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man:dlsym

DLSYM(3) Linux Programmer's Manual DLSYM(3)

NAME

     dlsym,  dlvsym  - obtain address of a symbol in a shared object or exe-
     cutable

SYNOPSIS

     #include <dlfcn.h>
     void *dlsym(void *handle, const char *symbol);
     #define _GNU_SOURCE
     #include <dlfcn.h>
     void *dlvsym(void *handle, char *symbol, char *version);
     Link with -ldl.

DESCRIPTION

     The function dlsym() takes a "handle" of a dynamic loaded shared object
     returned  by  dlopen(3)  along  with a null-terminated symbol name, and
     returns the address where that symbol is loaded into  memory.   If  the
     symbol  is  not  found,  in  the  specified object or any of the shared
     objects that were automatically loaded by dlopen(3)  when  that  object
     was  loaded, dlsym() returns NULL.  (The search performed by dlsym() is
     breadth first through the dependency tree of these shared objects.)
     Since the value of the symbol could actually be NULL (so  that  a  NULL
     return  from  dlsym()  need  not indicate an error), the correct way to
     test for an error is to call dlerror(3) to clear any old  error  condi-
     tions,  then  call  dlsym(), and then call dlerror(3) again, saving its
     return value into a variable, and check whether this saved value is not
     NULL.
     There are two special pseudo-handles that may be specified in handle:
     RTLD_DEFAULT
            Find  the  first  occurrence  of  the  desired  symbol using the
            default shared object search order.   The  search  will  include
            global  symbols  in the executable and its dependencies, as well
            as symbols in shared objects that were dynamically  loaded  with
            the RTLD_GLOBAL flag.
     RTLD_NEXT
            Find  the  next  occurrence  of the desired symbol in the search
            order after the current object.  This allows one  to  provide  a
            wrapper around a function in another shared object, so that, for
            example, the definition of a  function  in  a  preloaded  shared
            object  (see  LD_PRELOAD  in  ld.so(8))  can find and invoke the
            "real" function provided in another shared object (or  for  that
            matter,  the  "next"  definition  of the function in cases where
            there are multiple layers of preloading).
     The _GNU_SOURCE feature test macro must be defined in order  to  obtain
     the definitions of RTLD_DEFAULT and RTLD_NEXT from <dlfcn.h>.
     The  function  dlvsym()  does  the  same as dlsym() but takes a version
     string as an additional argument.

RETURN VALUE

     On success, these functions return the address associated with  symbol.
     On  failure,  they return NULL; the cause of the error can be diagnosed
     using dlerror(3).

VERSIONS

     dlsym() is present in glibc 2.0 and later.  dlvsym() first appeared  in
     glibc 2.1.

ATTRIBUTES

     For   an   explanation   of   the  terms  used  in  this  section,  see
     attributes(7).
     +------------------+---------------+---------+
     |Interface         | Attribute     | Value   |
     +------------------+---------------+---------+
     |dlsym(), dlvsym() | Thread safety | MT-Safe |
     +------------------+---------------+---------+

CONFORMING TO

     POSIX.1-2001 describes dlsym().  The dlvsym() function is a GNU  exten-
     sion.

NOTES

 History
     The  dlsym()  function  is  part of the dlopen API, derived from SunOS.
     That system does not have dlvsym().

EXAMPLE

     See dlopen(3).

SEE ALSO

     dl_iterate_phdr(3),  dladdr(3),   dlerror(3),   dlinfo(3),   dlopen(3),
     ld.so(8)

COLOPHON

     This  page  is  part of release 4.16 of the Linux man-pages project.  A
     description of the project, information about reporting bugs,  and  the
     latest     version     of     this    page,    can    be    found    at
     https://www.kernel.org/doc/man-pages/.

Linux 2017-09-15 DLSYM(3)

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