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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 May 28, 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 *handle" "const char *symbol" 49.Ft dlfunc_t 50.Fn dlfunc "void *handle" "const char *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 63.Fn dlopen 64provides access to the shared object in 65.Fa path , 66returning a descriptor that can be used for later 67references to the object in calls to 68.Fn dlsym 69and 70.Fn dlclose . 71If 72.Fa path 73was not in the address space prior to the call to 74.Fn dlopen , 75it is placed in the address space. 76When an object is first loaded into the address space in this way, its 77function 78.Fn _init , 79if any, is called by the dynamic linker. 80If 81.Fa path 82has already been placed in the address space in a previous call to 83.Fn dlopen , 84it is not added a second time, although a reference count of 85.Fn dlopen 86operations on 87.Fa path 88is maintained. 89A null pointer supplied for 90.Fa path 91is interpreted as a reference to the main 92executable of the process. 93.Fa mode 94controls the way in which external function references from the 95loaded object are bound to their referents. 96It must contain one of the following values, possibly ORed with 97additional flags which will be described subsequently: 98.Bl -tag -width RTLD_LAZYX 99.It Dv RTLD_LAZY 100Each external function reference is resolved when the function is first 101called. 102.It Dv RTLD_NOW 103All external function references are bound immediately by 104.Fn dlopen . 105.El 106.Pp 107.Dv RTLD_LAZY 108is normally preferred, for reasons of efficiency. 109However, 110.Dv RTLD_NOW 111is useful to ensure that any undefined symbols are discovered during the 112call to 113.Fn dlopen . 114.Pp 115One of the following flags may be ORed into the 116.Fa mode 117argument: 118.Bl -tag -width RTLD_GLOBALX 119.It Dv RTLD_GLOBAL 120Symbols from this shared object and its directed acyclic graph (DAG) 121of needed objects will be available for resolving undefined references 122from all other shared objects. 123.It Dv RTLD_LOCAL 124Symbols in this shared object and its DAG of needed objects will be 125available for resolving undefined references only from other objects 126in the same DAG. This is the default, but it may be specified 127explicitly with this flag. 128.It Dv RTLD_TRACE 129When set, causes dynamic linker to exit after loading all objects 130needed by this shared object and printing a summary which includes 131the absolute pathnames of all objects, to standard output. 132With this flag 133.Fn dlopen 134will return to the caller only in the case of error. 135.El 136.Pp 137If 138.Fn dlopen 139fails, it returns a null pointer, and sets an error condition which may 140be interrogated with 141.Fn dlerror . 142.Pp 143.Fn dlsym 144returns the address binding of the symbol described in the null-terminated 145character string 146.Fa symbol , 147as it occurs in the shared object identified by 148.Fa handle . 149The symbols exported by objects added to the address space by 150.Fn dlopen 151can be accessed only through calls to 152.Fn dlsym . 153Such symbols do not supersede any definition of those symbols already present 154in the address space when the object is loaded, nor are they available to 155satisfy normal dynamic linking references. 156.Pp 157If 158.Fn dlsym 159is called with the special 160.Fa handle 161.Dv NULL , 162it is interpreted as a reference to the executable or shared object 163from which the call 164is being made. Thus a shared object can reference its own symbols. 165.Pp 166If 167.Fn dlsym 168is called with the special 169.Fa handle 170.Dv RTLD_DEFAULT , 171the search for the symbol follows the algorithm used for resolving 172undefined symbols when objects are loaded. The objects searched are 173as follows, in the given order: 174.Bl -enum 175.It 176The referencing object itself (or the object from which the call to 177.Fn dlsym 178is made), if that object was linked using the 179.Fl Wsymbolic 180option to 181.Xr ld 1 . 182.It 183All objects loaded at program start-up. 184.It 185All objects loaded via 186.Fn dlopen 187which are in needed-object DAGs that also contain the referencing object. 188.It 189All objects loaded via 190.Fn dlopen 191with the 192.Dv RTLD_GLOBAL 193flag set in the 194.Fa mode 195argument. 196.El 197.Pp 198If 199.Fn dlsym 200is called with the special 201.Fa handle 202.Dv RTLD_NEXT , 203then the search for the symbol is limited to the shared objects 204which were loaded after the one issuing the call to 205.Fn dlsym . 206Thus, if the function is called from the main program, all 207the shared libraries are searched. 208If it is called from a shared library, all subsequent shared 209libraries are searched. 210.Dv RTLD_NEXT 211is useful for implementing wrappers around library functions. 212For example, a wrapper function 213.Fn getpid 214could access the 215.Dq real 216.Fn getpid 217with 218.Li dlsym(RTLD_NEXT, \&"getpid\&") . 219(Actually, the 220.Fn dlfunc 221interface, below, should be used, since 222.Fn getpid 223is a function and not a data object.) 224.Pp 225.Fn dlsym 226returns a null pointer if the symbol cannot be found, and sets an error 227condition which may be queried with 228.Fn dlerror . 229.Pp 230.Fn dlfunc 231implements all of the behavior of 232.Fn dlsym , 233but has a return type which can be cast to a function pointer without 234triggering compiler diagnostics. 235.Po Fn dlsym 236returns a data pointer; in the C standard, conversions between 237data and function pointer types are undefined. Some compilers and 238.Xr lint 1 239utilities warn about such casts. 240.Pc 241The precise return type of 242.Fn dlfunc 243is unspecified; applications must cast it to an appropriate function pointer 244type. 245.Pp 246.Fn dlerror 247returns a null-terminated character string describing the last error that 248occurred during a call to 249.Fn dlopen , 250.Fn dlsym , 251.Fn dlfunc , 252or 253.Fn dlclose . 254If no such error has occurred, 255.Fn dlerror 256returns a null pointer. 257At each call to 258.Fn dlerror , 259the error indication is reset. Thus in the case of two calls 260to 261.Fn dlerror , 262where the second call follows the first immediately, the second call 263will always return a null pointer. 264.Pp 265.Fn dlclose 266deletes a reference to the shared object referenced by 267.Fa handle . 268If the reference count drops to 0, the object is removed from the 269address space, and 270.Fa handle 271is rendered invalid. 272Just before removing a shared object in this way, the dynamic linker 273calls the object's 274.Fn _fini 275function, if such a function is defined by the object. 276If 277.Fn dlclose 278is successful, it returns a value of 0. 279Otherwise it returns -1, and sets an error condition that can be 280interrogated with 281.Fn dlerror . 282.Pp 283The object-intrinsic functions 284.Fn _init 285and 286.Fn _fini 287are called with no arguments, and are not expected to return values. 288.Sh NOTES 289ELF executables need to be linked 290using the 291.Fl export-dynamic 292option to 293.Xr ld 1 294for symbols defined in the executable to become visible to 295.Fn dlsym . 296.Pp 297In previous implementations, it was necessary to prepend an underscore 298to all external symbols in order to gain symbol 299compatibility with object code compiled from the C language. This is 300still the case when using the (obsolete) 301.Fl aout 302option to the C language compiler. 303.Sh ERRORS 304.Fn dlopen , 305.Fn dlsym , 306and 307.Fn dlfunc 308return a null pointer in the event of errors. 309.Fn dlclose 310returns 0 on success, or -1 if an error occurred. 311Whenever an error has been detected, a message detailing it can be 312retrieved via a call to 313.Fn dlerror . 314.Sh SEE ALSO 315.Xr ld 1 , 316.Xr rtld 1 , 317.Xr dladdr 3 , 318.Xr link 5 319