DLSYM(3) Linux Programmer's Manual DLSYM(3)
dlsym, dlvsym - obtain address of a symbol in a shared object or exe-
void *dlsym(void *handle, const char *symbol);
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 condi-
tions, then call dlsym(), and then call dlerror(3) again, saving its
return value into a variable, and check whether this saved value is not
There are two special pseudo-handles that may be specified in handle:
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.
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
dlsym() is present in glibc 2.0 and later. dlvsym() first appeared in
For an explanation of the terms used in this section, see
|Interface | Attribute | Value |
|dlsym(), dlvsym() | Thread safety | MT-Safe |
POSIX.1-2001 describes dlsym(). The dlvsym() function is a GNU exten-
The dlsym() function is part of the dlopen API, derived from SunOS.
That system does not have dlvsym().
dl_iterate_phdr(3), dladdr(3), dlerror(3), dlinfo(3), dlopen(3),
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