dlerror()

The function dlerror() returns a human readable string describing the most recent error that occurred from dlopen(), dlsym() or dlclose() since the last call to dlerror(). It returns NULL if no errors have occurred since initialization or since it was last called.

dlopen()

The function dlopen() loads the dynamic library file named by the null-terminated string filename and returns an opaque "handle" for the dynamic library. If filename is NULL, then the returned handle is for the main program. If filename contains a slash ("/"), then it is interpreted as a (relative or absolute) pathname. Otherwise, the dynamic linker searches for the library as follows (see ld.so(8) for further details):

If the library has dependencies on other shared libraries, then these are also automatically loaded by the dynamic linker using the same rules. (This process may occur recursively, if those libraries in turn have dependencies, and so on.)

One of the following two values must be included in flag:

Zero or more of the following values may also be ORed in flag:

If filename is a NULL pointer, then the returned handle is for the main program. When given to dlsym(), this handle causes a search for a symbol in the main program, followed by all shared libraries loaded at program startup, and then all shared libraries loaded by dlopen() with the flag RTLD_GLOBAL.

External references in the library are resolved using the libraries in that library's dependency list and any other libraries previously opened with the RTLD_GLOBAL flag. If the executable was linked with the flag "−rdynamic" (or, synonymously, "−−export−dynamic"), then the global symbols in the executable will also be used to resolve references in a dynamically loaded library.

If the same library is loaded again with dlopen(), the same file handle is returned. The dl library maintains reference counts for library handles, so a dynamic library is not deallocated until dlclose() has been called on it as many times as dlopen() has succeeded on it. The _init() routine, if present, is only called once. But a subsequent call with RTLD_NOW may force symbol resolution for a library earlier loaded with RTLD_LAZY.

If dlopen() fails for any reason, it returns NULL.

dlsym()

The function dlsym() takes a "handle" of a dynamic library returned by dlopen() and the null-terminated symbol name, returning the address where that symbol is loaded into memory. If the symbol is not found, in the specified library or any of the libraries that were automatically loaded by dlopen() when that library was loaded, dlsym() returns NULL. (The search performed by dlsym() is breadth first through the dependency tree of these libraries.) 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() to clear any old error conditions, then call dlsym(), and then call dlerror() again, saving its return value into a variable, and check whether this saved value is not NULL.

There are two special pseudo-handles, RTLD_DEFAULT and RTLD_NEXT. The former will find the first occurrence of the desired symbol using the default library search order. The latter will find the next occurrence of a function in the search order after the current library. This allows one to provide a wrapper around a function in another shared library.

dlclose()

The function dlclose() decrements the reference count on the dynamic library handle handle. If the reference count drops to zero and no other loaded libraries use symbols in it, then the dynamic library is unloaded.

The function dlclose() returns 0 on success, and nonzero on error.

The obsolete symbols _init() and _fini()

The linker recognizes special symbols _init and _fini. If a dynamic library exports a routine named _init(), then that code is executed after the loading, before dlopen() returns. If the dynamic library exports a routine named _fini(), then that routine is called just before the library is unloaded. In case you need to avoid linking against the system startup files, this can be done by using the gcc(1) −nostartfiles command-line option.

Using these routines, or the gcc −nostartfiles or −nostdlib options, is not recommended. Their use may result in undesired behavior, since the constructor/destructor routines will not be executed (unless special measures are taken).

Instead, libraries should export routines using the __attribute__((constructor)) and __attribute__((destructor)) function attributes. See the gcc info pages for information on these. Constructor routines are executed before dlopen() returns, and destructor routines are executed before dlclose() returns.

Glibc extensions: dladdr() and dlvsym()

Glibc adds two functions not described by POSIX, with prototypes

#define _GNU_SOURCE         /* See feature_test_macros(7) */
#include <dlfcn.h>

  int dladdr(void *addr, Dl_info *info);

  void *dlvsym(void *handle, char *symbol, char *version);

The function dladdr() takes a function pointer and tries to resolve name and file where it is located. Information is stored in the Dl_info structure:

If no symbol matching addr could be found, then dli_sname and dli_saddr are set to NULL.

dladdr() returns 0 on error, and nonzero on success.

The function dlvsym(), provided by glibc since version 2.1, does the same as dlsym() but takes a version string as an additional argument.

History

The dlopen interface standard comes from SunOS. That system also has dladdr(), but not dlvsym().