NetBSD/share/man/man3/dlfcn.3

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.Dd September 30, 1995
.Dt DLFCN 3
.Os
.Sh NAME
.Nm dlopen ,
.Nm dlclose ,
.Nm dlsym ,
.Nm dlctl ,
.Nm dlerror
.Nd dynamic link interface
.Sh LIBRARY
(These functions are not in a library.  They are included in every
dynamically linked program automatically.)
.Sh SYNOPSIS
.Fd #include <dlfcn.h>
.Ft "void *"
.Fn dlopen "const char *path" "int mode"
.Ft "int"
.Fn dlclose "void *handle"
.Ft "void *"
.Fn dlsym "void *handle" "const char *symbol"
.Ft "int"
.Fn dladdr "void *addr" "Dl_info *dli"
.Ft "int"
.Fn dlctl "void *handle" "int cmd" "void *data"
.Ft "char *"
.Fn dlerror "void"
.Sh DESCRIPTION
These functions provide an interface to the run-time linker
.Xr ld.so 1 .
They allow new shared objects to be loaded into the process' address space
under program control.
The
.Fn dlopen
function takes a name of a shared object as the first argument.
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 argument can either be an absolute pathname or it can be of the form
.Sm off
.Do Xo lib Ao name Ac .so
.Op .xx Op .yy Xc
.Dc
.Sm on
in which case the same library search rules apply that are used for
.Dq intrinsic
shared library searches.
The second argument has currently no effect, but should be set to
.Dv DL_LAZY
for future compatibility.
.Fn dlopen
returns a handle to be used in calls to
.Fn dlclose ,
.Fn dlsym
and
.Fn dlctl .
If the named shared object has already
been loaded by a previous call to
.Fn dlopen
.Pq and not yet unloaded by Fn dlclose ,
a handle refering to the resident copy is returned.
.Pp
.Fn dlclose
unlinks and removes the object referred to by
.Fa handle
from the process address space.
If multiple calls to
.fn dlopen
have been done on this object
.Po or the object was one loaded at startup time
.Pc
the object is removed when its reference count drops to zero.
.Pp
.Fn dlsym
looks for a definition of
.Fa symbol
in the shared object designated by
.Fa handle .
The symbols address is returned.
If the symbol cannot be resolved,
.Dv NULL
is returned.
.Pp
.Fn dladdr
examines all currently mapped shared objects for a symbol whose address --
as mapped in the proces address space -- is closest to but not exceeding
the value passed in the first argument
.Fa addr .
The symbols of a shared object are only eligible if
.Va addr
is between the base address of the shared object and the value of the
symbol
.Dq _end
in the same shared object. If no object for which this condition holds
true can be found,
.Fn dladdr
will return 0. Otherwise, a non-zero value is returned and the
.Fa dli
argument will be used to provide information on the selected symbol
and the shared object it is contained in.
The
.Fa dli
argument points at a caller-provided
.Va Dl_info
structure defined as follows:
.Bd -literal -offset indent
typedef struct {
	const char  *dli_fname;     /* File defining the symbol */
	void	    *dli_fbase;     /* Base address */
	const char  *dli_sname;     /* Symbol name */
	void	    *dli_saddr;     /* Symbol address */
} Dl_info;
.Ed
.Pp
The member
.Va dli_sname
points at the nul-terminated name of the selected symbol, and
.Va dli_saddr
is the actual address
.Pq as it appears in the process address space
of the symbol.
The member
.Va dli_fname
points at the file name corresponding to the shared object in which the
symbol was found, while
.Va dli_fbase
is the base address at which this shared object is loaded in the process
address space.
.Va dli_fname
and
.Va dli_fbase
may be zero if the symbol was found in the internally generated
.Dq copy
section
.Po
see
.Xr link 5
.Pc
which is not associated with a file.
Note: both strings pointed at by
.Va dli_fname
and
.Va dli_sname
reside in memory private to the run-time linker module and should not
be modified by the caller.
.Pp
.Fn dlctl
provides an interface similar to
.Xr ioctl 2
to control several aspects of the run-time linker's operation.
This interface
is
.Ud .
.Pp
.Fn dlerror
return 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 occured since the last invocation of
.Fn dlerror ,
.Fn dlerror
returns
.Dv NULL .
Thus, invoking
.Fn dlerror
a second time, immediately following a prior invocation, will result in
.Dv NULL
being returned.
.Sh SEE ALSO
.Xr ld 1 ,
.Xr rtld 1 ,
.Xr link 5
.Sh HISTORY
Some of the
.Nm dl*
functions first appeared in SunOS 4.
.Sh BUGS
An error that occurs while processing a
.Fn dlopen
request results in the termination of the program.