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 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 sym-
bols 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 wrap-
per around a function in another shared object, so that, for ex-
ample, the definition of a function in a preloaded shared object
(see LD_PRELOAD in ld.so(8)) can find and invoke the "real" func-
tion provided in another shared object (or for that matter, the
"next" definition of the function in cases where there are multi-
ple 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 attributes(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. Undefined weak sym-
bols 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 dler-
ror() 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 6.9.1 2024-05-02 dlsym(3)
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