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KTRACE(2)		  FreeBSD System Calls Manual		     KTRACE(2)

     ktrace -- process tracing

     Standard C	Library	(libc, -lc)

     #include <sys/param.h>
     #include <sys/time.h>
     #include <sys/uio.h>
     #include <sys/ktrace.h>

     ktrace(const char *tracefile, int ops, int	trpoints, int pid);

     The ktrace() system call enables or disables tracing of one or more pro-
     cesses.  Users may	only trace their own processes.	 Only the super-user
     can trace setuid or setgid	programs.

     The 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 call-
     ing 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),	tracefile may
     be	NULL.

     The ops argument specifies	the requested ktrace operation.	 The defined
     operations	are:

	   KTROP_SET		 Enable	trace points specified in trpoints.
	   KTROP_CLEAR		 Disable trace points specified	in trpoints.
	   KTROP_CLEARFILE	 Stop all tracing.
	   KTRFLAG_DESCEND	 The tracing change should apply to the	speci-
				 fied process and all its current children.

     The trpoints argument specifies the trace points of interest.  The
     defined trace points are:

	   KTRFAC_SYSCALL	 Trace system calls.
	   KTRFAC_SYSRET	 Trace return values from system calls.
	   KTRFAC_NAMEI		 Trace name lookup operations.
	   KTRFAC_GENIO		 Trace all I/O (note that this option can gen-
				 erate much output).
	   KTRFAC_PSIG		 Trace posted signals.
	   KTRFAC_CSW		 Trace context switch points.
	   KTRFAC_USER		 Trace application-specific events.
	   KTRFAC_STRUCT	 Trace certain data structures.
	   KTRFAC_SYSCTL	 Trace sysctls.
	   KTRFAC_PROCCTOR	 Trace process construction.
	   KTRFAC_PROCDTOR	 Trace process destruction.
	   KTRFAC_INHERIT	 Inherit tracing to future children.

     Each tracing event	outputs	a record composed of a generic header followed
     by	a trace	point specific structure.  The generic header is:

     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 */

     The ktr_len field specifies the length of the ktr_type data that follows
     this header.  The ktr_pid and ktr_comm fields specify the process and
     command generating	the record.  The ktr_time field	gives the time (with
     microsecond resolution) that the record was generated.  The ktr_tid field
     holds a thread id.

     The generic header	is followed by ktr_len bytes of	a ktr_type record.
     The type specific records are defined in the <sys/ktrace.h> include file.

     The following sysctl(8) tunables influence	the behaviour of ktrace():

	     bounds the	amount of data a traced	I/O request will log to	the
	     trace file.

	     bounds the	number of trace	events being logged at a time.

     Sysctl tunables that control process debuggability	(as determined by
     p_candebug(9)) also affect	the operation of ktrace().

     The ktrace() function returns the value 0 if successful; otherwise	the
     value -1 is returned and the global variable errno	is set to indicate the

     The ktrace() system call will fail	if:

     [ENOTDIR]		A component of the path	prefix is not a	directory.

     [ENAMETOOLONG]	A component of a pathname exceeded 255 characters, or
			an entire path name exceeded 1023 characters.

     [ENOENT]		The named tracefile does not exist.

     [EACCES]		Search permission is denied for	a component of the
			path prefix.

     [ELOOP]		Too many symbolic links	were encountered in translat-
			ing the	pathname.

     [EIO]		An I/O error occurred while reading from or writing to
			the file system.

     [ENOSYS]		The kernel was not compiled with ktrace	support.

     A thread may be unable to log one or more tracing events due to a tempo-
     rary shortage of resources.  This condition is remembered by the kernel,
     and the next tracing request that succeeds	will have the flag KTR_DROP
     set in its	ktr_type field.

     kdump(1), ktrace(1), utrace(2), sysctl(8),	p_candebug(9)

     The ktrace() system call first appeared in	4.4BSD.

FreeBSD	11.1			October	9, 2011			  FreeBSD 11.1


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