uclibc-ng/ldso/man/dlopen.3

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.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 .