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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 January 2, 2019 36.Dt DLOPEN 3 37.Os 38.Sh NAME 39.Nm dlopen , 40.Nm fdlopen , 41.Nm dlsym , 42.Nm dlvsym , 43.Nm dlfunc , 44.Nm dlerror , 45.Nm dlclose 46.Nd programmatic interface to the dynamic linker 47.Sh LIBRARY 48.Lb libc 49.Sh SYNOPSIS 50.In dlfcn.h 51.Ft void * 52.Fn dlopen "const char *path" "int mode" 53.Ft void * 54.Fn fdlopen "int fd" "int mode" 55.Ft void * 56.Fn dlsym "void * restrict handle" "const char * restrict symbol" 57.Ft void * 58.Fn dlvsym "void * restrict handle" "const char * restrict symbol" "const char * restrict version" 59.Ft dlfunc_t 60.Fn dlfunc "void * restrict handle" "const char * restrict symbol" 61.Ft char * 62.Fn dlerror "void" 63.Ft int 64.Fn dlclose "void *handle" 65.Sh DESCRIPTION 66These functions provide a simple programmatic interface to the services of the 67dynamic linker. 68Operations are provided to add new shared objects to a 69program's address space, to obtain the address bindings of symbols 70defined by such 71objects, and to remove such objects when their use is no longer required. 72.Pp 73The 74.Fn dlopen 75function 76provides access to the shared object in 77.Fa path , 78returning a descriptor that can be used for later 79references to the object in calls to 80.Fn dlsym , 81.Fn dlvsym 82and 83.Fn dlclose . 84If 85.Fa path 86was not in the address space prior to the call to 87.Fn dlopen , 88it is placed in the address space. 89When an object is first loaded into the address space in this way, its 90function 91.Fn _init , 92if any, is called by the dynamic linker. 93If 94.Fa path 95has already been placed in the address space in a previous call to 96.Fn dlopen , 97it is not added a second time, although a reference count of 98.Fn dlopen 99operations on 100.Fa path 101is maintained. 102A null pointer supplied for 103.Fa path 104is interpreted as a reference to the main 105executable of the process. 106The 107.Fa mode 108argument 109controls the way in which external function references from the 110loaded object are bound to their referents. 111It must contain one of the following values, possibly ORed with 112additional flags which will be described subsequently: 113.Bl -tag -width RTLD_LAZYX 114.It Dv RTLD_LAZY 115Each external function reference is resolved when the function is first 116called. 117.It Dv RTLD_NOW 118All external function references are bound immediately by 119.Fn dlopen . 120.El 121.Pp 122.Dv RTLD_LAZY 123is normally preferred, for reasons of efficiency. 124However, 125.Dv RTLD_NOW 126is useful to ensure that any undefined symbols are discovered during the 127call to 128.Fn dlopen . 129.Pp 130One of the following flags may be ORed into the 131.Fa mode 132argument: 133.Bl -tag -width RTLD_NODELETE 134.It Dv RTLD_GLOBAL 135Symbols from this shared object and its directed acyclic graph (DAG) 136of needed objects will be available for resolving undefined references 137from all other shared objects. 138.It Dv RTLD_LOCAL 139Symbols in this shared object and its DAG of needed objects will be 140available for resolving undefined references only from other objects 141in the same DAG. 142This is the default, but it may be specified 143explicitly with this flag. 144.It Dv RTLD_TRACE 145When set, causes dynamic linker to exit after loading all objects 146needed by this shared object and printing a summary which includes 147the absolute pathnames of all objects, to standard output. 148With this flag 149.Fn dlopen 150will return to the caller only in the case of error. 151.It Dv RTLD_NODELETE 152Prevents unload of the loaded object on 153.Fn dlclose . 154The same behaviour may be requested by 155.Fl "z nodelete" 156option of the static linker 157.Xr ld 1 . 158.It Dv RTLD_NOLOAD 159Only return valid handle for the object if it is already loaded in 160the process address space, otherwise 161.Dv NULL 162is returned. 163Other mode flags may be specified, which will be applied for promotion 164for the found object. 165.El 166.Pp 167If 168.Fn dlopen 169fails, it returns a null pointer, and sets an error condition which may 170be interrogated with 171.Fn dlerror . 172.Pp 173The 174.Fn fdlopen 175function is similar to 176.Fn dlopen , 177but it takes the file descriptor argument 178.Fa fd , 179which is used for the file operations needed to load an object 180into the address space. 181The file descriptor 182.Fa fd 183is not closed by the function regardless a result of execution, 184but a duplicate of the file descriptor is. 185This may be important if a 186.Xr lockf 3 187lock is held on the passed descriptor. 188The 189.Fa fd 190argument -1 is interpreted as a reference to the main 191executable of the process, similar to 192.Va NULL 193value for the 194.Fa name 195argument to 196.Fn dlopen . 197The 198.Fn fdlopen 199function can be used by the code that needs to perform 200additional checks on the loaded objects, to prevent races with 201symlinking or renames. 202.Pp 203The 204.Fn dlsym 205function 206returns the address binding of the symbol described in the null-terminated 207character string 208.Fa symbol , 209as it occurs in the shared object identified by 210.Fa handle . 211The symbols exported by objects added to the address space by 212.Fn dlopen 213can be accessed only through calls to 214.Fn dlsym . 215Such symbols do not supersede any definition of those symbols already present 216in the address space when the object is loaded, nor are they available to 217satisfy normal dynamic linking references. 218.Pp 219If 220.Fn dlsym 221is called with the special 222.Fa handle 223.Dv NULL , 224it is interpreted as a reference to the executable or shared object 225from which the call 226is being made. 227Thus a shared object can reference its own symbols. 228.Pp 229If 230.Fn dlsym 231is called with the special 232.Fa handle 233.Dv RTLD_DEFAULT , 234the search for the symbol follows the algorithm used for resolving 235undefined symbols when objects are loaded. 236The objects searched are 237as follows, in the given order: 238.Bl -enum 239.It 240The referencing object itself (or the object from which the call to 241.Fn dlsym 242is made), if that object was linked using the 243.Fl Bsymbolic 244option to 245.Xr ld 1 . 246.It 247All objects loaded at program start-up. 248.It 249All objects loaded via 250.Fn dlopen 251with the 252.Dv RTLD_GLOBAL 253flag set in the 254.Fa mode 255argument. 256.It 257All objects loaded via 258.Fn dlopen 259which are in needed-object DAGs that also contain the referencing object. 260.El 261.Pp 262If 263.Fn dlsym 264is called with the special 265.Fa handle 266.Dv RTLD_NEXT , 267then the search for the symbol is limited to the shared objects 268which were loaded after the one issuing the call to 269.Fn dlsym . 270Thus, if the function is called from the main program, all 271the shared libraries are searched. 272If it is called from a shared library, all subsequent shared 273libraries are searched. 274.Dv RTLD_NEXT 275is useful for implementing wrappers around library functions. 276For example, a wrapper function 277.Fn getpid 278could access the 279.Dq real 280.Fn getpid 281with 282.Li dlsym(RTLD_NEXT, \&"getpid\&") . 283(Actually, the 284.Fn dlfunc 285interface, below, should be used, since 286.Fn getpid 287is a function and not a data object.) 288.Pp 289If 290.Fn dlsym 291is called with the special 292.Fa handle 293.Dv RTLD_SELF , 294then the search for the symbol is limited to the shared object 295issuing the call to 296.Fn dlsym 297and those shared objects which were loaded after it. 298.Pp 299The 300.Fn dlsym 301function 302returns a null pointer if the symbol cannot be found, and sets an error 303condition which may be queried with 304.Fn dlerror . 305.Pp 306The 307.Fn dlvsym 308function behaves like 309.Fn dlsym , 310but takes an extra argument 311.Fa version : 312a null-terminated character string which is used to request a specific version 313of 314.Fa symbol . 315.Pp 316The 317.Fn dlfunc 318function 319implements all of the behavior of 320.Fn dlsym , 321but has a return type which can be cast to a function pointer without 322triggering compiler diagnostics. 323(The 324.Fn dlsym 325function 326returns a data pointer; in the C standard, conversions between 327data and function pointer types are undefined. 328Some compilers and 329.Xr lint 1 330utilities warn about such casts.) 331The precise return type of 332.Fn dlfunc 333is unspecified; applications must cast it to an appropriate function pointer 334type. 335.Pp 336The 337.Fn dlerror 338function 339returns a null-terminated character string describing the last error that 340occurred during a call to 341.Fn dlopen , 342.Fn dladdr , 343.Fn dlinfo , 344.Fn dlsym , 345.Fn dlvsym , 346.Fn dlfunc , 347or 348.Fn dlclose . 349If no such error has occurred, 350.Fn dlerror 351returns a null pointer. 352At each call to 353.Fn dlerror , 354the error indication is reset. 355Thus in the case of two calls 356to 357.Fn dlerror , 358where the second call follows the first immediately, the second call 359will always return a null pointer. 360.Pp 361The 362.Fn dlclose 363function 364deletes a reference to the shared object referenced by 365.Fa handle . 366If the reference count drops to 0, the object is removed from the 367address space, and 368.Fa handle 369is rendered invalid. 370Just before removing a shared object in this way, the dynamic linker 371calls the object's 372.Fn _fini 373function, if such a function is defined by the object. 374If 375.Fn dlclose 376is successful, it returns a value of 0. 377Otherwise it returns -1, and sets an error condition that can be 378interrogated with 379.Fn dlerror . 380.Pp 381The object-intrinsic functions 382.Fn _init 383and 384.Fn _fini 385are called with no arguments, and are not expected to return values. 386.Sh NOTES 387ELF executables need to be linked 388using the 389.Fl export-dynamic 390option to 391.Xr ld 1 392for symbols defined in the executable to become visible to 393.Fn dlsym , 394.Fn dlvsym 395or 396.Fn dlfunc 397.Pp 398Other ELF platforms require linking with 399.Lb libdl 400to provide 401.Fn dlopen 402and other functions. 403.Fx 404does not require linking with the library, but supports it for compatibility. 405.Pp 406In previous implementations, it was necessary to prepend an underscore 407to all external symbols in order to gain symbol 408compatibility with object code compiled from the C language. 409This is 410still the case when using the (obsolete) 411.Fl aout 412option to the C language compiler. 413.Sh ERRORS 414The 415.Fn dlopen , 416.Fn fdlopen , 417.Fn dlsym , 418.Fn dlvsym , 419and 420.Fn dlfunc 421functions 422return a null pointer in the event of errors. 423The 424.Fn dlclose 425function 426returns 0 on success, or -1 if an error occurred. 427Whenever an error has been detected, a message detailing it can be 428retrieved via a call to 429.Fn dlerror . 430.Sh SEE ALSO 431.Xr ld 1 , 432.Xr rtld 1 , 433.Xr dladdr 3 , 434.Xr dlinfo 3 , 435.Xr link 5 436