dlsym(3) — Linux manual page


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

NAME         top

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

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:

              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.

              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

       │Interface         Attribute     Value   │
       │dlsym(), dlvsym() │ Thread safety │ MT-Safe │

CONFORMING TO         top

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

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.

       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),

COLOPHON         top

       This page is part of release 5.08 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

Linux                            2020-06-09                         DLSYM(3)

Pages that refer to this page: dladdr1(3)dladdr(3)dlclose(3)dlerror(3)dlinfo(3)dlmopen(3)dlopen(3)rtld-audit(7)