DLSYM(3) Linux Programmer's Manual DLSYM(3)
NAME
dlsym, dlvsym - obtain address of a symbol in a shared object or exe-
cutable
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 re-
turns the address where that symbol is loaded into memory. If the sym-
bol 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 er-
ror. 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 de-
fault 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 or-
der 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 ob-
ject (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 at-
tributes(7).
+------------------+---------------+---------+
|Interface | Attribute | Value |
+------------------+---------------+---------+
|dlsym(), dlvsym() | Thread safety | MT-Safe |
+------------------+---------------+---------+
CONFORMING TO
POSIX.1-2001 describes dlsym(). The dlvsym() function is a GNU exten-
sion.
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. Unde-
fined 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 be-
havior 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
This page is part of release 5.07 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 2020-06-09 DLSYM(3)