dlsym(3) Library Functions Manual dlsym(3)
NAME
dlsym, dlvsym - obtain address of a symbol in a shared object or exe-
cutable
LIBRARY
Dynamic linking library (libdl, -ldl)
SYNOPSIS
#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);
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).
ATTRIBUTES
For an explanation of the terms used in this section, see attrib-
utes(7).
┌───────────────────────────────────────────┬───────────────┬─────────┐
│ Interface │ Attribute │ Value │
├───────────────────────────────────────────┼───────────────┼─────────┤
│ dlsym(), dlvsym() │ Thread safety │ MT-Safe │
└───────────────────────────────────────────┴───────────────┴─────────┘
STANDARDS
dlsym()
POSIX.1-2008.
dlvsym()
GNU.
HISTORY
dlsym()
glibc 2.0. POSIX.1-2001.
dlvsym()
glibc 2.1.
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. Un-
defined 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
See dlopen(3).
SEE ALSO
dl_iterate_phdr(3), dladdr(3), dlerror(3), dlinfo(3), dlopen(3),
ld.so(8)
Linux man-pages 6.7 2023-10-31 dlsym(3)
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