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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 14, 2020 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.It Dv RTLD_DEEPBIND 166Symbols from the loaded library are put before global symbols when 167resolving symbolic references originated from the library. 168.El 169.Pp 170If 171.Fn dlopen 172fails, it returns a null pointer, and sets an error condition which may 173be interrogated with 174.Fn dlerror . 175.Pp 176The 177.Fn fdlopen 178function is similar to 179.Fn dlopen , 180but it takes the file descriptor argument 181.Fa fd , 182which is used for the file operations needed to load an object 183into the address space. 184The file descriptor 185.Fa fd 186is not closed by the function regardless a result of execution, 187but a duplicate of the file descriptor is. 188This may be important if a 189.Xr lockf 3 190lock is held on the passed descriptor. 191The 192.Fa fd 193argument -1 is interpreted as a reference to the main 194executable of the process, similar to 195.Va NULL 196value for the 197.Fa name 198argument to 199.Fn dlopen . 200The 201.Fn fdlopen 202function can be used by the code that needs to perform 203additional checks on the loaded objects, to prevent races with 204symlinking or renames. 205.Pp 206The 207.Fn dlsym 208function 209returns the address binding of the symbol described in the null-terminated 210character string 211.Fa symbol , 212as it occurs in the shared object identified by 213.Fa handle . 214The symbols exported by objects added to the address space by 215.Fn dlopen 216can be accessed only through calls to 217.Fn dlsym . 218Such symbols do not supersede any definition of those symbols already present 219in the address space when the object is loaded, nor are they available to 220satisfy normal dynamic linking references. 221.Pp 222If 223.Fn dlsym 224is called with the special 225.Fa handle 226.Dv NULL , 227it is interpreted as a reference to the executable or shared object 228from which the call 229is being made. 230Thus a shared object can reference its own symbols. 231.Pp 232If 233.Fn dlsym 234is called with the special 235.Fa handle 236.Dv RTLD_DEFAULT , 237the search for the symbol follows the algorithm used for resolving 238undefined symbols when objects are loaded. 239The objects searched are 240as follows, in the given order: 241.Bl -enum 242.It 243The referencing object itself (or the object from which the call to 244.Fn dlsym 245is made), if that object was linked using the 246.Fl Bsymbolic 247option to 248.Xr ld 1 . 249.It 250All objects loaded at program start-up. 251.It 252All objects loaded via 253.Fn dlopen 254with the 255.Dv RTLD_GLOBAL 256flag set in the 257.Fa mode 258argument. 259.It 260All objects loaded via 261.Fn dlopen 262which are in needed-object DAGs that also contain the referencing object. 263.El 264.Pp 265If 266.Fn dlsym 267is called with the special 268.Fa handle 269.Dv RTLD_NEXT , 270then the search for the symbol is limited to the shared objects 271which were loaded after the one issuing the call to 272.Fn dlsym . 273Thus, if the function is called from the main program, all 274the shared libraries are searched. 275If it is called from a shared library, all subsequent shared 276libraries are searched. 277.Dv RTLD_NEXT 278is useful for implementing wrappers around library functions. 279For example, a wrapper function 280.Fn getpid 281could access the 282.Dq real 283.Fn getpid 284with 285.Li dlsym(RTLD_NEXT, \&"getpid\&") . 286(Actually, the 287.Fn dlfunc 288interface, below, should be used, since 289.Fn getpid 290is a function and not a data object.) 291.Pp 292If 293.Fn dlsym 294is called with the special 295.Fa handle 296.Dv RTLD_SELF , 297then the search for the symbol is limited to the shared object 298issuing the call to 299.Fn dlsym 300and those shared objects which were loaded after it. 301.Pp 302The 303.Fn dlsym 304function 305returns a null pointer if the symbol cannot be found, and sets an error 306condition which may be queried with 307.Fn dlerror . 308.Pp 309The 310.Fn dlvsym 311function behaves like 312.Fn dlsym , 313but takes an extra argument 314.Fa version : 315a null-terminated character string which is used to request a specific version 316of 317.Fa symbol . 318.Pp 319The 320.Fn dlfunc 321function 322implements all of the behavior of 323.Fn dlsym , 324but has a return type which can be cast to a function pointer without 325triggering compiler diagnostics. 326(The 327.Fn dlsym 328function 329returns a data pointer; in the C standard, conversions between 330data and function pointer types are undefined. 331Some compilers and 332.Xr lint 1 333utilities warn about such casts.) 334The precise return type of 335.Fn dlfunc 336is unspecified; applications must cast it to an appropriate function pointer 337type. 338.Pp 339The 340.Fn dlerror 341function 342returns a null-terminated character string describing the last error that 343occurred during a call to 344.Fn dlopen , 345.Fn dladdr , 346.Fn dlinfo , 347.Fn dlsym , 348.Fn dlvsym , 349.Fn dlfunc , 350or 351.Fn dlclose . 352If no such error has occurred, 353.Fn dlerror 354returns a null pointer. 355At each call to 356.Fn dlerror , 357the error indication is reset. 358Thus in the case of two calls 359to 360.Fn dlerror , 361where the second call follows the first immediately, the second call 362will always return a null pointer. 363.Pp 364The 365.Fn dlclose 366function 367deletes a reference to the shared object referenced by 368.Fa handle . 369If the reference count drops to 0, the object is removed from the 370address space, and 371.Fa handle 372is rendered invalid. 373Just before removing a shared object in this way, the dynamic linker 374calls the object's 375.Fn _fini 376function, if such a function is defined by the object. 377If 378.Fn dlclose 379is successful, it returns a value of 0. 380Otherwise it returns -1, and sets an error condition that can be 381interrogated with 382.Fn dlerror . 383.Pp 384The object-intrinsic functions 385.Fn _init 386and 387.Fn _fini 388are called with no arguments, and are not expected to return values. 389.Sh NOTES 390ELF executables need to be linked 391using the 392.Fl export-dynamic 393option to 394.Xr ld 1 395for symbols defined in the executable to become visible to 396.Fn dlsym , 397.Fn dlvsym 398or 399.Fn dlfunc 400.Pp 401Other ELF platforms require linking with 402.Lb libdl 403to provide 404.Fn dlopen 405and other functions. 406.Fx 407does not require linking with the library, but supports it for compatibility. 408.Pp 409In previous implementations, it was necessary to prepend an underscore 410to all external symbols in order to gain symbol 411compatibility with object code compiled from the C language. 412This is 413still the case when using the (obsolete) 414.Fl aout 415option to the C language compiler. 416.Sh ERRORS 417The 418.Fn dlopen , 419.Fn fdlopen , 420.Fn dlsym , 421.Fn dlvsym , 422and 423.Fn dlfunc 424functions 425return a null pointer in the event of errors. 426The 427.Fn dlclose 428function 429returns 0 on success, or -1 if an error occurred. 430Whenever an error has been detected, a message detailing it can be 431retrieved via a call to 432.Fn dlerror . 433.Sh SEE ALSO 434.Xr ld 1 , 435.Xr rtld 1 , 436.Xr dladdr 3 , 437.Xr dlinfo 3 , 438.Xr link 5 439