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.\"     @(#)ktrace.2	8.1 (Berkeley) 6/4/93
.\"
.Dd $Mdocdate: September 1 2021 $
.Dt KTRACE 2
.Os
.Sh NAME
.Nm ktrace
.Nd process tracing
.Sh SYNOPSIS
.In sys/types.h
.In sys/uio.h
.In sys/ktrace.h
.Ft int
.Fn ktrace "const char *tracefile" "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 superuser can trace setuid or setgid programs.
This function is only available on kernels compiled with the
.Cm KTRACE
option.
.Pp
.Fa tracefile
gives the pathname of the file to be used for tracing.
The file must exist, be writable by the calling process, and
not be a symbolic link.
If tracing points are being disabled (see
.Dv KTROP_CLEAR
below),
.Fa tracefile
must be
.Dv NULL .
.Pp
Trace records are always appended to the file, ignoring the file offset,
so the caller will usually want to truncate the file before calling
these functions.
.Pp
The
.Fa ops
parameter specifies the requested ktrace operation.
The defined operations are:
.Pp
.Bl -tag -width KTRFLAG_DESCEND -offset indent -compact
.It Dv KTROP_SET
Enable trace points specified in
.Fa trpoints .
.It Dv KTROP_CLEAR
Disable trace points specified in
.Fa trpoints .
.It Dv KTROP_CLEARFILE
Stop all tracing to the trace file.
.It Dv KTRFLAG_DESCEND
The tracing change should apply to the
specified process and all its current children.
.El
.Pp
The
.Fa trpoints
parameter specifies the trace points of interest.
The defined trace points are:
.Pp
.Bl -tag -width KTRFAC_EXECARGS -offset indent -compact
.It Dv KTRFAC_SYSCALL
Trace system calls.
.It Dv KTRFAC_SYSRET
Trace return values from system calls.
.It Dv KTRFAC_NAMEI
Trace name lookup operations.
.It Dv KTRFAC_GENIO
Trace all I/O
(note that this option can generate much output).
.It Dv KTRFAC_PSIG
Trace posted signals.
.It Dv KTRFAC_STRUCT
Trace various structs.
.It Dv KTRFAC_USER
Trace user data coming from
.Xr utrace 2
calls.
.It Dv KTRFAC_EXECARGS
Trace argument vector in
.Xr execve 2
calls.
.It Dv KTRFAC_EXECENV
Trace environment vector in
.Xr execve 2
calls.
.It Dv KTRFAC_PLEDGE
Trace violations of
.Xr pledge 2
restrictions.
.It Dv KTRFAC_INHERIT
Inherit tracing to future children.
.El
.Pp
The
.Fa pid
parameter refers to a process ID.
If it is negative,
it refers to a process group ID.
.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 {
	uint	ktr_type;		/* trace record type */
	pid_t	ktr_pid;		/* process id */
	pid_t	ktr_tid;		/* thread id */
	struct	timespec ktr_time;	/* timestamp */
	char	ktr_comm[MAXCOMLEN+1];	/* command name */
	size_t	ktr_len;		/* length of buf */
};
.Ed
.Pp
The
.Fa ktr_len
field specifies the length of the
.Fa ktr_type
data that follows this header.
The
.Fa ktr_pid , ktr_tid ,
and
.Fa ktr_comm
fields specify the process, thread, and command generating the record.
The
.Fa ktr_time
field gives the time (with nanosecond resolution)
that the record was generated.
.Pp
The generic header is followed by
.Fa ktr_len
bytes of a
.Fa ktr_type
record.
The type specific records are defined in the
.In sys/ktrace.h
include file.
.Sh RETURN VALUES
.Rv -std
.Sh ERRORS
.Fn ktrace
will fail if:
.Bl -tag -width EINVALAA
.It Bq Er EINVAL
No trace points were selected.
.It Bq Er EPERM
The tracing process is not the superuser and either its effective
user ID does not match the real user ID of the receiving process,
its effective group ID does not match the real group ID of the
receiving process,
the receiving process is currently being traced by the superuser,
or the receiving process has changed its UIDs or GIDs.
When tracing multiple processes,
this error is returned if none of the targetted processes could be traced.
When clearing a trace file with
.Dv KTROP_CLEARFILE ,
this error is returned if it could not stop tracing any of the processes
tracing to the file.
.It Bq Er ESRCH
No process can be found corresponding to that specified by
.Fa pid .
.It Bq Er EACCES
The named file is a device or FIFO.
.It Bq Er EIO
An I/O error occurred while reading from or writing to the file system.
.El
.Pp
Additionally,
.Fn ktrace
will fail if:
.Bl -tag -width ENAMETOOLONGAA
.It Bq Er ENOTDIR
A component of the path prefix is not a directory.
.It Bq Er ENAMETOOLONG
A component of a pathname exceeded
.Dv NAME_MAX
characters, or an entire pathname (including the terminating NUL)
exceeded
.Dv PATH_MAX
bytes.
.It Bq Er ENOENT
The named tracefile does not exist.
.It Bq Er EACCES
Search permission is denied for a component of the path prefix or the
path refers to a symbolic link.
.It Bq Er ELOOP
Too many symbolic links were encountered in translating the pathname.
.It Bq Er EFAULT
.Fa tracefile
points outside the process's allocated address space.
.El
.Sh SEE ALSO
.Xr kdump 1 ,
.Xr ktrace 1 ,
.Xr utrace 2
.Sh HISTORY
A
.Fn ktrace
function call first appeared in
.Bx 4.4 .