dlsym(3)                                              Library Functions Manual                                             dlsym(3)

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

LIBRARY
       Dynamic linking library (libdl, -ldl)

SYNOPSIS
       #include <dlfcn.h>

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

       #define _GNU_SOURCE
       #include <dlfcn.h>

       void *dlvsym(void *restrict handle, const char *restrict symbol,
                    const char *restrict version);

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 in‐
              clude global symbols in the executable and its dependencies, as well as symbols in shared objects that  were  dynami‐
              cally 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 pro‐
              vide 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 │
       └─────────────────────────────────────────────────────────────────────────────────────────────────┴───────────────┴─────────┘

STANDARDS
       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 ad‐
       dress 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  behav‐
       ior 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)

Linux man-pages 6.03                                         2023-01-07                                                    dlsym(3)