dlsym(3p) — Linux manual page


DLSYM(3P)                 POSIX Programmer's Manual                DLSYM(3P)

PROLOG         top

       This manual page is part of the POSIX Programmer's Manual.  The Linux
       implementation of this interface may differ (consult the
       corresponding Linux manual page for details of Linux behavior), or
       the interface may not be implemented on Linux.

NAME         top

       dlsym — get the address of a symbol from a symbol table handle

SYNOPSIS         top

       #include <dlfcn.h>

       void *dlsym(void *restrict handle, const char *restrict name);

DESCRIPTION         top

       The dlsym() function shall obtain the address of a symbol (a function
       identifier or a data object identifier) defined in the symbol table
       identified by the handle argument. The handle argument is a symbol
       table handle returned from a call to dlopen() (and which has not
       since been released by a call to dlclose()), and name is the symbol's
       name as a character string. The return value from dlsym(), cast to a
       pointer to the type of the named symbol, can be used to call (in the
       case of a function) or access the contents of (in the case of a data
       object) the named symbol.

       The dlsym() function shall search for the named symbol in the symbol
       table referenced by handle.  If the symbol table was created with
       lazy loading (see RTLD_LAZY in dlopen()), load ordering shall be used
       in dlsym() operations to relocate executable object files needed to
       resolve the symbol. The symbol resolution algorithm used shall be
       dependency order as described in dlopen().

       The RTLD_DEFAULT and RTLD_NEXT symbolic constants (which may be
       defined in <dlfcn.h>) are reserved for future use as special values
       that applications may be allowed to use for handle.

RETURN VALUE         top

       Upon successful completion, if name names a function identifier,
       dlsym() shall return the address of the function converted from type
       pointer to function to type pointer to void; otherwise, dlsym() shall
       return the address of the data object associated with the data object
       identifier named by name converted from a pointer to the type of the
       data object to a pointer to void.  If handle does not refer to a
       valid symbol table handle or if the symbol named by name cannot be
       found in the symbol table associated with handle, dlsym() shall
       return a null pointer.

       More detailed diagnostic information shall be available through

ERRORS         top

       No errors are defined.

       The following sections are informative.

EXAMPLES         top

       The following example shows how dlopen() and dlsym() can be used to
       access either a function or a data object. For simplicity, error
       checking has been omitted.

           void *handle;
           int (*fptr)(int), *iptr, result;
           /* open the needed symbol table */
           handle = dlopen("/usr/home/me/libfoo.so", RTLD_LOCAL | RTLD_LAZY);
           /* find the address of the function my_function */
           fptr = (int (*)(int))dlsym(handle, "my_function");
           /* find the address of the data object my_object */
           iptr = (int *)dlsym(handle, "my_OBJ");
           /* invoke my_function, passing the value of my_OBJ as the parameter */
           result = (*fptr)(*iptr);


       The following special purpose values for handle are reserved for
       future use and have the indicated meanings:

                   The identifier lookup happens in the normal global scope;
                   that is, a search for an identifier using handle would
                   find the same definition as a direct use of this
                   identifier in the program code.

       RTLD_NEXT   Specifies the next executable object file after this one
                   that defines name.  This one refers to the executable
                   object file containing the invocation of dlsym().  The
                   next executable object file is the one found upon the
                   application of a load order symbol resolution algorithm
                   (see dlopen()).  The next symbol is either one of global
                   scope (because it was introduced as part of the original
                   process image or because it was added with a dlopen()
                   operation including the RTLD_GLOBAL flag), or is in an
                   executable object file that was included in the same
                   dlopen() operation that loaded this one.

       The RTLD_NEXT flag is useful to navigate an intentionally created
       hierarchy of multiply-defined symbols created through interposition.
       For example, if a program wished to create an implementation of
       malloc() that embedded some statistics gathering about memory
       allocations, such an implementation could use the real malloc()
       definition to perform the memory allocation — and itself only embed
       the necessary logic to implement the statistics gathering function.

       Note that conversion from a void * pointer to a function pointer as

           fptr = (int (*)(int))dlsym(handle, "my_function");

       is not defined by the ISO C standard. This standard requires this
       conversion to work correctly on conforming implementations.

RATIONALE         top




SEE ALSO         top

       dlclose(3p), dlerror(3p), dlopen(3p)

       The Base Definitions volume of POSIX.1‐2008, dlfcn.h(0p)

COPYRIGHT         top

       Portions of this text are reprinted and reproduced in electronic form
       from IEEE Std 1003.1, 2013 Edition, Standard for Information
       Technology -- Portable Operating System Interface (POSIX), The Open
       Group Base Specifications Issue 7, Copyright (C) 2013 by the
       Institute of Electrical and Electronics Engineers, Inc and The Open
       Group.  (This is POSIX.1-2008 with the 2013 Technical Corrigendum 1
       applied.) 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.unix.org/online.html .

       Any typographical or formatting errors that appear in this page are
       most likely to have been introduced during the conversion of the
       source files to man page format. To report such errors, see
       https://www.kernel.org/doc/man-pages/reporting_bugs.html .

IEEE/The Open Group                 2013                           DLSYM(3P)

Pages that refer to this page: dlfcn.h(0p)dlclose(3p)dlerror(3p)dlopen(3p)