man-pages/man3p/dlsym.3p

.\" Copyright (c) 2001-2003 The Open Group, All Rights Reserved 
.TH "DLSYM" 3P 2003 "IEEE/The Open Group" "POSIX Programmer's Manual"
.\" dlsym 
.SH NAME
dlsym \- obtain the address of a symbol from a dlopen object
.SH SYNOPSIS
.LP
\fB#include <dlfcn.h>
.br
.sp
void *dlsym(void *restrict\fP \fIhandle\fP\fB, const char *restrict\fP
\fIname\fP\fB); \fP
\fB
.br
\fP
.SH DESCRIPTION
.LP
The \fIdlsym\fP() function shall obtain the address of a symbol defined
within an object made accessible through a \fIdlopen\fP() call. The
\fIhandle\fP argument is the value returned from a call to \fIdlopen\fP()
(and which has not since been released via a call to \fIdlclose\fP()),
and \fIname\fP is the symbol's name as a character string.
.LP
The \fIdlsym\fP() function shall search for the named symbol in all
objects loaded automatically as a result of loading the
object referenced by \fIhandle\fP (see \fIdlopen\fP() ). Load ordering
is used in \fIdlsym\fP()
operations upon the global symbol object. The symbol resolution algorithm
used shall be dependency order as described in \fIdlopen\fP() .
.LP
The RTLD_DEFAULT and RTLD_NEXT flags are reserved for future use.
.SH RETURN VALUE
.LP
If \fIhandle\fP does not refer to a valid object opened by \fIdlopen\fP(),
or if the
named symbol cannot be found within any of the objects associated
with \fIhandle\fP, \fIdlsym\fP() shall return NULL. More
detailed diagnostic information shall be available through \fIdlerror\fP()
\&.
.SH ERRORS
.LP
No errors are defined.
.LP
\fIThe following sections are informative.\fP
.SH EXAMPLES
.LP
The following example shows how \fIdlopen\fP() and \fIdlsym\fP() can
be used to access
either function or data objects. For simplicity, error checking has
been omitted.
.sp
.RS
.nf

\fBvoid    *handle;
int     *iptr, (*fptr)(int);
.sp

/* open the needed object */
handle = dlopen("/usr/home/me/libfoo.so", RTLD_LOCAL | RTLD_LAZY);
.sp

/* find the address of function and data objects */
*(void **)(&fptr) = dlsym(handle, "my_function");
iptr = (int *)dlsym(handle, "my_object");
.sp

/* invoke function, passing value of integer as a parameter */
(*fptr)(*iptr);
\fP
.fi
.RE
.SH APPLICATION USAGE
.LP
Special purpose values for \fIhandle\fP are reserved for future use.
These values and their meanings are:
.TP 7
RTLD_DEFAULT
The symbol lookup happens in the normal global scope; that is, a search
for a symbol using this handle would find the same
definition as a direct use of this symbol in the program code.
.TP 7
RTLD_NEXT
Specifies the next object after this one that defines \fIname\fP.
\fIThis one\fP refers to the object containing the
invocation of \fIdlsym\fP(). The \fInext\fP object is the one found
upon the application of a load order symbol resolution
algorithm (see \fIdlopen\fP() ). The next object is either one of
global scope (because it was
introduced as part of the original process image or because it was
added with a \fIdlopen\fP() operation including the RTLD_GLOBAL flag),
or is an object that was included in the
same \fIdlopen\fP() operation that loaded this one. 
.LP
The RTLD_NEXT flag is useful to navigate an intentionally created
hierarchy of multiply-defined symbols created through
\fIinterposition\fP. For example, if a program wished to create an
implementation of \fImalloc\fP() that embedded some statistics gathering
about memory allocations, such an
implementation could use the real \fImalloc\fP() definition to perform
the memory
allocation-and itself only embed the necessary logic to implement
the statistics gathering function.
.sp
.SH RATIONALE
.LP
The ISO\ C standard does not require that pointers to functions can
be cast back and forth to pointers to data. Indeed, the
ISO\ C standard does not require that an object of type \fBvoid *\fP
can hold a pointer to a function. Implementations
supporting the XSI extension, however, do require that an object of
type \fBvoid *\fP can hold a pointer to a function. The result
of converting a pointer to a function into a pointer to another data
type (except \fBvoid *\fP) is still undefined, however. Note
that compilers conforming to the ISO\ C standard are required to generate
a warning if a conversion from a \fBvoid *\fP
pointer to a function pointer is attempted as in:
.sp
.RS
.nf

\fBfptr = (int (*)(int))dlsym(handle, "my_function");
\fP
.fi
.RE
.LP
Due to the problem noted here, a future version may either add a new
function to return function pointers, or the current
interface may be deprecated in favor of two new functions: one that
returns data pointers and the other that returns function
pointers.
.SH FUTURE DIRECTIONS
.LP
None.
.SH SEE ALSO
.LP
\fIdlclose\fP() , \fIdlerror\fP() , \fIdlopen\fP() , the Base Definitions
volume of IEEE\ Std\ 1003.1-2001, \fI<dlfcn.h>\fP
.SH COPYRIGHT
Portions of this text are reprinted and reproduced in electronic form
from IEEE Std 1003.1, 2003 Edition, Standard for Information Technology
-- Portable Operating System Interface (POSIX), The Open Group Base
Specifications Issue 6, Copyright (C) 2001-2003 by the Institute of
Electrical and Electronics Engineers, Inc and The Open Group. In the
event of any discrepancy between this version and the original IEEE and
The Open Group Standard, the original IEEE and The Open Group Standard
is the referee document. The original Standard can be obtained online at
http://www.opengroup.org/unix/online.html .