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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: src/lib/libc/gen/dlopen.3,v 1.8.2.10 2003/03/15 15:11:05 trhodes Exp $ 34.\" $DragonFly: src/lib/libc/gen/dlopen.3,v 1.2 2003/06/17 04:26:42 dillon Exp $ 35.\" 36.Dd September 24, 1989 37.Os 38.Dt DLOPEN 3 39.Sh NAME 40.Nm dlopen , 41.Nm dlsym , 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 *handle" "const char *symbol" 53.Ft dlfunc_t 54.Fn dlfunc "void *handle" "const char *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 200which are in needed-object DAGs that also contain the referencing object. 201.It 202All objects loaded via 203.Fn dlopen 204with the 205.Dv RTLD_GLOBAL 206flag set in the 207.Fa mode 208argument. 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.Pp 233If 234.Fn dlsym 235is called with the special 236.Fa handle 237.Dv RTLD_SELF , 238then the search for the symbol is limited to the shared object 239issuing the call to 240.Fn dlsym 241and those shared objects which were loaded after it. 242.Pp 243The 244.Fn dlsym 245function 246returns a null pointer if the symbol cannot be found, and sets an error 247condition which may be queried with 248.Fn dlerror . 249.Pp 250The 251.Fn dlerror 252function 253returns a null-terminated character string describing the last error that 254occurred during a call to 255.Fn dlopen , 256.Fn dladdr , 257.Fn dlinfo , 258.Fn dlsym , 259or 260.Fn dlclose . 261If no such error has occurred, 262.Fn dlerror 263returns a null pointer. 264At each call to 265.Fn dlerror , 266the error indication is reset. 267Thus in the case of two calls 268to 269.Fn dlerror , 270where the second call follows the first immediately, the second call 271will always return a null pointer. 272.Pp 273The 274.Fn dlclose 275function 276deletes a reference to the shared object referenced by 277.Fa handle . 278If the reference count drops to 0, the object is removed from the 279address space, and 280.Fa handle 281is rendered invalid. 282Just before removing a shared object in this way, the dynamic linker 283calls the object's 284.Fn _fini 285function, if such a function is defined by the object. 286If 287.Fn dlclose 288is successful, it returns a value of 0. 289Otherwise it returns -1, and sets an error condition that can be 290interrogated with 291.Fn dlerror . 292.Pp 293The object-intrinsic functions 294.Fn _init 295and 296.Fn _fini 297are called with no arguments, and are not expected to return values. 298.Sh NOTES 299ELF executables need to be linked 300using the 301.Fl export-dynamic 302option to 303.Xr ld 1 304for symbols defined in the executable to become visible to 305.Fn dlsym . 306.Pp 307In previous implementations, it was necessary to prepend an underscore 308to all external symbols in order to gain symbol 309compatibility with object code compiled from the C language. 310This is 311still the case when using the (obsolete) 312.Fl aout 313option to the C language compiler. 314.Sh ERRORS 315The 316.Fn dlopen 317and 318.Fn dlsym 319functions 320return a null pointer in the event of errors. 321The 322.Fn dlclose 323function 324returns 0 on success, or -1 if an error occurred. 325Whenever an error has been detected, a message detailing it can be 326retrieved via a call to 327.Fn dlerror . 328.Sh SEE ALSO 329.Xr ld 1 , 330.Xr rtld 1 , 331.Xr dladdr 3 , 332.Xr dlinfo 3 , 333.Xr link 5 334