libc/gen/dlinfo.3

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.\" Copyright (c) 2003 Alexey Zelkin <phantom@FreeBSD.org>
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.Dd May 21, 2020
.Dt DLINFO 3
.Os
.Sh NAME
.Nm dlinfo
.Nd information about dynamically loaded object
.Sh LIBRARY
.Lb libc
.Sh SYNOPSIS
.In link.h
.In dlfcn.h
.Ft int
.Fn dlinfo "void * restrict handle" "int request" "void * restrict p"
.Sh DESCRIPTION
The
.Fn dlinfo
function provides information about dynamically loaded object.
The action taken by
.Fn dlinfo
and exact meaning and type of
.Fa p
argument depend on value of the
.Fa request
argument provided by caller.
.Pp
The
.Fa handle
argument is either the value returned from the
.Xr dlopen 3
function call or special handle
.Dv RTLD_SELF .
If
.Fa handle
is the value returned from
.Xr dlopen 3 ,
the information returned by the
.Fn dlinfo
function pertains to the specified object.
If handle is the special handle
.Dv RTLD_SELF ,
the information returned pertains to the caller itself.
.Pp
Possible values for the
.Fa request
argument are:
.Bl -tag -width indent
.It Dv RTLD_DI_LINKMAP
Retrieve the
.Vt Link_map
.Pq Vt "struct link_map"
structure pointer for the specified
.Fa handle .
On successful return, the
.Fa p
argument is filled with the pointer to the
.Vt Link_map
structure
.Pq Fa "Link_map **p"
describing a shared object specified by the
.Fa handle
argument.
The
.Vt Link_map
structures are maintained as a doubly linked list by
.Xr ld.so 1 ,
in the same order as
.Xr dlopen 3
and
.Xr dlclose 3
are called.
See
.Sx EXAMPLES ,
example 1.
.Pp
The
.Vt Link_map
structure is defined in
.In link.h
and has the following members:
.Bd -literal -offset indent
caddr_t         l_base;    /* Base Address of library */
const char      *l_name;   /* Absolute Path to Library */
const void      *l_ld;     /* Pointer to .dynamic in memory */
struct link_map *l_next,   /* linked list of mapped libs */
                *l_prev;
caddr_t         l_addr;     /* Load Offset of library */
const char      *l_refname; /* Object this one filters for */
.Ed
.Bl -tag -width ".Va l_addr"
.It Va l_base
The base address of the object loaded into memory.
.It Va l_name
The full name of the loaded shared object.
.It Va l_ld
The address of the dynamic linking information segment
.Pq Dv PT_DYNAMIC
loaded into memory.
.It Va l_next
The next
.Vt Link_map
structure on the link-map list.
.It Va l_prev
The previous
.Vt Link_map
structure on the link-map list.
.It Va l_addr
The load offset of the object, that is, the difference between
the actual load address and the base virtual address the object
was linked at.
.It Va l_refname
A name of the object this object filters for, if any.
If there are more then one filtee, a name from the first
.Dv DT_FILTER
dynamic entry is supplied.
.El
.It Dv RTLD_DI_SERINFO
Retrieve the library search paths associated with the given
.Fa handle
argument.
The
.Fa p
argument should point to
.Vt Dl_serinfo
structure buffer
.Pq Fa "Dl_serinfo *p" .
The
.Vt Dl_serinfo
structure must be initialized first with the
.Dv RTLD_DI_SERINFOSIZE
request.
.Pp
The returned
.Vt Dl_serinfo
structure contains
.Va dls_cnt
.Vt Dl_serpath
entries.
Each entry's
.Va dlp_name
field points to the search path.
The corresponding
.Va dlp_info
field contains one of more flags indicating the origin of the path (see the
.Dv LA_SER_*
flags defined in the
.In link.h
header file).
See
.Sx EXAMPLES ,
example 2, for a usage example.
.It Dv RTLD_DI_SERINFOSIZE
Initialize a
.Vt Dl_serinfo
structure for use in a
.Dv RTLD_DI_SERINFO
request.
Both the
.Va dls_cnt
and
.Va dls_size
fields are returned to indicate the number of search paths applicable
to the handle, and the total size of a
.Vt Dl_serinfo
buffer required to hold
.Va dls_cnt
.Vt Dl_serpath
entries and the associated search path strings.
See
.Sx EXAMPLES ,
example 2, for a usage example.
.It Va RTLD_DI_ORIGIN
Retrieve the origin of the dynamic object associated with the handle.
On successful return,
.Fa p
argument is filled with the
.Vt char
pointer
.Pq Fa "char *p" .
.El
.Sh RETURN VALUES
The
.Fn dlinfo
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
.Xr dlerror 3 .
.Sh EXAMPLES
Example 1: Using
.Fn dlinfo
to retrieve
.Vt Link_map
structure.
.Pp
The following example shows how dynamic library can detect the list
of shared libraries loaded after caller's one.
For simplicity, error checking has been omitted.
.Bd -literal -offset indent
Link_map *map;

dlinfo(RTLD_SELF, RTLD_DI_LINKMAP, &map);

while (map != NULL) {
	printf("%p: %s\\n", map->l_addr, map->l_name);
	map = map->l_next;
}
.Ed
.Pp
Example 2: Using
.Fn dlinfo
to retrieve the library search paths.
.Pp
The following example shows how a dynamic object can inspect the library
search paths that would be used to locate a simple filename with
.Xr dlopen 3 .
For simplicity, error checking has been omitted.
.Bd -literal -offset indent
Dl_serinfo	 _info, *info = &_info;
Dl_serpath	*path;
unsigned int	 cnt;

/* determine search path count and required buffer size */
dlinfo(RTLD_SELF, RTLD_DI_SERINFOSIZE, (void *)info);

/* allocate new buffer and initialize */
info = malloc(_info.dls_size);
info->dls_size = _info.dls_size;
info->dls_cnt = _info.dls_cnt;

/* obtain sarch path information */
dlinfo(RTLD_SELF, RTLD_DI_SERINFO, (void *)info);

path = &info->dls_serpath[0];

for (cnt = 1; cnt <= info->dls_cnt; cnt++, path++) {
	(void) printf("%2d: %s\\n", cnt, path->dls_name);
}
.Ed
.Sh SEE ALSO
.Xr rtld 1 ,
.Xr dladdr 3 ,
.Xr dlopen 3 ,
.Xr dlsym 3
.Sh HISTORY
The
.Fn dlinfo
function first appeared in the Solaris operating system.
In
.Fx ,
it first appeared in
.Fx 4.8 .
.Sh AUTHORS
.An -nosplit
The
.Fx
implementation of the
.Fn dlinfo
function was originally written by
.An Alexey Zelkin Aq Mt phantom@FreeBSD.org
and later extended and improved by
.An Alexander Kabaev Aq Mt kan@FreeBSD.org .
.Pp
The manual page for this function was written by
.An Alexey Zelkin Aq Mt phantom@FreeBSD.org .