dlsym(3) — Linux manual page

NAME | SYNOPSIS | DESCRIPTION | RETURN VALUE | VERSIONS | ATTRIBUTES | CONFORMING TO | NOTES | EXAMPLES | 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
       executable

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.)

       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         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

       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         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 5.10 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                          2020-06-09                       DLSYM(3)

Pages that refer to this page: dladdr(3)dlerror(3)dlinfo(3)dlopen(3)rtld-audit(7)