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NAME | SYNOPSIS | DESCRIPTION | RETURN VALUE | VERSIONS | ATTRIBUTES | CONFORMING TO | NOTES | EXAMPLE | SEE ALSO | COLOPHON |
DLSYM(3) Linux Programmer's Manual DLSYM(3)
dlsym, dlvsym - obtain address of a symbol in a shared object or exe‐
cutable
#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.
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 _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.
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).
dlsym() is present in glibc 2.0 and later. dlvsym() first appeared
in glibc 2.1.
For an explanation of the terms used in this section, see
attributes(7).
┌──────────────────┬───────────────┬─────────┐
│Interface │ Attribute │ Value │
├──────────────────┼───────────────┼─────────┤
│dlsym(), dlvsym() │ Thread safety │ MT-Safe │
└──────────────────┴───────────────┴─────────┘
POSIX.1-2001 describes dlsym(). The dlvsym() function is a GNU
extension.
History
The dlsym() function is part of the dlopen API, derived from SunOS.
That system does not have dlvsym().
See dlopen(3).
dl_iterate_phdr(3), dladdr(3), dlerror(3), dlinfo(3), dlopen(3),
ld.so(8)
This page is part of release 4.12 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 2017-07-13 DLSYM(3)
Pages that refer to this page: dladdr(3), dlerror(3), dlinfo(3), dlopen(3), rtld-audit(7)