DLFCN(3) BSD Programmer's Manual DLFCN(3)
dlopen, dlclose, dlsym, dladdr, dlctl, dlerror - dynamic link interface
#include <dlfcn.h> void * dlopen(const char *path, int mode); int dlclose(void *handle); void * dlsym(void *handle, const char *symbol); int dladdr(const void *addr, Dl_info *info); int dlctl(void *handle, int cmd, void *data); const char * dlerror(void);
These functions provide an interface to the run-time linker ld.so(1). They allow new shared objects to be loaded into a process's address space under program control. The dlopen() function takes a name of a shared object as its first argu- ment. The shared object is mapped into the address space, relocated, and its external references are resolved in the same way as is done with the implicitly loaded shared libraries at program startup. The path argument can either be an absolute pathname or it can be of the form "lib<name>.so[.xx[.yy]]" in which case the same library search rules apply that are used for "intrinsic" shared library searches. A null pointer supplied for path is interpreted as a reference to the main exe- cutable of the process. The second argument currently has no effect, but should be set to DL_LAZY for future compatibility. dlopen() returns a handle to be used in calls to dlclose(), dlsym(), and dlctl(). If the named shared object has already been loaded by a previous call to dlopen() (and not yet unloaded by dlclose()), a handle referring to the resident copy is returned. dlclose() unlinks and removes the object referred to by handle from the process address space. If multiple calls to dlopen() have been done on this object (or the object was once loaded at startup time) the object is removed when its reference count drops to zero. dlsym() looks for a definition of symbol in the shared object designated by handle. The symbol's address is returned. If the symbol cannot be resolved, NULL is returned. If dlsym() is called with the special handle NULL, it is interpreted as a reference to the executable or shared object from which the call is being made. Thus a shared object can reference its own symbols. If dlsym() is called with the special handle RTLD_DEFAULT, all the shared objects will be searched in the order they were loaded. If dlsym() is called with the special handle RTLD_NEXT, then the search for the symbol is limited to the shared objects which were loaded after the one issuing the call to dlsym(). Thus, if the function is called from the main program, all the shared libraries are searched. If it is called from a shared library, all subsequent shared libraries are searched. If dlsym() is called with the special handle RTLD_SELF, then the search for the symbol is limited to the shared object issuing the call to dlsym() and those shared objects which were loaded after it. dladdr() queries the dynamic linker for information about the shared ob- ject containing the address addr. The information is returned in the structure specified by info. The structure contains at least the follow- ing members: const char *dli_fname The pathname of the shared object containing the address addr. void *dli_fbase The base address at which the shared object is mapped into the address space of the calling process. const char *dli_sname The name of the nearest run-time symbol with a address less than or equal to addr. If no symbol with a suitable address is found, both this field and dli_saddr are set to NULL. void *dli_saddr The address of the symbol returned in dli_sname. If a mapped shared object containing addr cannot be found, dladdr() re- turns 0. In that case, a message detailing the failure can be retrieved by calling dlerror(). On success, a non-zero value is returned. Note: both strings pointed at by dli_fname and dli_sname reside in memory private to the run-time linker module and should not be modified by the caller. In dynamically linked programs, the address of a global function will point to its program linkage table entry, rather than to the entry point of the function itself. This causes most global functions to appear to be defined within the main executable, rather than in the shared libraries where the actual code resides. dlctl() provides an interface similar to ioctl(2) to control several as- pects of the run-time linker's operation. This interface is currently under development. dlerror() returns a character string representing the most recent error that has occurred while processing one of the other functions described here. If no dynamic linking errors have occurred since the last invoca- tion of dlerror(), dlerror() returns NULL. Thus, invoking dlerror() a second time, immediately following a prior invocation, will result in NULL being returned.
ld(1), ld.so(1), link(5)
Some of the dl* functions first appeared in SunOS 4.
An error that occurs while processing a dlopen() request results in the termination of the program. MirOS BSD #10-current September 30, 1995 1
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