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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.\" 34.Dd May 14, 2020 35.Dt DLOPEN 3 36.Os 37.Sh NAME 38.Nm dlopen , 39.Nm fdlopen , 40.Nm dlsym , 41.Nm dlvsym , 42.Nm dlfunc , 43.Nm dlerror , 44.Nm dlclose 45.Nd programmatic interface to the dynamic linker 46.Sh LIBRARY 47.Lb libc 48.Sh SYNOPSIS 49.In dlfcn.h 50.Ft void * 51.Fn dlopen "const char *path" "int mode" 52.Ft void * 53.Fn fdlopen "int fd" "int mode" 54.Ft void * 55.Fn dlsym "void * restrict handle" "const char * restrict symbol" 56.Ft void * 57.Fn dlvsym "void * restrict handle" "const char * restrict symbol" "const char * restrict version" 58.Ft dlfunc_t 59.Fn dlfunc "void * restrict handle" "const char * restrict symbol" 60.Ft char * 61.Fn dlerror "void" 62.Ft int 63.Fn dlclose "void *handle" 64.Sh DESCRIPTION 65These functions provide a simple programmatic interface to the services of the 66dynamic linker. 67Operations are provided to add new shared objects to a 68program's address space, to obtain the address bindings of symbols 69defined by such 70objects, and to remove such objects when their use is no longer required. 71.Pp 72The 73.Fn dlopen 74function 75provides access to the shared object in 76.Fa path , 77returning a descriptor that can be used for later 78references to the object in calls to 79.Fn dlsym , 80.Fn dlvsym 81and 82.Fn dlclose . 83If 84.Fa path 85was not in the address space prior to the call to 86.Fn dlopen , 87it is placed in the address space. 88When an object is first loaded into the address space in this way, its 89function 90.Fn _init , 91if any, is called by the dynamic linker. 92If 93.Fa path 94has already been placed in the address space in a previous call to 95.Fn dlopen , 96it is not added a second time, although a reference count of 97.Fn dlopen 98operations on 99.Fa path 100is maintained. 101A null pointer supplied for 102.Fa path 103is interpreted as a reference to the main 104executable of the process. 105The 106.Fa mode 107argument 108controls the way in which external function references from the 109loaded object are bound to their referents. 110It must contain one of the following values, possibly ORed with 111additional flags which will be described subsequently: 112.Bl -tag -width RTLD_LAZYX 113.It Dv RTLD_LAZY 114Each external function reference is resolved when the function is first 115called. 116.It Dv RTLD_NOW 117All external function references are bound immediately by 118.Fn dlopen . 119.El 120.Pp 121.Dv RTLD_LAZY 122is normally preferred, for reasons of efficiency. 123However, 124.Dv RTLD_NOW 125is useful to ensure that any undefined symbols are discovered during the 126call to 127.Fn dlopen . 128.Pp 129One of the following flags may be ORed into the 130.Fa mode 131argument: 132.Bl -tag -width RTLD_NODELETE 133.It Dv RTLD_GLOBAL 134Symbols from this shared object and its directed acyclic graph (DAG) 135of needed objects will be available for resolving undefined references 136from all other shared objects. 137.It Dv RTLD_LOCAL 138Symbols in this shared object and its DAG of needed objects will be 139available for resolving undefined references only from other objects 140in the same DAG. 141This is the default, but it may be specified 142explicitly with this flag. 143.It Dv RTLD_TRACE 144When set, causes dynamic linker to exit after loading all objects 145needed by this shared object and printing a summary which includes 146the absolute pathnames of all objects, to standard output. 147With this flag 148.Fn dlopen 149will return to the caller only in the case of error. 150.It Dv RTLD_NODELETE 151Prevents unload of the loaded object on 152.Fn dlclose . 153The same behaviour may be requested by 154.Fl "z nodelete" 155option of the static linker 156.Xr ld 1 . 157.It Dv RTLD_NOLOAD 158Only return valid handle for the object if it is already loaded in 159the process address space, otherwise 160.Dv NULL 161is returned. 162Other mode flags may be specified, which will be applied for promotion 163for the found object. 164.It Dv RTLD_DEEPBIND 165Symbols from the loaded library are put before global symbols when 166resolving symbolic references originated from the library. 167.El 168.Pp 169If 170.Fn dlopen 171fails, it returns a null pointer, and sets an error condition which may 172be interrogated with 173.Fn dlerror . 174.Pp 175The 176.Fn fdlopen 177function is similar to 178.Fn dlopen , 179but it takes the file descriptor argument 180.Fa fd , 181which is used for the file operations needed to load an object 182into the address space. 183The file descriptor 184.Fa fd 185is not closed by the function regardless a result of execution, 186but a duplicate of the file descriptor is. 187This may be important if a 188.Xr lockf 3 189lock is held on the passed descriptor. 190The 191.Fa fd 192argument -1 is interpreted as a reference to the main 193executable of the process, similar to 194.Va NULL 195value for the 196.Fa name 197argument to 198.Fn dlopen . 199The 200.Fn fdlopen 201function can be used by the code that needs to perform 202additional checks on the loaded objects, to prevent races with 203symlinking or renames. 204.Pp 205The 206.Fn dlsym 207function 208returns the address binding of the symbol described in the null-terminated 209character string 210.Fa symbol , 211as it occurs in the shared object identified by 212.Fa handle . 213The symbols exported by objects added to the address space by 214.Fn dlopen 215can be accessed only through calls to 216.Fn dlsym . 217Such symbols do not supersede any definition of those symbols already present 218in the address space when the object is loaded, nor are they available to 219satisfy normal dynamic linking references. 220.Pp 221If 222.Fn dlsym 223is called with the special 224.Fa handle 225.Dv NULL , 226it is interpreted as a reference to the executable or shared object 227from which the call 228is being made. 229Thus a shared object can reference its own symbols. 230.Pp 231If 232.Fn dlsym 233is called with the special 234.Fa handle 235.Dv RTLD_DEFAULT , 236the search for the symbol follows the algorithm used for resolving 237undefined symbols when objects are loaded. 238The objects searched are 239as follows, in the given order: 240.Bl -enum 241.It 242The referencing object itself (or the object from which the call to 243.Fn dlsym 244is made), if that object was linked using the 245.Fl Bsymbolic 246option to 247.Xr ld 1 . 248.It 249All objects loaded at program start-up. 250.It 251All objects loaded via 252.Fn dlopen 253with the 254.Dv RTLD_GLOBAL 255flag set in the 256.Fa mode 257argument. 258.It 259All objects loaded via 260.Fn dlopen 261which are in needed-object DAGs that also contain the referencing object. 262.El 263.Pp 264If 265.Fn dlsym 266is called with the special 267.Fa handle 268.Dv RTLD_NEXT , 269then the search for the symbol is limited to the shared objects 270which were loaded after the one issuing the call to 271.Fn dlsym . 272Thus, if the function is called from the main program, all 273the shared libraries are searched. 274If it is called from a shared library, all subsequent shared 275libraries are searched. 276.Dv RTLD_NEXT 277is useful for implementing wrappers around library functions. 278For example, a wrapper function 279.Fn getpid 280could access the 281.Dq real 282.Fn getpid 283with 284.Li dlsym(RTLD_NEXT, \&"getpid\&") . 285(Actually, the 286.Fn dlfunc 287interface, below, should be used, since 288.Fn getpid 289is a function and not a data object.) 290.Pp 291If 292.Fn dlsym 293is called with the special 294.Fa handle 295.Dv RTLD_SELF , 296then the search for the symbol is limited to the shared object 297issuing the call to 298.Fn dlsym 299and those shared objects which were loaded after it. 300.Pp 301The 302.Fn dlsym 303function 304returns a null pointer if the symbol cannot be found, and sets an error 305condition which may be queried with 306.Fn dlerror . 307.Pp 308The 309.Fn dlvsym 310function behaves like 311.Fn dlsym , 312but takes an extra argument 313.Fa version : 314a null-terminated character string which is used to request a specific version 315of 316.Fa symbol . 317.Pp 318The 319.Fn dlfunc 320function 321implements all of the behavior of 322.Fn dlsym , 323but has a return type which can be cast to a function pointer without 324triggering compiler diagnostics. 325(The 326.Fn dlsym 327function 328returns an object pointer; in the C standard, conversions between 329object and function pointer types are undefined. 330Some compilers and lint utilities 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