ktrace man page on FreeBSD

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KTRACE(2)		    BSD System Calls Manual		     KTRACE(2)

NAME
     ktrace — process tracing

LIBRARY
     Standard C Library (libc, -lc)

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

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

DESCRIPTION
     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_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 threadid.

     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.

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

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

     kern.ktrace.request_pool
	     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().

RETURN VALUES
     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
     error.

ERRORS
     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.

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

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

BSD				 July 13, 2008				   BSD
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