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NAME

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

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 conditions, 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).

       ┌──────────────────┬───────────────┬─────────┐
       │InterfaceAttributeValue   │
       ├──────────────────┼───────────────┼─────────┤
       │dlsym(), dlvsym() │ Thread safety │ MT-Safe │
       └──────────────────┴───────────────┴─────────┘

CONFORMING TO

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

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

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