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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 10, 2002 36.Os 37.Dt DLOPEN 3 38.Sh NAME 39.Nm dlopen , dlsym , dlfunc , 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 * restrict handle" "const char * restrict symbol" 49.Ft dlfunc_t 50.Fn dlfunc "void * restrict handle" "const char * restrict symbol" 51.Ft const char * 52.Fn dlerror "void" 53.Ft int 54.Fn dlclose "void *handle" 55.Sh DESCRIPTION 56These functions provide a simple programmatic interface to the services of the 57dynamic linker. 58Operations are provided to add new shared objects to a 59program's address space, to obtain the address bindings of symbols 60defined by such 61objects, and to remove such objects when their use is no longer required. 62.Pp 63The 64.Fn dlopen 65function 66provides access to the shared object in 67.Fa path , 68returning a descriptor that can be used for later 69references to the object in calls to 70.Fn dlsym 71and 72.Fn dlclose . 73If 74.Fa path 75was not in the address space prior to the call to 76.Fn dlopen , 77it is placed in the address space. 78When an object is first loaded into the address space in this way, its 79function 80.Fn _init , 81if any, is called by the dynamic linker. 82If 83.Fa path 84has already been placed in the address space in a previous call to 85.Fn dlopen , 86it is not added a second time, although a reference count of 87.Fn dlopen 88operations on 89.Fa path 90is maintained. 91A null pointer supplied for 92.Fa path 93is interpreted as a reference to the main 94executable of the process. 95The 96.Fa mode 97argument 98controls the way in which external function references from the 99loaded object are bound to their referents. 100It must contain one of the following values, possibly ORed with 101additional flags which will be described subsequently: 102.Bl -tag -width RTLD_LAZYX 103.It Dv RTLD_LAZY 104Each external function reference is resolved when the function is first 105called. 106.It Dv RTLD_NOW 107All external function references are bound immediately by 108.Fn dlopen . 109.El 110.Pp 111.Dv RTLD_LAZY 112is normally preferred, for reasons of efficiency. 113However, 114.Dv RTLD_NOW 115is useful to ensure that any undefined symbols are discovered during the 116call to 117.Fn dlopen . 118.Pp 119One of the following flags may be ORed into the 120.Fa mode 121argument: 122.Bl -tag -width RTLD_GLOBALX 123.It Dv RTLD_GLOBAL 124Symbols from this shared object and its directed acyclic graph (DAG) 125of needed objects will be available for resolving undefined references 126from all other shared objects. 127.It Dv RTLD_LOCAL 128Symbols in this shared object and its DAG of needed objects will be 129available for resolving undefined references only from other objects 130in the same DAG. 131This is the default, but it may be specified 132explicitly with this flag. 133.It Dv RTLD_TRACE 134When set, causes dynamic linker to exit after loading all objects 135needed by this shared object and printing a summary which includes 136the absolute pathnames of all objects, to standard output. 137With this flag 138.Fn dlopen 139will return to the caller only in the case of error. 140.El 141.Pp 142If 143.Fn dlopen 144fails, it returns a null pointer, and sets an error condition which may 145be interrogated with 146.Fn dlerror . 147.Pp 148The 149.Fn dlsym 150function 151returns the address binding of the symbol described in the null-terminated 152character string 153.Fa symbol , 154as it occurs in the shared object identified by 155.Fa handle . 156The symbols exported by objects added to the address space by 157.Fn dlopen 158can be accessed only through calls to 159.Fn dlsym . 160Such symbols do not supersede any definition of those symbols already present 161in the address space when the object is loaded, nor are they available to 162satisfy normal dynamic linking references. 163.Pp 164If 165.Fn dlsym 166is called with the special 167.Fa handle 168.Dv NULL , 169it is interpreted as a reference to the executable or shared object 170from which the call 171is being made. 172Thus a shared object can reference its own symbols. 173.Pp 174If 175.Fn dlsym 176is called with the special 177.Fa handle 178.Dv RTLD_DEFAULT , 179the search for the symbol follows the algorithm used for resolving 180undefined symbols when objects are loaded. 181The objects searched are 182as follows, in the given order: 183.Bl -enum 184.It 185The referencing object itself (or the object from which the call to 186.Fn dlsym 187is made), if that object was linked using the 188.Fl Wsymbolic 189option to 190.Xr ld 1 . 191.It 192All objects loaded at program start-up. 193.It 194All objects loaded via 195.Fn dlopen 196which are in needed-object DAGs that also contain the referencing object. 197.It 198All objects loaded via 199.Fn dlopen 200with the 201.Dv RTLD_GLOBAL 202flag set in the 203.Fa mode 204argument. 205.El 206.Pp 207If 208.Fn dlsym 209is called with the special 210.Fa handle 211.Dv RTLD_NEXT , 212then the search for the symbol is limited to the shared objects 213which were loaded after the one issuing the call to 214.Fn dlsym . 215Thus, if the function is called from the main program, all 216the shared libraries are searched. 217If it is called from a shared library, all subsequent shared 218libraries are searched. 219.Dv RTLD_NEXT 220is useful for implementing wrappers around library functions. 221For example, a wrapper function 222.Fn getpid 223could access the 224.Dq real 225.Fn getpid 226with 227.Li dlsym(RTLD_NEXT, \&"getpid\&") . 228(Actually, the 229.Fn dlfunc 230interface, below, should be used, since 231.Fn getpid 232is a function and not a data object.) 233.Pp 234The 235.Fn dlsym 236function 237returns a null pointer if the symbol cannot be found, and sets an error 238condition which may be queried with 239.Fn dlerror . 240.Pp 241The 242.Fn dlfunc 243function 244implements all of the behavior of 245.Fn dlsym , 246but has a return type which can be cast to a function pointer without 247triggering compiler diagnostics. 248(The 249.Fn dlsym 250function 251returns a data pointer; in the C standard, conversions between 252data and function pointer types are undefined. 253Some compilers and 254.Xr lint 1 255utilities warn about such casts.) 256The precise return type of 257.Fn dlfunc 258is unspecified; applications must cast it to an appropriate function pointer 259type. 260.Pp 261The 262.Fn dlerror 263function 264returns a null-terminated character string describing the last error that 265occurred during a call to 266.Fn dlopen , 267.Fn dlsym , 268.Fn dlfunc , 269or 270.Fn dlclose . 271If no such error has occurred, 272.Fn dlerror 273returns a null pointer. 274At each call to 275.Fn dlerror , 276the error indication is reset. 277Thus in the case of two calls 278to 279.Fn dlerror , 280where the second call follows the first immediately, the second call 281will always return a null pointer. 282.Pp 283The 284.Fn dlclose 285function 286deletes a reference to the shared object referenced by 287.Fa handle . 288If the reference count drops to 0, the object is removed from the 289address space, and 290.Fa handle 291is rendered invalid. 292Just before removing a shared object in this way, the dynamic linker 293calls the object's 294.Fn _fini 295function, if such a function is defined by the object. 296If 297.Fn dlclose 298is successful, it returns a value of 0. 299Otherwise it returns -1, and sets an error condition that can be 300interrogated with 301.Fn dlerror . 302.Pp 303The object-intrinsic functions 304.Fn _init 305and 306.Fn _fini 307are called with no arguments, and are not expected to return values. 308.Sh NOTES 309ELF executables need to be linked 310using the 311.Fl export-dynamic 312option to 313.Xr ld 1 314for symbols defined in the executable to become visible to 315.Fn dlsym . 316.Pp 317In previous implementations, it was necessary to prepend an underscore 318to all external symbols in order to gain symbol 319compatibility with object code compiled from the C language. 320This is 321still the case when using the (obsolete) 322.Fl aout 323option to the C language compiler. 324.Sh ERRORS 325The 326.Fn dlopen , 327.Fn dlsym , 328and 329.Fn dlfunc 330functions 331return a null pointer in the event of errors. 332The 333.Fn dlclose 334function 335returns 0 on success, or -1 if an error occurred. 336Whenever an error has been detected, a message detailing it can be 337retrieved via a call to 338.Fn dlerror . 339.Sh SEE ALSO 340.Xr ld 1 , 341.Xr rtld 1 , 342.Xr dladdr 3 , 343.Xr link 5 344