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@(#) dlopen.3 1.6 90/01/31 SMI
.Dd September 24, 1989 .Os FreeBSD .Dt DLOPEN 3 .Sh NAME .Nm dlopen, dlsym, dlerror, dlclose .Nd programmatic interface to the dynamic linker .Sh SYNOPSIS .Fd #include <dlfcn.h> .Ft void * .Fn dlopen "const char *path" "int mode" .Ft void * .Fn dlsym "void *handle" "const char *symbol" .Ft const char * .Fn dlerror "void" .Ft int .Fn dlclose "void *handle" .Sh DESCRIPTION These functions provide a simple programmatic interface to the services of the dynamic linker. Operations are provided to add new shared objects to a program's address space, to obtain the address bindings of symbols defined by such objects, and to remove such objects when their use is no longer required.
p .Fn dlopen provides access to the shared object in .Fa path , returning a descriptor that can be used for later references to the object in calls to .Fn dlsym and .Fn dlclose . If .Fa path was not in the address space prior to the call to .Fn dlopen , it is placed in the address space. When an object is first loaded into the address space in this way, its function .Fn _init , if any, is called by the dynamic linker. (Note that .Ql _init is the name as expressed in the C language. From assembly language, the name would appear as .Ql __init instead.) If .Fa path has already been placed in the address space in a previous call to .Fn dlopen , it is not added a second time, although a reference count of .Fn dlopen operations on .Fa path is maintained. A null pointer supplied for .Fa path is interpreted as a reference to the main executable of the process. .Fa mode controls the way in which external function references from the loaded object are bound to their referents. It must contain one of the following values: l -tag -width RTLD_LAZYX t Dv RTLD_LAZY Each external function reference is resolved when the function is first called. t Dv RTLD_NOW All external function references are bound immediately by .Fn dlopen . .El
p .Dv RTLD_LAZY is normally preferred, for reasons of efficiency. However, .Dv RTLD_NOW is useful to ensure that any undefined symbols are discovered during the call to .Fn dlopen . If .Fn dlopen fails, it returns a null pointer, and sets an error condition which may be interrogated with .Fn dlerror .
p .Fn dlsym returns the address binding of the symbol described in the null-terminated character string .Fa symbol , as it occurs in the shared object identified by .Fa handle . Note that .Fa symbol is the assembly language representation of the symbol name. The assembly language representation of a C language symbol contains an extra underscore at the beginning. For example, the symbol .Ql foo in C would appear as .Ql _foo in assembly language, and in the .Fa symbol argument to .Fn dlsym . The symbols exported by objects added to the address space by .Fn dlopen can be accessed only through calls to .Fn dlsym . Such symbols do not supersede any definition of those symbols already present in the address space when the object is loaded, nor are they available to satisfy normal dynamic linking references. A null pointer supplied as the value of .Fa handle is interpreted as a reference to the executable from which the call to .Fn dlsym is being made. Thus a shared object can reference its own symbols. .Fn dlsym returns a null pointer if the symbol cannot be found, and sets an error condition which may be queried with .Fn dlerror .
p If .Fn dlsym is called with the special .Fa handle .Dv RTLD_NEXT , then the search for the symbol is limited to the shared objects which were loaded after the one issuing the call to .Fn dlsym . Thus, if the function is called from the main program, all the shared libraries are searched. If it is called from a shared library, all subsequent shared libraries are searched. .Dv RTLD_NEXT is useful for implementing wrappers around library functions. For example, a wrapper function .Fn getpid could access the .Dq real .Fn getpid with .Li dlsym(RTLD_NEXT, "_getpid") .
p .Fn dlerror returns a null-terminated character string describing the last error that occurred during a call to .Fn dlopen , .Fn dlsym , or .Fn dlclose . If no such error has occurred, .Fn dlerror returns a null pointer. At each call to .Fn dlerror , the error indication is reset. Thus in the case of two calls to .Fn dlerror , where the second call follows the first immediately, the second call will always return a null pointer.
p .Fn dlclose deletes a reference to the shared object referenced by .Fa handle . If the reference count drops to 0, the object is removed from the address space, and .Fa handle is rendered invalid. Just before removing a shared object in this way, the dynamic linker calls the object's .Fn _fini function, if such a function is defined by the object. As with .Ql _init , .Ql _fini is the C language name of the function. If .Fn dlclose is successful, it returns a value of 0. Otherwise it returns -1, and sets an error condition that can be interrogated with .Fn dlerror .
p The object-intrinsic functions .Fn _init and .Fn _fini are called with no arguments, and are not expected to return values. .Sh NOTES ELF executables need to be linked using the .Fl export-dynamic option to .Xr ld 1 for symbols defined in the executable to become visible to .Fn dlsym . .Sh ERRORS .Fn dlopen and .Fn dlsym return the null pointer in the event of errors. .Fn dlclose returns 0 on success, or -1 if an error occurred. Whenever an error has been detected, a message detailing it can be retrieved via a call to .Fn dlerror . .Sh SEE ALSO .Xr ld 1 , .Xr rtld 1 , .Xr link 5