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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 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.In 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, possibly ORed with 95additional flags which will be described subsequently: 96.Bl -tag -width RTLD_LAZYX 97.It Dv RTLD_LAZY 98Each external function reference is resolved when the function is first 99called. 100.It Dv RTLD_NOW 101All external function references are bound immediately by 102.Fn dlopen . 103.El 104.Pp 105.Dv RTLD_LAZY 106is normally preferred, for reasons of efficiency. 107However, 108.Dv RTLD_NOW 109is useful to ensure that any undefined symbols are discovered during the 110call to 111.Fn dlopen . 112.Pp 113One of the following flags may be ORed into the 114.Fa mode 115argument: 116.Bl -tag -width RTLD_GLOBALX 117.It Dv RTLD_GLOBAL 118Symbols from this shared object and its directed acyclic graph (DAG) 119of needed objects will be available for resolving undefined references 120from all other shared objects. 121.It Dv RTLD_LOCAL 122Symbols in this shared object and its DAG of needed objects will be 123available for resolving undefined references only from other objects 124in the same DAG. This is the default, but it may be specified 125explicitly with this flag. 126.El 127.Pp 128If 129.Fn dlopen 130fails, it returns a null pointer, and sets an error condition which may 131be interrogated with 132.Fn dlerror . 133.Pp 134.Fn dlsym 135returns the address binding of the symbol described in the null-terminated 136character string 137.Fa symbol , 138as it occurs in the shared object identified by 139.Fa handle . 140The symbols exported by objects added to the address space by 141.Fn dlopen 142can be accessed only through calls to 143.Fn dlsym . 144Such symbols do not supersede any definition of those symbols already present 145in the address space when the object is loaded, nor are they available to 146satisfy normal dynamic linking references. 147.Pp 148If 149.Fn dlsym 150is called with the special 151.Fa handle 152.Dv NULL , 153it is interpreted as a reference to the executable or shared object 154from which the call 155is being made. Thus a shared object can reference its own symbols. 156.Pp 157If 158.Fn dlsym 159is called with the special 160.Fa handle 161.Dv RTLD_DEFAULT , 162the search for the symbol follows the algorithm used for resolving 163undefined symbols when objects are loaded. The objects searched are 164as follows, in the given order: 165.Bl -enum 166.It 167The referencing object itself (or the object from which the call to 168.Fn dlsym 169is made), if that object was linked using the 170.Fl Wsymbolic 171option to 172.Xr ld 1 . 173.It 174All objects loaded at program start-up. 175.It 176All objects loaded via 177.Fn dlopen 178which are in needed-object DAGs that also contain the referencing object. 179.It 180All objects loaded via 181.Fn dlopen 182with the 183.Dv RTLD_GLOBAL 184flag set in the 185.Fa mode 186argument. 187.El 188.Pp 189If 190.Fn dlsym 191is called with the special 192.Fa handle 193.Dv RTLD_NEXT , 194then the search for the symbol is limited to the shared objects 195which were loaded after the one issuing the call to 196.Fn dlsym . 197Thus, if the function is called from the main program, all 198the shared libraries are searched. 199If it is called from a shared library, all subsequent shared 200libraries are searched. 201.Dv RTLD_NEXT 202is useful for implementing wrappers around library functions. 203For example, a wrapper function 204.Fn getpid 205could access the 206.Dq real 207.Fn getpid 208with 209.Li dlsym(RTLD_NEXT, \&"getpid\&") . 210.Pp 211.Fn dlsym 212returns a null pointer if the symbol cannot be found, and sets an error 213condition which may be queried with 214.Fn dlerror . 215.Pp 216.Fn dlerror 217returns a null-terminated character string describing the last error that 218occurred during a call to 219.Fn dlopen , 220.Fn dlsym , 221or 222.Fn dlclose . 223If no such error has occurred, 224.Fn dlerror 225returns a null pointer. 226At each call to 227.Fn dlerror , 228the error indication is reset. Thus in the case of two calls 229to 230.Fn dlerror , 231where the second call follows the first immediately, the second call 232will always return a null pointer. 233.Pp 234.Fn dlclose 235deletes a reference to the shared object referenced by 236.Fa handle . 237If the reference count drops to 0, the object is removed from the 238address space, and 239.Fa handle 240is rendered invalid. 241Just before removing a shared object in this way, the dynamic linker 242calls the object's 243.Fn _fini 244function, if such a function is defined by the object. 245If 246.Fn dlclose 247is successful, it returns a value of 0. 248Otherwise it returns -1, and sets an error condition that can be 249interrogated with 250.Fn dlerror . 251.Pp 252The object-intrinsic functions 253.Fn _init 254and 255.Fn _fini 256are called with no arguments, and are not expected to return values. 257.Sh NOTES 258ELF executables need to be linked 259using the 260.Fl export-dynamic 261option to 262.Xr ld 1 263for symbols defined in the executable to become visible to 264.Fn dlsym . 265.Pp 266In previous implementations, it was necessary to prepend an underscore 267to all external symbols in order to gain symbol 268compatibility with object code compiled from the C language. This is 269still the case when using the (obsolete) 270.Fl aout 271option to the C language compiler. 272.Sh ERRORS 273.Fn dlopen 274and 275.Fn dlsym 276return the null pointer in the event of errors. 277.Fn dlclose 278returns 0 on success, or -1 if an error occurred. 279Whenever an error has been detected, a message detailing it can be 280retrieved via a call to 281.Fn dlerror . 282.Sh SEE ALSO 283.Xr ld 1 , 284.Xr rtld 1 , 285.Xr dladdr 3 , 286.Xr link 5 287