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.)

       In  unusual  cases  (see NOTES) the value of the symbol could actually be NULL.  Therefore, a NULL return
       from dlsym() need not indicate an error.  The correct way to distinguish an error  from  a  symbol  whose
       value  is  NULL 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).
       ┌───────────────────┬───────────────┬─────────┐
       │ Interface         │ Attribute     │ Value   │
       ├───────────────────┼───────────────┼─────────┤
       │ dlsym(), dlvsym() │ Thread safety │ MT-Safe │
       └───────────────────┴───────────────┴─────────┘

CONFORMING TO

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

NOTES

       There  are  several  scenarios when the address of a global symbol is NULL.  For example, a symbol can be
       placed at zero address by the linker, via a linker script or with --defsym command-line option. Undefined
       weak symbols also have NULL value.  Finally, the symbol value  may  be  the  result  of  a  GNU  indirect
       function  (IFUNC)  resolver function that returns NULL as the resolved value. In the latter case, dlsym()
       also returns NULL without error. However, in the former two cases, the behavior of GNU dynamic linker  is
       inconsistent:  relocation  processing  succeeds  and  the  symbol can be observed to have NULL value, but
       dlsym() fails and dlerror() indicates a lookup error.

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

EXAMPLES

       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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Linux                                              2020-06-09                                           DLSYM(3)