NAME

dlopen — load and link a dynamic library or bundle

SYNOPSIS

#include <dlfcn.h>

void*
dlopen(const char* path, int mode);

DESCRIPTION

dlopen() examines the mach-o file specified by path. If the file is compatible with the current process and has not already been loaded into the current process, it is loaded and linked. After being linked, if it contains any initializer functions, they are called, before dlopen() returns. dlopen() can load dynamic libraries and bundles. It returns a handle that can be used with dlsym() and dlclose(). A second call to dlopen() with the same path will return the same handle, but the internal reference count for the handle will be incremented. Therefore all dlopen() calls should be balanced with a dlclose() call.

If a null pointer is passed in path, dlopen() returns a handle equivalent to RTLD_DEFAULT.

mode contains options to dlopen(). It must contain one or more of the following values, possibly ORed together:

RTLD_LAZY

Each external function reference is bound the first time the function is called.

RTLD_NOW

All external function references are bound immediately during the call to dlopen().

RTLD_LAZY is normally preferred, for reasons of efficiency. However, RTLD_NOW is useful to ensure that any undefined symbols are discovered during the call to dlopen(). If neither RTLD_LAZY nor RTLD_NOW is specified, the default is RTLD_LAZY.

One of the following flags may be ORed into the mode argument:

RTLD_GLOBAL

Symbols exported from this image (dynamic library or bundle) will be available to any images build with -flat_namespace option to ld(1) or to calls to dlsym() when using a special handle.

RTLD_LOCAL

Symbols exported from this image (dynamic library or bundle) are generally hidden and only availble to dlsym() when directly using the handle returned by this call to dlopen().

If neither RTLD_GLOBAL nor RTLD_LOCAL is specified, the default is RTLD_GLOBAL.

One of the following may be ORed into the mode argument:

RTLD_NOLOAD

The specified image is not loaded. However, a valid handle is returned if the image already exists in the process. This provides a way to query if an image is already loaded. The handle returned is ref-counted, so you eventually need a corresponding call to dlclose()

RTLD_NODELETE

The specified image is tagged so that will never be removed from the address space, even after all clients have released it via dlclose()

Additionally, the following may be ORed into the mode argument:

RTLD_FIRST

The retuned handle is tagged so that any dlsym() calls on the handle will only search the image specified, and not subsequent images. If path is NULL and the option RTLD_FIRST is used, the handle returned will only search the main executable.

SEARCHING

In general, dyld does not search for dylibs. Dylibs are specified via a full path, either as a static dependent dylib in a mach-o file, or as a path passed to dlopen() , But during development the env vars DYLD_LIBRARY_PATH and DYLD_FRAMEWORK_PATH can be used to override the specified path and look for the leaf framework/dylib name in the specified directories.

When path does not contain a slash character (i.e. it is just a leaf name), dlopen() will do searching. If $DYLD_LIBRARY_PATH was set at launch, dyld will first look in that directory. Next, if the calling mach-o file or the main executable specify an LC_RPATH, then dyld will look in those directories. Next, if the process is unrestricted, dyld will search in the current working directory. Lastly, for old binaries, dyld will try some fallbacks. If $DYLD_FALLBACK_LIBRARY_PATH was set at launch, dyld will search in those directories, otherwise, dyld will look in /usr/local/lib/ (if the process is unrestricted), and then in /usr/lib/.

When path looks like a framework path (e.g. /stuff/foo.framework/foo), if $DYLD_FRAMEWORK_PATH was set at launch, dyld will first look in that directory for the framework partial path (e.g. foo.framework/foo). Next, dyld will try the supplied path as-is (using current working directory for relative paths). Lastly, for old binaries, dyld will try some fallbacks. If $DYLD_FALLBACK_FRAMEWORK_PATH was set at launch, dyld will search those directories. Otherwise, it will search /Library/Frameworks (on macOS if process is unrestricted), then /System/Library/Frameworks.

When path contains a slash but is not a framework path (i.e. a full path or a partial path to a dylib), dlopen() first looks in (if set) in $DYLD_LIBRARY_PATH (with leaf part from path ). Next, dyld tries the supplied path (using current working directory for relative paths (but only for unrestricted processes)). Lastly, for older binaries, dyld will try fallbacks. If $DYLD_FALLBACK_LIBRARY_PATH was set at launch, dyld will search in those directories, otherwise, dyld will look in /usr/local/lib/ (if the process is unrestricted), and then in /usr/lib/.

If you want to find a dylib via the dynamic run path, use @rpath (e.g. dlopen("@rpath/libfoo.dylib",xxx)),

Note: There are no configuration files to control dlopen searching.

Note: If the main executable is a set[ug]id binary or codesigned with entitlements, then all environment variables are ignored, and only a full path can be used.

Note: Apple platforms use "universal" files to combine 32-bit and 64-bit libraries. This means there are no separate 32-bit and 64-bit search paths.

Note: On Apple platforms most OS dylibs are combined into the dyld cache and do not exist on disk. Therefore, calling stat() to preflight if an OS dylib exists won't work.

RETURN VALUES

If dlopen() fails, it returns a null pointer, and sets an error condition which may be interrogated with dlerror().

SEE ALSO

dlopen_preflight(3) dlclose(3) dlsym(3) dlerror(3) dyld(1) ld(1)