dlsym (3)
Leading comments
Copyright 1995 Yggdrasil Computing, Incorporated. and Copyright 2003, 2015 Michael Kerrisk <mtk.manpages@gmail.com> %%%LICENSE_START(GPLv2+_DOC_FULL) This is free documentation; you can redistribute it and/or modify it under the terms of the GNU General Public License as published by the Free Software Foundation; either version 2 of the License, or (at your option) any later version. The GNU General Public License's references to "object code" and "executables" are to be interpreted as the ...
NAME
dlsym, dlvsym - obtain address of a symbol in a shared object or executableSYNOPSIS
#include <dlfcn.h>void *dlsym(void *handle, const char *symbol);
#define _GNU_SOURCE
#include <dlfcn.h>
void *dlvsym(void *handle, char *symbol, char *version);
Link with -ldl.
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.)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 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 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 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 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).| Interface | Attribute | Value |
| dlsym(), dlvsym() | Thread safety | MT-Safe |