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Dd March 7, 2024
Dt DLFCN 3
Os
Sh NAME
Nm _dlauxinfo ,
Nm dlopen ,
Nm dlclose ,
Nm dlsym ,
Nm dlvsym ,
Nm dladdr ,
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
In dlfcn.h
Ft "void *"
Fn _dlauxinfo "void"
Ft "void *"
Fn dlopen "const char *path" "int mode"
Ft "int"
Fn dlclose "void *handle"
Ft "void *"
Fn dlsym "void * restrict handle" "const char * restrict symbol"
Ft "void *"
Fn dlvsym "void * restrict handle" "const char * restrict symbol" "const char *version"
Ft "int"
Fn dladdr "void * restrict addr" "Dl_info * restrict 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.
Pp
The
Fn _dlauxinfo
function returns a pointer to the
Xr elf 5
array of
Vt AuxInfo
structures for the current executable.
Pp
The
Fn dlopen
function takes the name of a shared object as the first argument.
The
Fa path
argument can be specified as either an absolute pathname to a shared object
or just the name of the shared object itself.
When an absolute pathname is specified,
only the path provided will be searched.
When just a shared object name is specified, the same search rules apply that
are used for
Dq intrinsic
shared object searches
Po
see
Xr ld.elf_so 1
Pc .
Pp
Shared libraries take the following form:
Sm off
Ic lib\^ Ao Ar name Ac Ic .so Oo Ic \&. Ar xx\^ Oo Ic \&. Ar yy\^ Oc Oc .
Sm on
Pp
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.
Pp
If the first argument is
Dv NULL ,
Fn dlopen
returns a
Fa handle
on the global symbol object.
This object
provides access to all symbols from an ordered set of objects consisting
of the original program image and any dependencies loaded during startup.
Pp
The
Fa mode
parameter specifies symbol resolution time and symbol visibility.
One of the following values may be used to specify symbol resolution time:
Bl -tag -width ".Dv RTLD_NODELETE" -offset indent
It Dv RTLD_NOW
Symbols are resolved immediately.
It Dv RTLD_LAZY
Symbols are resolved when they are first referred to.
This is the default value if resolution time is unspecified.
El
Pp
One of the following values may be used to specify symbol visibility:
Bl -tag -width ".Dv RTLD_NODELETE" -offset indent
It Dv RTLD_GLOBAL
The object's symbols and the symbols of its dependencies will be visible to
other objects.
It Dv RTLD_LOCAL
The object's symbols and the symbols of its dependencies will not be visible to
other objects.
This is the default value if visibility is unspecified.
El
Pp
To specify both resolution time and visibility, bitwise inclusive
Tn OR
one of each of the above values together.
If an object was opened with
Dv RTLD_LOCAL
and later opened with
Dv RTLD_GLOBAL ,
then it is promoted to
Dv RTLD_GLOBAL .
Pp
Additionally, one of the following flags may be
Tn OR Ap ed
into the
Fa mode
argument:
Bl -tag -width ".Dv RTLD_NODELETE" -offset indent
It Dv RTLD_NODELETE
Prevents unload of the loaded object on
Fn dlclose .
The same behaviour may be requested by
Fl z Cm 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 do not load the object and return
Dv NULL .
El
Pp
Fn dlopen
returns a
Fa handle
to be used in calls to
Fn dlclose ,
Fn dlsym ,
Fn dlvsym ,
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
Fa handle
referring 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, or the object was one loaded at startup time,
or the object is a dependency of another object
then the object is removed when its reference count drops to zero.
Fn dlclose
returns 0 on success and non-zero on failure.
Pp
Fn dlsym
looks for a definition of
Fa symbol
in the shared object designated by
Fa handle ,
and all shared objects that are listed as dependencies.
The symbol's address is returned.
If the symbol cannot be resolved,
Dv NULL
is returned.
Pp
Fn dlsym
may also be called with special
Fa handle
values.
Fn dlsym
respects symbol visibility as specified by the
Fn dlopen
Fa mode
parameter.
However, the symbols of an object's dependencies are always visible to it.
All shared objects loaded at program startup are globally visible.
Only the symbols in the main executable that are referenced by a
shared object at link time will be visible unless it has been linked
with the
Fl Fl export-dynamic
option where all of its symbols will be visible.
The following special
Fa handle
values may be used with
Fn dlsym :
Bl -tag -width ".Dv RTLD_DEFAULT" -offset indent
It Dv NULL
Interpreted as a reference to the executable or shared object
from which the call is being made.
Thus an object can reference its own symbols and the symbols of its
dependencies without calling
Fn dlopen .
It Dv RTLD_DEFAULT
All the visible shared objects and the executable will be searched in the order
they were loaded.
It Dv RTLD_NEXT
The search for
Fa symbol
is limited to the visible shared objects which were loaded after the one
issuing the call to
Fn dlsym .
Thus, if
Fn dlsym
is called from the main program, all the visible shared libraries are searched.
If it is called from a shared library, all subsequently visible shared
libraries are searched.
It Dv RTLD_SELF
The search for
Fa symbol
is limited to the shared object issuing the call to
Fn dlsym
and those shared objects which were loaded after it that are visible.
El
Pp
Fn dlvsym
does the same as
Fn dlsym
but takes a
Fa version
string as an additional argument.
Both the
Fa symbol
and the
Fa version
must match in order for the symbol to be resolved.
Pp
Fn dladdr
examines all currently mapped shared objects for a symbol whose address \(em
as mapped in the process address space \(em 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
Fa addr
is between the base address of the shared object and the value of the
symbol
Va _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
Vt 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 */
       const void  *dli_saddr;     /* Symbol address */
} Dl_info;
Ed
Pp
The structure members are further described as follows:
Bl -tag -width Fa
It Fa dli_fname
The pathname of the shared object containing the address
Fa addr .
It Fa dli_fbase
The base address at which this shared object is loaded in the process
address space.
This 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.
It Fa dli_sname
points at the nul-terminated name of the selected symbol
It Fa dli_saddr
is the actual address
Pq as it appears in the process address space
of the symbol.
El
Pp
Em Note :
both strings pointed at by
Fa dli_fname
and
Fa dli_sname
reside in memory private to the run-time linker module and should not
be modified by the caller.
Pp
In dynamically linked programs, the address of a global function will
point to its program linkage table entry, rather than to the entry
point of the function itself.
This causes most global functions to appear to be defined within the
main executable, rather than in the shared libraries where the actual
code resides.
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
returns 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 occurred 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 ERRORS
Bl -diag
It Cannot dlopen non-loadable /usr/lib/libpthread.so.1
A program tries to
Fn dlopen
a module that needs
Lb libpthread
but the program isn't linked against it itself.
El
Sh SEE ALSO
Xr ld 1 ,
Xr rtld 1 ,
Xr dlinfo 3 ,
Xr link 5
Sh HISTORY
Some of the
Nm dl*
functions first appeared in SunOS\~4.

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