.\" -*- nroff -*- .\" Copyright 1995 Yggdrasil Computing, Incorporated. .\" written by Adam J. Richter (adam@yggdrasil.com), .\" with typesetting help from Daniel Quinlan (quinlan@yggdrasil.com). .\" .\" This is free documentation; you can redistribute it and/or .\" modify it under the terms of the GNU General Public License as .\" published by the Free Software Foundation; either version 2 of .\" the License, or (at your option) any later version. .\" .\" The GNU General Public License's references to "object code" .\" and "executables" are to be interpreted as the output of any .\" document formatting or typesetting system, including .\" intermediate and printed output. .\" .\" This manual is distributed in the hope that it will be useful, .\" but WITHOUT ANY WARRANTY; without even the implied warranty of .\" MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the .\" GNU General Public License for more details. .\" .\" You should have received a copy of the GNU General Public .\" License along with this manual; if not, write to the Free .\" Software Foundation, Inc., 675 Mass Ave, Cambridge, MA 02139, .\" USA. .\" .TH DLOPEN 3 "16 May 1995" "Linux" "Linux Programmer's Manual" .SH NAME dlclose, dlerror, dlopen, dlsym \- Programming interface to dynamic linking loader. .SH SYNOPSIS .B #include <dlfcn.h> .sp .BI "void *dlopen (const char *" "filename" ", int " flag "); .br .BI "const char *dlerror(void);" .br .BI "void *dlsym(void *"handle ", char *"symbol ");" .br .BI "int dladdr(void *"address ", Dl_info *"dlip ");" .br .BI "int dlclose (void *"handle "); .sp Special symbols: .BR "_init" ", " "_fini" ". " .SH DESCRIPTION .B dlopen loads a dynamic library from the file named by the null terminated string .I filename and returns an opaque "handle" for the dynamic library. If .I filename is not an absolute path (i.e., it does not begin with a "/"), then the file is searched for in the following locations: .RS .PP A colon-separated list of directories in the user's \fBLD_LIBRARY\fP path environment variable. .PP The list of libraries specified in \fI/etc/ld.so.cache\fP. .PP \fI/usr/lib\fP, followed by \fI/lib\fP. .RE .PP If .I filename is a NULL pointer, then the returned handle is for the main program. .PP External references in the library are resolved using the libraries in that library's dependency list and any other libraries previously opened with the .B 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. .PP .I flag must be either .BR RTLD_LAZY , meaning resolve undefined symbols as code from the dynamic library is executed, or .BR RTLD_NOW , meaning resolve all undefined symbols before .B dlopen returns, and fail if this cannot be done. Optionally, .B RTLD_GLOBAL may be or'ed with .IR flag, in which case the external symbols defined in the library will be made available to subsequently loaded libraries. .PP If the library exports a routine named .BR _init , then that code is executed before dlopen returns. If the same library is loaded twice with .BR 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 .B dlclose has been called on it as many times as .B dlopen has succeeded on it. .PP If .B 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 .BR dlerror() . .B dlerror returns NULL if no errors have occurred since initialization or since it was last called. (Calling .B dlerror() twice consecutively, will always result in the second call returning NULL.) .B 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, .B dlsym returns NULL; however, the correct way to test for an error from .B dlsym is to save the result of .B 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 .B dlerror into a variable because if .B dlerror is called again, it will return NULL. .PP .B dladdr returns information about the shared library containing the memory location specified by .IR address . .B dladdr returns zero on success and non-zero on error. .PP .B dlclose decrements the reference count on the dynamic library handle .IR 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 .BR _fini , then that routine is called just before the library is unloaded. .SH EXAMPLES .B Load the math library, and print the cosine of 2.0: .RS .nf .if t .ft CW #include <dlfcn.h> int main(int argc, char **argv) { void *handle = dlopen ("/lib/libm.so", RTLD_LAZY); double (*cosine)(double) = dlsym(handle, "cos"); printf ("%f\\n", (*cosine)(2.0)); dlclose(handle); } .if t .ft P .fi .PP If this program were in a file named "foo.c", you would build the program with the following command: .RS .LP gcc -rdynamic -o foo foo.c -ldl .RE .RE .LP .B Do the same thing, but check for errors at every step: .RS .nf .if t .ft CW #include <stdio.h> #include <dlfcn.h> int main(int argc, char **argv) { void *handle; double (*cosine)(double); char *error; handle = dlopen ("/lib/libm.so", RTLD_LAZY); if (!handle) { fputs (dlerror(), stderr); exit(1); } cosine = dlsym(handle, "cos"); if ((error = dlerror()) != NULL) { fputs(error, stderr); exit(1); } printf ("%f\\n", (*cosine)(2.0)); dlclose(handle); } .if t .ft P .fi .RE .SH 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. .SH SEE ALSO .BR ld(1) , .BR ld.so(8) , .BR ldconfig(8) , .BR ldd(1) , .BR ld.so.info .