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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 , 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 const 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_GLOBALX 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.El 145.Pp 146If 147.Fn dlopen 148fails, it returns a null pointer, and sets an error condition which may 149be interrogated with 150.Fn dlerror . 151.Pp 152The 153.Fn dlsym 154function 155returns the address binding of the symbol described in the null-terminated 156character string 157.Fa symbol , 158as it occurs in the shared object identified by 159.Fa handle . 160The symbols exported by objects added to the address space by 161.Fn dlopen 162can be accessed only through calls to 163.Fn dlsym . 164Such symbols do not supersede any definition of those symbols already present 165in the address space when the object is loaded, nor are they available to 166satisfy normal dynamic linking references. 167.Pp 168If 169.Fn dlsym 170is called with the special 171.Fa handle 172.Dv NULL , 173it is interpreted as a reference to the executable or shared object 174from which the call 175is being made. 176Thus a shared object can reference its own symbols. 177.Pp 178If 179.Fn dlsym 180is called with the special 181.Fa handle 182.Dv RTLD_DEFAULT , 183the search for the symbol follows the algorithm used for resolving 184undefined symbols when objects are loaded. 185The objects searched are 186as follows, in the given order: 187.Bl -enum 188.It 189The referencing object itself (or the object from which the call to 190.Fn dlsym 191is made), if that object was linked using the 192.Fl Wsymbolic 193option to 194.Xr ld 1 . 195.It 196All objects loaded at program start-up. 197.It 198All objects loaded via 199.Fn dlopen 200with the 201.Dv RTLD_GLOBAL 202flag set in the 203.Fa mode 204argument. 205.It 206All objects loaded via 207.Fn dlopen 208which are in needed-object DAGs that also contain the referencing object. 209.El 210.Pp 211If 212.Fn dlsym 213is called with the special 214.Fa handle 215.Dv RTLD_NEXT , 216then the search for the symbol is limited to the shared objects 217which were loaded after the one issuing the call to 218.Fn dlsym . 219Thus, if the function is called from the main program, all 220the shared libraries are searched. 221If it is called from a shared library, all subsequent shared 222libraries are searched. 223.Dv RTLD_NEXT 224is useful for implementing wrappers around library functions. 225For example, a wrapper function 226.Fn getpid 227could access the 228.Dq real 229.Fn getpid 230with 231.Li dlsym(RTLD_NEXT, \&"getpid\&") . 232(Actually, the 233.Fn dlfunc 234interface, below, should be used, since 235.Fn getpid 236is a function and not a data object.) 237.Pp 238If 239.Fn dlsym 240is called with the special 241.Fa handle 242.Dv RTLD_SELF , 243then the search for the symbol is limited to the shared object 244issuing the call to 245.Fn dlsym 246and those shared objects which were loaded after it. 247.Pp 248The 249.Fn dlsym 250function 251returns a null pointer if the symbol cannot be found, and sets an error 252condition which may be queried with 253.Fn dlerror . 254.Pp 255The 256.Fn dlfunc 257function 258implements all of the behavior of 259.Fn dlsym , 260but has a return type which can be cast to a function pointer without 261triggering compiler diagnostics. 262(The 263.Fn dlsym 264function 265returns a data pointer; in the C standard, conversions between 266data and function pointer types are undefined. 267Some compilers and 268.Xr lint 1 269utilities warn about such casts.) 270The precise return type of 271.Fn dlfunc 272is unspecified; applications must cast it to an appropriate function pointer 273type. 274.Pp 275The 276.Fn dlerror 277function 278returns a null-terminated character string describing the last error that 279occurred during a call to 280.Fn dlopen , 281.Fn dladdr , 282.Fn dlinfo , 283.Fn dlsym , 284.Fn dlfunc , 285or 286.Fn dlclose . 287If no such error has occurred, 288.Fn dlerror 289returns a null pointer. 290At each call to 291.Fn dlerror , 292the error indication is reset. 293Thus in the case of two calls 294to 295.Fn dlerror , 296where the second call follows the first immediately, the second call 297will always return a null pointer. 298.Pp 299The 300.Fn dlclose 301function 302deletes a reference to the shared object referenced by 303.Fa handle . 304If the reference count drops to 0, the object is removed from the 305address space, and 306.Fa handle 307is rendered invalid. 308Just before removing a shared object in this way, the dynamic linker 309calls the object's 310.Fn _fini 311function, if such a function is defined by the object. 312If 313.Fn dlclose 314is successful, it returns a value of 0. 315Otherwise it returns -1, and sets an error condition that can be 316interrogated with 317.Fn dlerror . 318.Pp 319The object-intrinsic functions 320.Fn _init 321and 322.Fn _fini 323are called with no arguments, and are not expected to return values. 324.Sh NOTES 325ELF executables need to be linked 326using the 327.Fl export-dynamic 328option to 329.Xr ld 1 330for symbols defined in the executable to become visible to 331.Fn dlsym . 332.Pp 333In previous implementations, it was necessary to prepend an underscore 334to all external symbols in order to gain symbol 335compatibility with object code compiled from the C language. 336This is 337still the case when using the (obsolete) 338.Fl aout 339option to the C language compiler. 340.Sh ERRORS 341The 342.Fn dlopen , 343.Fn dlsym , 344and 345.Fn dlfunc 346functions 347return a null pointer in the event of errors. 348The 349.Fn dlclose 350function 351returns 0 on success, or -1 if an error occurred. 352Whenever an error has been detected, a message detailing it can be 353retrieved via a call to 354.Fn dlerror . 355.Sh SEE ALSO 356.Xr ld 1 , 357.Xr rtld 1 , 358.Xr dladdr 3 , 359.Xr dlinfo 3 , 360.Xr link 5 361