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dlsym(3) [osx man page]

DLSYM(3)						   BSD Library Functions Manual 						  DLSYM(3)

dlsym -- get address of a symbol SYNOPSIS
#include <dlfcn.h> void* dlsym(void* handle, const char* symbol); DESCRIPTION
dlsym() returns the address of the code or data location specified by the null-terminated character string symbol. Which libraries and bun- dles are searched depends on the handle parameter. If dlsym() is called with a handle, returned by dlopen() then only that image and any libraries it depends on are searched for symbol. If dlsym() is called with the special handle RTLD_DEFAULT, then all mach-o images in the process (except those loaded with dlopen(xxx, RTLD_LOCAL)) are searched in the order they were loaded. This can be a costly search and should be avoided. If dlsym() is called with the special handle RTLD_NEXT, then dyld searches for the symbol in the dylibs the calling image linked against when built. It is usually used when you intentionally have multiply defined symbol across images and want to find the "next" definition. It searches other images for the definition that the caller would be using if it did not have a definition. The exact search algorithm depends on whether the caller's image was linked -flat_namespace or -twolevel_namespace. For flat linked images, the search starts in the load ordered list of all images, in the image right after the caller's image. For two-level images, the search simulates how the static linker would have searched for the symbol when linking the caller's image. If dlsym() is called with the special handle RTLD_SELF, then the search for the symbol starts with the image that called dlsym(). If it is not found, the search continues as if RTLD_NEXT was used. If dlsym() is called with the special handle RTLD_MAIN_ONLY, then it only searches for symbol in the main executable. RETURN VALUES
The dlsym() function returns a null pointer if the symbol cannot be found, and sets an error condition which may be queried with dlerror(). NOTES
The symbol name passed to dlsym() is the name used in C source code. For example to find the address of function foo(), you would pass "foo" as the symbol name. This is unlike the older dyld APIs which required a leading underscore. If you looking up a C++ symbol, you need to use the mangled C++ symbol name. SEE ALSO
dlopen(3) dlerror(3) dyld(3) ld(1) cc(1) August 28, 2008

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DLSYM(3)						     Linux Programmer's Manual							  DLSYM(3)

dlsym, dlvsym - obtain address of a symbol in a shared object or executable SYNOPSIS
#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 sym- bols 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 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 <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 diag- nosed 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 | +------------------+---------------+---------+ CONFORMING TO
POSIX.1-2001 describes dlsym(). The dlvsym() function is a GNU extension. NOTES
History The dlsym() function is part of the dlopen API, derived from SunOS. That system does not have dlvsym(). EXAMPLE
See dlopen(3). SEE ALSO
dl_iterate_phdr(3), dladdr(3), dlerror(3), dlinfo(3), dlopen(3), COLOPHON
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