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 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:

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).

VERSIONS

dlsym() is present in glibc 2.0 and later. dlvsym() first appeared in glibc 2.1.

ATTRIBUTES

For an explanation of the terms used in this section, see attributes(7).

CONFORMING TO

POSIX.1-2001 describes dlsym(). The dlvsym() function is a GNU extension.

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 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.

EXAMPLES

See dlopen(3).

SEE ALSO

dl_iterate_phdr(3), dladdr(3), dlerror(3), dlinfo(3), dlopen(3), ld.so(8)