dlopen.3x

早期freebsd实现

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.\" @(#) dlopen.3x 1.6 90/01/31 SMI
.TH DLOPEN 3X "24 September 1989"
.SH NAME
dlopen, dlsym, dlerror, dlclose \- simple programmatic interface to the dynamic linker
.SH SYNOPSIS
.nf
.ft B
#include <dlfcn.h>
.LP
.ft B
.nf
void *dlopen(path, mode)
char *path; int mode;
.fi
.ft R
.LP
.ft B
.nfvoid *dlopen(path, mode)
.fi
.ft R
.LP
.ft B
.nf
void *dlsym(handle, symbol)
void *handle; char *symbol;
.fi
.ft R
.LP
.ft B
.nf
char *dlerror(\|)
.fi
.ft R
.LP
.ft B
.nf
int dlclose(handle);
void *handle;
.fi
.ft R
.ft R
.fi
.SH DESCRIPTION
.IX "dlopen()" "" "\fLdlopen()\fP \(em dynamically load a shared object"
.IX "programmatic interface to dynamic linker" dlopen() "" \fLdlopen()\fP
.IX "dlsym()" "" "\fLdlsym()\fP \(em dynamically lookup a symbol"
.IX "programmatic interface to dynamic linker" dlsym "" \fLdlsym()\fP
.IX "dlerror()" "" "\fLdlerror()\fP \(em dynamic linking error string"
.IX "programmatic interface to dynamic linker" dlerror() "" \fLdlerror()\fP
.IX "dlclose()" "" "\fLdlclose()\fP \(em unload a shared object"
.IX "programmatic interface to dynamic linker" dlclose() "" \fLdlclose()\fP
.LP
These functions provide a simple programmatic interface to the services of the
dynamic link-editor.
Operations are provided to add a new shared object to an
program's address space, obtain the address bindings of symbols defined by such
objects, and to remove such objects when their use is no longer required.
.LP
.B dlopen(\|)
provides access to the shared object in 
.IR path ,
returning a descriptor that can be used for later
references to the object in calls to 
.B dlsym(\|)
and
.BR dlclose(\|) .
If
.I path
was not in the address space prior to the call to
.BR dlopen(\|) ,
then it will be placed in the address space, and if it defines a function
with the name 
.I _init
that function will be called by 
.BR dlopen(\|) .
If, however,
.I path
has already been placed in the address space in a previous call to
.BR dlopen(\|) , 
then it will not be added a
second time, although a count of 
.B dlopen(\|)
operations on
.I path
will be maintained.
.I mode
is an integer containing flags describing options to be applied to the opening
and loading process \(em it is reserved for future expansion and must always have
the value 1.
A null pointer supplied for 
.I path
is interpreted as a reference to the \*(lqmain\*(rq
executable of the process.
If 
.B dlopen(\|)
fails, it will return a null pointer.
.LP
.B dlsym(\|)
returns the address binding of the symbol described in the null-terminated
character string
.I symbol
as it occurs in the shared object identified by
.IR handle .
The symbols exported by objects added to the address space by 
.B dlopen(\|)
can be accessed 
.I only
through calls to
.BR 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 \*(lqnormal\*(rq dynamic linking references.
.B dlsym(\|)
returns a null pointer if the symbol can not be found.
A null pointer supplied as the value of 
.I handle
is interpreted as a reference to the executable from which the call to 
.B dlsym(\|)
is being made \(em thus a shared object can reference its own symbols.
.LP
.B dlerror
returns a null-terminated character string describing the last error that
occurred during a
.BR dlopen(\|) ,
.BR dlsym(\|) ,
or
.BR dlclose(\|) .
If no such error has occurred, then 
.B dlerror(\|)
will return a null pointer.
At each call to
.BR dlerror(\|) ,
the \*(lqlast error\*(rq indication will be reset, thus in the case of two calls
to
.BR dlerror(\|) ,
and where the second call follows the first immediately, the second call
will always return a null pointer.
.LP
.B dlclose(\|)
deletes a reference to the shared object referenced by
.IR handle .
If the reference count drops to 0, then if the object referenced by 
.I handle
defines a function 
.IR _fini ,
that function will be called, the object
removed from the address space, and 
.I handle
destroyed.
If 
.B dlclose(\|)
is successful, it will return a value of 0.
A failing call to
.B dlclose(\|)
will return a non-zero value.
.LP
The object-intrinsic functions 
.I _init
and
.I _fini
are called with no arguments and treated as though their types were
.BR void .
.LP
These functions are obtained by specifying
.B \-ldl
as an option to 
.BR ld (1).
.SH SEE ALSO
.BR ld (1),
.BR link (5)