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.\"     @(#)ktrace.2	8.1 (Berkeley) 6/4/93
.\"
.Dd March 19, 2016
.Dt KTRACE 2
.Os
.Sh NAME
.Nm ktrace
.Nd process tracing
.Sh LIBRARY
.Lb libc
.Sh SYNOPSIS
.In sys/param.h
.In sys/ktrace.h
.Ft int
.Fn ktrace "const char *tracefile" "int ops" "int trpoints" "pid_t pid"
.Ft int
.Fn fktrace "int fd" "int ops" "int trpoints" "pid_t pid"
.Sh DESCRIPTION
The
.Fn ktrace
function enables or disables tracing of one or more processes.
Users may only trace their own processes.
Only the super-user can trace setuid or setgid programs.
.Pp
The
.Ar tracefile
gives the pathname of the file to be used for tracing.
The file must exist and be writable by the calling process.
All trace records are always appended to the file,
so the file must be truncated to zero length to discard
previous trace data.
If tracing points are being disabled (see KTROP_CLEAR below),
.Ar tracefile
may be NULL.
If using
.Fn fktrace
then instead of passing a filename as
.Ar tracefile ,
a file descriptor is passed as
.Ar fd
and behaviour is otherwise the same.
.Pp
The
.Nm ops
parameter specifies the requested ktrace operation.
The defined operations are:
.Bl -column KTRFLAG_DESCENDXXX -offset indent
.It KTROP_SET	Enable trace points specified in Ar trpoints .
.It KTROP_CLEAR	Disable trace points specified in Ar trpoints .
.It KTROP_CLEARFILE	Stop all tracing.
.It KTRFLAG_DESCEND	The tracing change should apply to the
specified process and all its current children.
.El
.Pp
The
.Nm trpoints
parameter specifies the trace points of interest.
The defined trace points are:
.Bl -column KTRFAC_SYSCALLXXX -offset indent
.It KTRFAC_SYSCALL	Trace system calls.
.It KTRFAC_SYSRET	Trace return values from system calls.
.It KTRFAC_NAMEI	Trace name lookup operations.
.It KTRFAC_GENIO	Trace all I/O (note that this option can
generate much output).
.It KTRFAC_PSIG	Trace posted signals.
.It KTRFAC_CSW	Trace context switch points.
.It KTRFAC_EMUL	Trace emulation changes.
.It KTRFAC_INHERIT	Inherit tracing to future children.
.El
.Pp
Each tracing event outputs a record composed of a generic header
followed by a trace point specific structure.
The generic header is:
.Bd -literal
struct ktr_header {
	int	ktr_len;		/* length of buf */
	short	ktr_type;		/* trace record type */
	short	ktr_version;		/* trace record version */
	pid_t	ktr_pid;		/* process id */
	char	ktr_comm[MAXCOMLEN+1];	/* command name */
	struct	timespec ktr_time;	/* timestamp */
	lwpid_t	ktr_lid;
};
.Ed
.Pp
The
.Nm ktr_len
field specifies the length of the data that follows this header.
The
.Nm ktr_type
and
.Nm ktr_version
fields (whose ordering in the structure depends on byte order)
specify the format of this data.
The
.Nm ktr_pid ,
.Nm ktr_lid ,
and
.Nm ktr_comm
fields specify the process and command generating the record.
The
.Nm ktr_time
field gives the time (with nanosecond resolution)
that the record was generated.
.Pp
The generic header is followed by
.Nm ktr_len
bytes of a
.Nm ktr_type
record of version
.Nm ktr_version .
The type specific records are defined in the
.Aq Pa sys/ktrace.h
include file.
.Sh RETURN VALUES
On successful completion a value of 0 is returned.
Otherwise, a value of \-1 is returned and
.Va errno
is set to show the error.
.Sh ERRORS
.Fn ktrace
will fail if:
.Bl -tag -width ENAMETOOLONGAA
.It Bq Er EACCES
Search permission is denied for a component of the path prefix.
.It Bq Er EINVAL
The pathname contains a character with the high-order bit set.
.It Bq Er EIO
An I/O error occurred while reading from or writing to the file system.
.It Bq Er ELOOP
Too many symbolic links were encountered in translating the pathname.
.It Bq Er ENAMETOOLONG
A component of a pathname exceeded
.Brq Dv NAME_MAX
characters,
or an entire path name exceeded
.Brq Dv PATH_MAX
characters.
.It Bq Er ENOENT
The named tracefile does not exist.
.It Bq Er ENOTDIR
A component of the path prefix is not a directory.
.El
.Sh SEE ALSO
.Xr kdump 1 ,
.Xr ktrace 1
.Sh HISTORY
A
.Nm ktrace
function call first appeared in
.Bx 4.4 .