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libpdel

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.\" Author: Archie Cobbs <archie@freebsd.org>
.\"
.\" $Id: logfile.3,v 1.1.1.1 2012/02/21 23:25:53 misho Exp $
.\"
.Dd April 22, 2002
.Dt LOGFILE 3
.Os
.Sh NAME
.Nm logfile
.Nd circular log files
.Sh LIBRARY
PDEL Library (libpdel, \-lpdel)
.Sh SYNOPSIS
.In sys/types.h
.In pdel/sys/logfile.h
.Ft "struct logfile *"
.Fn logfile_open "const char *path" "int flags" "u_int32_t maxent" "u_int32_t maxdata"
.Ft void
.Fn logfile_close "struct logfile **lfp"
.Ft u_int32_t
.Fn logfile_num_entries "struct logfile *lf"
.Ft "const void *"
.Fn logfile_get "struct logfile *lf" "int which" "int *lenp"
.Ft int
.Fn logfile_put "struct logfile *lf" "const void *data" "int len"
.Ft void
.Fn logfile_trim "struct logfile *lf" "int num"
.Ft void
.Fn logfile_sync "struct logfile *lf"
.Sh DESCRIPTION
These functions provide support for circular log files.
A
.Nm logfile
file contains the most recently added entries, where an entry
is any opaque chunk of data supplied by the application.
When the file becomes full, new entries overwrite the oldest
entries in a circular fashion.
.Pp
Each
.Nm logfile
file has a fixed size and is accessed using
.Xr mmap 2
for efficiency.
.Pp
.Fn logfile_open
opens the log file with pathname
.Fa path .
.Fa flags
may be equal to zero,
.Dv O_SHLOCK ,
or
.Dv O_EXLOCK
for locking purposes
(see
.Xr open 2
for details).
If the file is locked,
.Fn logfile_open
does not block, but instead returns
.Dv NULL
immediately with
.Va errno
set to
.Er EWOULDBLOCK .
.Pp
If the named file exists, it must be a valid
.Nm logfile
file and
.Fa maxent
and
.Fa maxdata
are ignored.
Otherwise, it is created using those parameters:
.Fa maxent
limits the total number of entries that the file may contain, and
.Fa maxdata
limit the total amount of entry data (in bytes) that the file
may contain.
When full, the file's ultimate size will be approximately
.Fa maxdata
+ (8 *
.Fa maxent )
+ 20.
.Pp
The 
.Fa path
may be
.Dv NULL ,
in which case the logfile is not stored in the file system and
therefore is not persistent.
.Pp
.Fn logfile_close
closes a log file previously opened using
.Fn logfile_open .
Upon return,
.Fa "*lfp"
will be set to
.Dv NULL .
If
.Fa "*lfp"
is already equal to
.Dv NULL
when
.Fn logfile_close
is invoked, nothing happens.
.Pp
.Fn logfile_num_entries
returns the number of valid entries contained in a log file.
.Pp
.Fn logfile_get
retrieves an entry from a log file.
.Fa which
must be a negative number: -1 is the most recently added entry,
-2 is the second most recently added, etc.
If
.Fa "lenp"
is not equal to
.Dv NULL ,
then
.Fa "*lenp"
is set to the length of the entry.
Entries returned by
.Fn logfile_get
are contiguous and suitably aligned for any type.
The caller should not free the returned pointer.
.Pp
.Fn logfile_put
adds a new entry to a log file.
The entry is pointed to by
.Fa data
and has length
.Fa len .
.Pp
.Fn logfile_trim
discards the oldest entries in a log file as necessary to make
the total number of entries be at most
.Fa num .
.Pp
.Fn logfile_sync
synchronously flushes any unwritten entries to permanent storage.
.Pp
Each
.Nm logfile
object maintains an internal mutex lock.
The functions
.Fn logfile_num_entries ,
.Fn logfile_get ,
.Fn logfile_put ,
.Fn logfile_trim ,
and
.Fn logfile_sync
may be safely called simultaneously from different threads.
.Sh RETURN VALUES
.Fn logfile_open
and
.Fn logfile_get
return
.Dv NULL
to indicate an error.
.Fn logfile_put
returns -1 to indicate an error.
In all error cases,
.Va errno
is set accordingly.
.Sh SEE ALSO
.Xr open 2 ,
.Xr alog 3 ,
.Xr libpdel 3 ,
.Xr typed_mem 3
.Sh HISTORY
The PDEL library was developed at Packet Design, LLC.
.Dv "http://www.packetdesign.com/"
.Sh AUTHORS
.An Archie Cobbs Aq archie@freebsd.org
.Sh BUGS
Meta information is stored in the logfile in host order.
Therefore,
.Nm logfile
files are not portable.