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.\" -*- nroff -*-

.\" Copyright 1995 Yggdrasil Computing, Incorporated.

.\" written by Adam J. Richter (adam@yggdrasil.com),

.\" with typesetting help from Daniel Quinlan (quinlan@yggdrasil.com).

.\"

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.\" GNU General Public License for more details.

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.\" You should have received a copy of the GNU General Public

.\" License along with this manual; if not, write to the Free

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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 

.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 

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 

#include 

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 .