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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 July 7, 2009 36.Dt DLOPEN 3 37.Os 38.Sh NAME 39.Nm dlopen , 40.Nm dlsym , 41.Nm dlfunc , 42.Nm dlerror , 43.Nm dlclose 44.Nd programmatic interface to the dynamic linker 45.Sh LIBRARY 46.Lb libc 47.Sh SYNOPSIS 48.In dlfcn.h 49.Ft void * 50.Fn dlopen "const char *path" "int mode" 51.Ft void * 52.Fn dlsym "void * restrict handle" "const char * restrict symbol" 53.Ft dlfunc_t 54.Fn dlfunc "void * restrict handle" "const char * restrict symbol" 55.Ft char * 56.Fn dlerror "void" 57.Ft int 58.Fn dlclose "void *handle" 59.Sh DESCRIPTION 60These functions provide a simple programmatic interface to the services of the 61dynamic linker. 62Operations are provided to add new shared objects to a 63program's address space, to obtain the address bindings of symbols 64defined by such 65objects, and to remove such objects when their use is no longer required. 66.Pp 67The 68.Fn dlopen 69function 70provides access to the shared object in 71.Fa path , 72returning a descriptor that can be used for later 73references to the object in calls to 74.Fn dlsym 75and 76.Fn dlclose . 77If 78.Fa path 79was not in the address space prior to the call to 80.Fn dlopen , 81it is placed in the address space. 82When an object is first loaded into the address space in this way, its 83function 84.Fn _init , 85if any, is called by the dynamic linker. 86If 87.Fa path 88has already been placed in the address space in a previous call to 89.Fn dlopen , 90it is not added a second time, although a reference count of 91.Fn dlopen 92operations on 93.Fa path 94is maintained. 95A null pointer supplied for 96.Fa path 97is interpreted as a reference to the main 98executable of the process. 99The 100.Fa mode 101argument 102controls the way in which external function references from the 103loaded object are bound to their referents. 104It must contain one of the following values, possibly ORed with 105additional flags which will be described subsequently: 106.Bl -tag -width RTLD_LAZYX 107.It Dv RTLD_LAZY 108Each external function reference is resolved when the function is first 109called. 110.It Dv RTLD_NOW 111All external function references are bound immediately by 112.Fn dlopen . 113.El 114.Pp 115.Dv RTLD_LAZY 116is normally preferred, for reasons of efficiency. 117However, 118.Dv RTLD_NOW 119is useful to ensure that any undefined symbols are discovered during the 120call to 121.Fn dlopen . 122.Pp 123One of the following flags may be ORed into the 124.Fa mode 125argument: 126.Bl -tag -width RTLD_NODELETE 127.It Dv RTLD_GLOBAL 128Symbols from this shared object and its directed acyclic graph (DAG) 129of needed objects will be available for resolving undefined references 130from all other shared objects. 131.It Dv RTLD_LOCAL 132Symbols in this shared object and its DAG of needed objects will be 133available for resolving undefined references only from other objects 134in the same DAG. 135This is the default, but it may be specified 136explicitly with this flag. 137.It Dv RTLD_TRACE 138When set, causes dynamic linker to exit after loading all objects 139needed by this shared object and printing a summary which includes 140the absolute pathnames of all objects, to standard output. 141With this flag 142.Fn dlopen 143will return to the caller only in the case of error. 144.It Dv RTLD_NODELETE 145Prevents unload of the loaded object on 146.Fn dlclose . 147The same behaviour may be requested by 148.Fl "z nodelete" 149option of the static linker 150.Xr ld 1 . 151.It Dv RTLD_NOLOAD 152Only return valid handle for the object if it is already loaded in 153the process address space, otherwise 154.Dv NULL 155is returned. 156Other mode flags may be specified, which will be applied for promotion 157for the found object. 158.El 159.Pp 160If 161.Fn dlopen 162fails, it returns a null pointer, and sets an error condition which may 163be interrogated with 164.Fn dlerror . 165.Pp 166The 167.Fn dlsym 168function 169returns the address binding of the symbol described in the null-terminated 170character string 171.Fa symbol , 172as it occurs in the shared object identified by 173.Fa handle . 174The symbols exported by objects added to the address space by 175.Fn dlopen 176can be accessed only through calls to 177.Fn dlsym . 178Such symbols do not supersede any definition of those symbols already present 179in the address space when the object is loaded, nor are they available to 180satisfy normal dynamic linking references. 181.Pp 182If 183.Fn dlsym 184is called with the special 185.Fa handle 186.Dv NULL , 187it is interpreted as a reference to the executable or shared object 188from which the call 189is being made. 190Thus a shared object can reference its own symbols. 191.Pp 192If 193.Fn dlsym 194is called with the special 195.Fa handle 196.Dv RTLD_DEFAULT , 197the search for the symbol follows the algorithm used for resolving 198undefined symbols when objects are loaded. 199The objects searched are 200as follows, in the given order: 201.Bl -enum 202.It 203The referencing object itself (or the object from which the call to 204.Fn dlsym 205is made), if that object was linked using the 206.Fl Wsymbolic 207option to 208.Xr ld 1 . 209.It 210All objects loaded at program start-up. 211.It 212All objects loaded via 213.Fn dlopen 214with the 215.Dv RTLD_GLOBAL 216flag set in the 217.Fa mode 218argument. 219.It 220All objects loaded via 221.Fn dlopen 222which are in needed-object DAGs that also contain the referencing object. 223.El 224.Pp 225If 226.Fn dlsym 227is called with the special 228.Fa handle 229.Dv RTLD_NEXT , 230then the search for the symbol is limited to the shared objects 231which were loaded after the one issuing the call to 232.Fn dlsym . 233Thus, if the function is called from the main program, all 234the shared libraries are searched. 235If it is called from a shared library, all subsequent shared 236libraries are searched. 237.Dv RTLD_NEXT 238is useful for implementing wrappers around library functions. 239For example, a wrapper function 240.Fn getpid 241could access the 242.Dq real 243.Fn getpid 244with 245.Li dlsym(RTLD_NEXT, \&"getpid\&") . 246(Actually, the 247.Fn dlfunc 248interface, below, should be used, since 249.Fn getpid 250is a function and not a data object.) 251.Pp 252If 253.Fn dlsym 254is called with the special 255.Fa handle 256.Dv RTLD_SELF , 257then the search for the symbol is limited to the shared object 258issuing the call to 259.Fn dlsym 260and those shared objects which were loaded after it. 261.Pp 262The 263.Fn dlsym 264function 265returns a null pointer if the symbol cannot be found, and sets an error 266condition which may be queried with 267.Fn dlerror . 268.Pp 269The 270.Fn dlfunc 271function 272implements all of the behavior of 273.Fn dlsym , 274but has a return type which can be cast to a function pointer without 275triggering compiler diagnostics. 276(The 277.Fn dlsym 278function 279returns a data pointer; in the C standard, conversions between 280data and function pointer types are undefined. 281Some compilers and 282.Xr lint 1 283utilities warn about such casts.) 284The precise return type of 285.Fn dlfunc 286is unspecified; applications must cast it to an appropriate function pointer 287type. 288.Pp 289The 290.Fn dlerror 291function 292returns a null-terminated character string describing the last error that 293occurred during a call to 294.Fn dlopen , 295.Fn dladdr , 296.Fn dlinfo , 297.Fn dlsym , 298.Fn dlfunc , 299or 300.Fn dlclose . 301If no such error has occurred, 302.Fn dlerror 303returns a null pointer. 304At each call to 305.Fn dlerror , 306the error indication is reset. 307Thus in the case of two calls 308to 309.Fn dlerror , 310where the second call follows the first immediately, the second call 311will always return a null pointer. 312.Pp 313The 314.Fn dlclose 315function 316deletes a reference to the shared object referenced by 317.Fa handle . 318If the reference count drops to 0, the object is removed from the 319address space, and 320.Fa handle 321is rendered invalid. 322Just before removing a shared object in this way, the dynamic linker 323calls the object's 324.Fn _fini 325function, if such a function is defined by the object. 326If 327.Fn dlclose 328is successful, it returns a value of 0. 329Otherwise it returns -1, and sets an error condition that can be 330interrogated with 331.Fn dlerror . 332.Pp 333The object-intrinsic functions 334.Fn _init 335and 336.Fn _fini 337are called with no arguments, and are not expected to return values. 338.Sh NOTES 339ELF executables need to be linked 340using the 341.Fl export-dynamic 342option to 343.Xr ld 1 344for symbols defined in the executable to become visible to 345.Fn dlsym . 346.Pp 347In previous implementations, it was necessary to prepend an underscore 348to all external symbols in order to gain symbol 349compatibility with object code compiled from the C language. 350This is 351still the case when using the (obsolete) 352.Fl aout 353option to the C language compiler. 354.Sh ERRORS 355The 356.Fn dlopen , 357.Fn dlsym , 358and 359.Fn dlfunc 360functions 361return a null pointer in the event of errors. 362The 363.Fn dlclose 364function 365returns 0 on success, or -1 if an error occurred. 366Whenever an error has been detected, a message detailing it can be 367retrieved via a call to 368.Fn dlerror . 369.Sh SEE ALSO 370.Xr ld 1 , 371.Xr rtld 1 , 372.Xr dladdr 3 , 373.Xr dlinfo 3 , 374.Xr link 5 375