dlsym, dlvsym - obtain address of a symbol in a shared object or executable
void *dlsym(void *restrict handle, const char *restrict symbol);
void *dlvsym(void *restrict handle, const char *restrict symbol,
const char *restrict 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.)
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
- 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
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
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).
POSIX.1-2001 describes dlsym(). The dlvsym() function is a GNU
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
The dlsym() function is part of the dlopen API, derived from SunOS. That
system does not have dlvsym().
This page is part of release 5.13 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/.
|dlsym (), dlvsym ()