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NAME | LIBRARY | SYNOPSIS | DESCRIPTION | RETURN VALUE | ATTRIBUTES | STANDARDS | HISTORY | NOTES | EXAMPLES | SEE ALSO | COLOPHON |
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dlsym(3) Library Functions Manual dlsym(3)
dlsym, dlvsym - obtain address of a symbol in a shared object or
executable
Dynamic linking library (libdl, -ldl)
#include <dlfcn.h>
void *dlsym(void *restrict handle, const char *restrict symbol);
#define _GNU_SOURCE
#include <dlfcn.h>
void *dlvsym(void *restrict handle, const char *restrict symbol,
const char *restrict version);
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.
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).
For an explanation of the terms used in this section, see
attributes(7).
┌─────────────────────────────────────┬───────────────┬─────────┐
│ Interface │ Attribute │ Value │
├─────────────────────────────────────┼───────────────┼─────────┤
│ dlsym(), dlvsym() │ Thread safety │ MT-Safe │
└─────────────────────────────────────┴───────────────┴─────────┘
dlsym()
POSIX.1-2008.
dlvsym()
GNU.
dlsym()
glibc 2.0. POSIX.1-2001.
dlvsym()
glibc 2.1.
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().
See dlopen(3).
dl_iterate_phdr(3), dladdr(3), dlerror(3), dlinfo(3), dlopen(3),
ld.so(8)
This page is part of the man-pages (Linux kernel and C library
user-space interface documentation) project. Information about
the project can be found at
⟨https://www.kernel.org/doc/man-pages/⟩. If you have a bug report
for this manual page, see
⟨https://git.kernel.org/pub/scm/docs/man-pages/man-pages.git/tree/CONTRIBUTING⟩.
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⟨https://mirrors.edge.kernel.org/pub/linux/docs/man-pages/⟩ on
2024-06-26. If you discover any rendering problems in this HTML
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man-pages@man7.org
Linux man-pages 6.9.1 2024-05-02 dlsym(3)
Pages that refer to this page: dladdr(3), dlerror(3), dlinfo(3), dlopen(3), rtld-audit(7)
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HTML rendering created 2024-06-26 by Michael Kerrisk, author of The Linux Programming Interface. For details of in-depth Linux/UNIX system programming training courses that I teach, look here. Hosting by jambit GmbH. |
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