dlclose(3) [osx man page]
DLCLOSE(3) BSD Library Functions Manual DLCLOSE(3) NAME
dlclose -- close a dynamic library or bundle SYNOPSIS
#include <dlfcn.h> int dlclose(void* handle); DESCRIPTION
dlclose() releases a reference to the dynamic library or bundle referenced by handle. If the reference count drops to 0, the bundle is removed from the address space, and handle is rendered invalid. Just before removing a dynamic library or bundle in this way, any termina- tion routines in it are called. handle is the value returned by a previous call to dlopen. Prior to Mac OS X 10.5, only bundles could be unloaded. Starting in Mac OS X 10.5, dynamic libraries may also be unloaded. There are a cou- ple of cases in which a dynamic library will never be unloaded: 1) the main executable links against it, 2) An API that does not supoort unloading (e.g. NSAddImage()) was used to load it or some other dynnamic library that depends on it, 3) the dynamic library is in dyld's shared cache. RETURN VALUES
If dlclose() is successful, it returns a value of 0. Otherwise it returns -1, and sets an error string that can be retrived with dlerror(). SEE ALSO
dlopen(3) dlsym(3) dlerror(3) dyld(3) ld(1) cc(1) Nov 6, 2006
Check Out this Related Man Page
DLOPEN(3) Linux Programmer's Manual DLOPEN(3) NAME
dlclose, dlerror, dlopen, dlsym - Programming interface to dynamic linking loader. SYNOPSIS
#include <dlfcn.h> void *dlopen(const char *filename, int flag); const char *dlerror(void); void *dlsym(void *handle, char *symbol); int dlclose(void *handle); Special symbols: _init, _fini. DESCRIPTION
dlopen loads a dynamic library from the file named by the null terminated string filename and returns an opaque "handle" for the dynamic library. If filename is not an absolute path (i.e., it does not begin with a "/"), then the file is searched for in the following loca- tions: A colon-separated list of directories in the user's LD_LIBRARY_PATH environment variable. The list of libraries cached in /etc/ld.so.cache. /lib, followed by /usr/lib. If filename is a NULL pointer, then the returned handle is for the main program. External references in the library are resolved using the libraries in that library's dependency list and any other libraries previously opened with the RTLD_GLOBAL flag. If the executable was linked with the flag "-rdynamic", then the global symbols in the executable will also be used to resolve references in a dynamically loaded library. flag must be either RTLD_LAZY, meaning resolve undefined symbols as code from the dynamic library is executed, or RTLD_NOW, meaning resolve all undefined symbols before dlopen returns, and fail if this cannot be done. Optionally, RTLD_GLOBAL may be or'ed with flag, in which case the external symbols defined in the library will be made available to subsequently loaded libraries. If the library exports a routine named _init, then that code is executed before dlopen returns. If the same library is loaded twice with dlopen(), the same file handle is returned. The dl library maintains link counts for dynamic file handles, so a dynamic library is not deallocated until dlclose has been called on it as many times as dlopen has succeeded on it. If dlopen fails for any reason, it returns NULL. A human readable string describing the most recent error that occurred from any of the dl routines (dlopen, dlsym or dlclose) can be extracted with dlerror(). dlerror returns NULL if no errors have occurred since initialization or since it was last called. (Calling dlerror() twice consecutively, will always result in the second call returning NULL.) dlsym takes a "handle" of a dynamic library returned by dlopen and the null terminated symbol name, returning the address where that symbol is loaded. If the symbol is not found, dlsym returns NULL; however, the correct way to test for an error from dlsym is to save the result of dlerror into a variable, and then check if saved value is not NULL. This is because the value of the symbol could actually be NULL. It is also necessary to save the results of dlerror into a variable because if dlerror is called again, it will return NULL. There are two special pseudo-handles, RTLD_DEFAULT and RTLD_NEXT. The former will find the first occurrence of the desired symbol using the default library search order. The latter, which is usable only from within a dynamic library, will find the next occurrence of a func- tion in the search order after the current library. This allows one to provide a wrapper around a function in another shared library. dlclose decrements the reference count on the dynamic library handle handle. If the reference count drops to zero and no other loaded libraries use symbols in it, then the dynamic library is unloaded. If the dynamic library exports a routine named _fini, then that routine is called just before the library is unloaded. RETURN VALUE
dlclose returns 0 on success, and non-zero on error. EXAMPLE
Load the math library, and print the cosine of 2.0: #include <stdio.h> #include <dlfcn.h> int main(int argc, char **argv) { void *handle; double (*cosine)(double); char *error; handle = dlopen ("libm.so", RTLD_LAZY); if (!handle) { fprintf (stderr, "%s ", dlerror()); exit(1); } cosine = dlsym(handle, "cos"); if ((error = dlerror()) != NULL) { fprintf (stderr, "%s ", error); exit(1); } printf ("%f ", (*cosine)(2.0)); dlclose(handle); return 0; } If this program were in a file named "foo.c", you would build the program with the following command: gcc -rdynamic -o foo foo.c -ldl NOTES
The symbols RTLD_DEFAULT and RTLD_NEXT are defined by <dlfcn.h> only when _GNU_SOURCE was defined before including it. ACKNOWLEDGEMENTS
The dlopen interface standard comes from Solaris. The Linux dlopen implementation was primarily written by Eric Youngdale with help from Mitch D'Souza, David Engel, Hongjiu Lu, Andreas Schwab and others. The manual page was written by Adam Richter. SEE ALSO
ld(1), ld.so(8), ldconfig(8), ldd(1), ld.so.info Linux 2001-12-14 DLOPEN(3)