NAME | SYNOPSIS | DESCRIPTION | RETURN VALUE | VERSIONS | ATTRIBUTES | CONFORMING TO | NOTES | EXAMPLE | SEE ALSO | COLOPHON

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

NAME         top

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

SYNOPSIS         top

       #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         top

       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 function dlvsym() does the same as dlsym() but takes a version
       string as an additional argument.

RETURN VALUE         top

       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         top

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

ATTRIBUTES         top

       For an explanation of the terms used in this section, see
       attributes(7).

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

CONFORMING TO         top

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

NOTES         top

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

EXAMPLE         top

       See dlopen(3).

SEE ALSO         top

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

COLOPHON         top

       This page is part of release 4.07 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                            2015-08-08                         DLSYM(3)