object's 189.Fn _fini 190function, if such a function is defined by the object. 191If 192.Fn dlclose 193is successful, it returns a value of 0. 194Otherwise it returns -1, and sets an error condition that can be 195interrogated with 196.Fn dlerror . 197.Pp 198The object-intrinsic functions 199.Fn _init 200and 201.Fn _fini 202are called with no arguments, and are not expected to return values. 203.Sh NOTES 204ELF executables need to be linked 205using the 206.Fl export-dynamic 207option to 208.Xr ld 1 209for symbols defined in the executable to become visible to 210.Fn dlsym . 211.Pp 212In previous implementations, it was necessary to prepend an underscore 213to all external symbols in order to gain symbol 214compatibility with object code compiled from the C language. This is 215still the case when using the (obsolete) 216.Fl aout 217option to the C language compiler. 218.Sh ERRORS 219.Fn dlopen 220and 221.Fn dlsym 222return the null pointer in the event of errors. 223.Fn dlclose 224returns 0 on success, or -1 if an error occurred. 225Whenever an error has been detected, a message detailing it can be 226retrieved via a call to 227.Fn dlerror . 228.Sh SEE ALSO 229.Xr ld 1 , 230.Xr rtld 1 , 231.Xr link 5 232 233