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.\"     @(#)ktrace.2	8.1 (Berkeley) 6/4/93
.\" $FreeBSD: releng/12.2/lib/libc/sys/ktrace.2 365473 2020-09-08 20:51:19Z jhb $
.\"
.Dd March 30, 2020
.Dt KTRACE 2
.Os
.Sh NAME
.Nm ktrace
.Nd process tracing
.Sh LIBRARY
.Lb libc
.Sh SYNOPSIS
.In sys/param.h
.In sys/time.h
.In sys/uio.h
.In sys/ktrace.h
.Ft int
.Fn ktrace "const char *tracefile" "int ops" "int trpoints" "int pid"
.Sh DESCRIPTION
The
.Fn ktrace
system call 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
.Fa tracefile
argument
gives the pathname of the file to be used for tracing.
The file must exist and be a regular file 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),
.Fa tracefile
may be NULL.
.Pp
The
.Fa ops
argument specifies the requested ktrace operation.
The defined operations are:
.Bl -column KTRFLAG_DESCENDXXX -offset indent
.It KTROP_SET Ta "Enable trace points specified in"
.Fa trpoints .
.It KTROP_CLEAR Ta "Disable trace points specified in"
.Fa trpoints .
.It KTROP_CLEARFILE Ta "Stop all tracing."
.It KTRFLAG_DESCEND Ta "The tracing change should apply to the"
specified process and all its current children.
.El
.Pp
The
.Fa trpoints
argument specifies the trace points of interest.
The defined trace points are:
.Bl -column KTRFAC_PROCCTORXXX -offset indent
.It KTRFAC_SYSCALL Ta "Trace system calls."
.It KTRFAC_SYSRET Ta "Trace return values from system calls."
.It KTRFAC_NAMEI Ta "Trace name lookup operations."
.It KTRFAC_GENIO Ta "Trace all I/O (note that this option can"
generate much output).
.It KTRFAC_PSIG Ta "Trace posted signals."
.It KTRFAC_CSW Ta "Trace context switch points."
.It KTRFAC_USER Ta "Trace application-specific events."
.It KTRFAC_STRUCT Ta "Trace certain data structures."
.It KTRFAC_SYSCTL Ta "Trace sysctls."
.It KTRFAC_PROCCTOR Ta "Trace process construction."
.It KTRFAC_PROCDTOR Ta "Trace process destruction."
.It KTRFAC_CAPFAIL Ta "Trace capability failures."
.It KTRFAC_FAULT Ta "Trace page faults."
.It KTRFAC_FAULTEND Ta "Trace the end of page faults."
.It KTRFAC_INHERIT Ta "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 */
	pid_t		ktr_pid;		/* process id */
	char		ktr_comm[MAXCOMLEN+1];	/* command name */
	struct timeval	ktr_time;		/* timestamp */
	intptr_t	ktr_tid;		/* was ktr_buffer */
};
.Ed
.Pp
The
.Va ktr_len
field specifies the length of the
.Va ktr_type
data that follows this header.
The
.Va ktr_pid
and
.Va ktr_comm
fields specify the process and command generating the record.
The
.Va ktr_time
field gives the time (with microsecond resolution)
that the record was generated.
The
.Va ktr_tid
field holds a thread id.
.Pp
The generic header is followed by
.Va ktr_len
bytes of a
.Va ktr_type
record.
The type specific records are defined in the
.In sys/ktrace.h
include file.
.Sh SYSCTL TUNABLES
The following
.Xr sysctl 8
tunables influence the behaviour of
.Fn ktrace :
.Bl -tag -width indent
.It Va kern.ktrace.geniosize
bounds the amount of data a traced I/O request will log
to the trace file.
.It Va kern.ktrace.request_pool
bounds the number of trace events being logged at a time.
.El
.Pp
Sysctl tunables that control process debuggability (as determined by
.Xr p_candebug 9 )
also affect the operation of
.Fn ktrace .
.Sh RETURN VALUES
.Rv -std ktrace
.Sh ERRORS
The
.Fn ktrace
system call
will fail if:
.Bl -tag -width Er
.It Bq Er ENOTDIR
A component of the path prefix is not a directory.
.It Bq Er ENAMETOOLONG
A component of a pathname exceeded 255 characters,
or an entire path name exceeded 1023 characters.
.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.
.It Bq Er ELOOP
Too many symbolic links were encountered in translating the pathname.
.It Bq Er EIO
An I/O error occurred while reading from or writing to the file system.
.It Bq Er EINTEGRITY
Corrupted data was detected while reading from the file system.
.It Bq Er ENOSYS
The kernel was not compiled with
.Nm
support.
.El
.Pp
A thread may be unable to log one or more tracing events due to a
temporary shortage of resources.
This condition is remembered by the kernel, and the next tracing request
that succeeds will have the flag
.Li KTR_DROP
set in its
.Va ktr_type
field.
.Sh SEE ALSO
.Xr kdump 1 ,
.Xr ktrace 1 ,
.Xr utrace 2 ,
.Xr sysctl 8 ,
.Xr p_candebug 9
.Sh HISTORY
The
.Fn ktrace
system call first appeared in
.Bx 4.4 .