libc/gen/dlopen.3

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.Dd May 7, 2024
.Dt DLOPEN 3
.Os
.Sh NAME
.Nm dlopen ,
.Nm fdlopen ,
.Nm dlsym ,
.Nm dlvsym ,
.Nm dlfunc ,
.Nm dlerror ,
.Nm dlclose
.Nd programmatic interface to the dynamic linker
.Sh LIBRARY
.Lb libc
.Sh SYNOPSIS
.In dlfcn.h
.Ft void *
.Fn dlopen "const char *path" "int mode"
.Ft void *
.Fn fdlopen "int fd" "int mode"
.Ft void *
.Fn dlsym "void * restrict handle" "const char * restrict symbol"
.Ft void *
.Fn dlvsym "void * restrict handle" "const char * restrict symbol" "const char * restrict version"
.Ft dlfunc_t
.Fn dlfunc "void * restrict handle" "const char * restrict symbol"
.Ft char *
.Fn dlerror "void"
.Ft int
.Fn dlclose "void *handle"
.Sh DESCRIPTION
These functions provide a simple programmatic interface to the services of the
dynamic linker.
Operations are provided to add new shared objects to a
program's address space, to obtain the address bindings of symbols
defined by such
objects, and to remove such objects when their use is no longer required.
.Pp
The
.Fn dlopen
function
provides access to the shared object in
.Fa path ,
returning a descriptor that can be used for later
references to the object in calls to
.Fn dlsym ,
.Fn dlvsym
and
.Fn dlclose .
If
.Fa path
was not in the address space prior to the call to
.Fn dlopen ,
it is placed in the address space.
When an object is first loaded into the address space in this way, its
function
.Fn _init ,
if any, is called by the dynamic linker.
If
.Fa path
has already been placed in the address space in a previous call to
.Fn dlopen ,
it is not added a second time, although a reference count of
.Fn dlopen
operations on
.Fa path
is maintained.
A null pointer supplied for
.Fa path
is interpreted as a reference to the main
executable of the process.
The
.Fa mode
argument
controls the way in which external function references from the
loaded object are bound to their referents.
It must contain one of the following values, possibly ORed with
additional flags which will be described subsequently:
.Bl -tag -width RTLD_LAZYX
.It Dv RTLD_LAZY
Each external function reference is resolved when the function is first
called.
.It Dv RTLD_NOW
All external function references are bound immediately by
.Fn dlopen .
.El
.Pp
.Dv RTLD_LAZY
is normally preferred, for reasons of efficiency.
However,
.Dv RTLD_NOW
is useful to ensure that any undefined symbols are discovered during the
call to
.Fn dlopen .
.Pp
One of the following flags may be ORed into the
.Fa mode
argument:
.Bl -tag -width RTLD_NODELETE
.It Dv RTLD_GLOBAL
Symbols from this shared object and its directed acyclic graph (DAG)
of needed objects will be available for resolving undefined references
from all other shared objects.
.It Dv RTLD_LOCAL
Symbols in this shared object and its DAG of needed objects will be
available for resolving undefined references only from other objects
in the same DAG.
This is the default, but it may be specified
explicitly with this flag.
.It Dv RTLD_TRACE
When set, causes dynamic linker to exit after loading all objects
needed by this shared object and printing a summary which includes
the absolute pathnames of all objects, to standard output.
With this flag
.Fn dlopen
will return to the caller only in the case of error.
.It Dv RTLD_NODELETE
Prevents unload of the loaded object on
.Fn dlclose .
The same behaviour may be requested by
.Fl "z nodelete"
option of the static linker
.Xr ld 1 .
.It Dv RTLD_NOLOAD
Only return valid handle for the object if it is already loaded in
the process address space, otherwise
.Dv NULL
is returned.
Other mode flags may be specified, which will be applied for promotion
for the found object.
.It Dv RTLD_DEEPBIND
Symbols from the loaded library are put before global symbols when
resolving symbolic references originated from the library.
.El
.Pp
If
.Fn dlopen
fails, it returns a null pointer, and sets an error condition which may
be interrogated with
.Fn dlerror .
.Pp
The
.Fn fdlopen
function is similar to
.Fn dlopen ,
but it takes the file descriptor argument
.Fa fd ,
which is used for the file operations needed to load an object
into the address space.
The file descriptor
.Fa fd
is not closed by the function regardless a result of execution,
but a duplicate of the file descriptor is.
This may be important if a
.Xr lockf 3
lock is held on the passed descriptor.
The
.Fa fd
argument -1 is interpreted as a reference to the main
executable of the process, similar to
.Va NULL
value for the
.Fa name
argument to
.Fn dlopen .
The
.Fn fdlopen
function can be used by the code that needs to perform
additional checks on the loaded objects, to prevent races with
symlinking or renames.
Applications sandboxed using
.Xr capsicum 4
can also make beneficial use of
.Fn fdlopen .
.Pp
The
.Fn dlsym
function
returns the address binding of the symbol described in the null-terminated
character string
.Fa symbol ,
as it occurs in the shared object identified by
.Fa handle .
The symbols exported by objects added to the address space by
.Fn dlopen
can be accessed only through calls to
.Fn dlsym .
Such symbols do not supersede any definition of those symbols already present
in the address space when the object is loaded, nor are they available to
satisfy normal dynamic linking references.
.Pp
If
.Fn dlsym
is called with the special
.Fa handle
.Dv 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.
.Pp
If
.Fn dlsym
is called with the special
.Fa handle
.Dv RTLD_DEFAULT ,
the search for the symbol follows the algorithm used for resolving
undefined symbols when objects are loaded.
The objects searched are
as follows, in the given order:
.Bl -enum
.It
The referencing object itself (or the object from which the call to
.Fn dlsym
is made), if that object was linked using the
.Fl Bsymbolic
option to
.Xr ld 1 .
.It
All objects loaded at program start-up.
.It
All objects loaded via
.Fn dlopen
with the
.Dv RTLD_GLOBAL
flag set in the
.Fa mode
argument.
.It
All objects loaded via
.Fn dlopen
which are in needed-object DAGs that also contain the referencing object.
.El
.Pp
If
.Fn dlsym
is called with the special
.Fa handle
.Dv RTLD_NEXT ,
then the search for the symbol is limited to the shared objects
which were loaded after the one issuing the call to
.Fn 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.
.Dv RTLD_NEXT
is useful for implementing wrappers around library functions.
For example, a wrapper function
.Fn getpid
could access the
.Dq real
.Fn getpid
with
.Li dlsym(RTLD_NEXT, \&"getpid\&") .
(Actually, the
.Fn dlfunc
interface, below, should be used, since
.Fn getpid
is a function and not a data object.)
.Pp
If
.Fn dlsym
is called with the special
.Fa handle
.Dv RTLD_SELF ,
then the search for the symbol is limited to the shared object
issuing the call to
.Fn dlsym
and those shared objects which were loaded after it.
.Pp
The
.Fn dlsym
function
returns a null pointer if the symbol cannot be found, and sets an error
condition which may be queried with
.Fn dlerror .
.Pp
The
.Fn dlvsym
function behaves like
.Fn dlsym ,
but takes an extra argument
.Fa version :
a null-terminated character string which is used to request a specific version
of
.Fa symbol .
.Pp
The
.Fn dlfunc
function
implements all of the behavior of
.Fn dlsym ,
but has a return type which can be cast to a function pointer without
triggering compiler diagnostics.
(The
.Fn dlsym
function
returns an object pointer; in the C standard, conversions between
object and function pointer types are undefined.
Some compilers and lint utilities warn about such casts.)
The precise return type of
.Fn dlfunc
is unspecified; applications must cast it to an appropriate function pointer
type.
.Pp
The
.Fn dlerror
function
returns a null-terminated character string describing the last error that
occurred during a call to
.Fn dlopen ,
.Fn dladdr ,
.Fn dlinfo ,
.Fn dlsym ,
.Fn dlvsym ,
.Fn dlfunc ,
or
.Fn dlclose .
If no such error has occurred,
.Fn dlerror
returns a null pointer.
At each call to
.Fn dlerror ,
the error indication is reset.
Thus in the case of two calls
to
.Fn dlerror ,
where the second call follows the first immediately, the second call
will always return a null pointer.
.Pp
The
.Fn dlclose
function
deletes a reference to the shared object referenced by
.Fa handle .
If the reference count drops to 0, the object is removed from the
address space, and
.Fa handle
is rendered invalid.
Just before removing a shared object in this way, the dynamic linker
calls the object's
.Fn _fini
function, if such a function is defined by the object.
If
.Fn dlclose
is successful, it returns a value of 0.
Otherwise it returns -1, and sets an error condition that can be
interrogated with
.Fn dlerror .
.Pp
The object-intrinsic functions
.Fn _init
and
.Fn _fini
are called with no arguments, and are not expected to return values.
.Sh NOTES
ELF executables need to be linked
using the
.Fl export-dynamic
option to
.Xr ld 1
for symbols defined in the executable to become visible to
.Fn dlsym ,
.Fn dlvsym
or
.Fn dlfunc
.Pp
Other ELF platforms require linking with
.Lb libdl
to provide
.Fn dlopen
and other functions.
.Fx
does not require linking with the library, but supports it for compatibility.
.Pp
In previous implementations, it was necessary to prepend an underscore
to all external symbols in order to gain symbol
compatibility with object code compiled from the C language.
This is
still the case when using the (obsolete)
.Fl aout
option to the C language compiler.
.Sh ERRORS
The
.Fn dlopen ,
.Fn fdlopen ,
.Fn dlsym ,
.Fn dlvsym ,
and
.Fn dlfunc
functions
return a null pointer in the event of errors.
The
.Fn dlclose
function
returns 0 on success, or -1 if an error occurred.
Whenever an error has been detected, a message detailing it can be
retrieved via a call to
.Fn dlerror .
.Sh SEE ALSO
.Xr ld 1 ,
.Xr rtld 1 ,
.Xr dladdr 3 ,
.Xr dlinfo 3 ,
.Xr link 5