xref: /freebsd/lib/libsys/ktrace.2 (revision 62a52c15422470f97fc7b311d89c83f910bcc1b1)
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28.Dd November 2, 2022
29.Dt KTRACE 2
30.Os
31.Sh NAME
32.Nm ktrace
33.Nd process tracing
34.Sh LIBRARY
35.Lb libc
36.Sh SYNOPSIS
37.In sys/param.h
38.In sys/time.h
39.In sys/uio.h
40.In sys/ktrace.h
41.Ft int
42.Fn ktrace "const char *tracefile" "int ops" "int trpoints" "int pid"
43.Sh DESCRIPTION
44The
45.Fn ktrace
46system call enables or disables tracing of one or more processes.
47Users may only trace their own processes.
48Only the super-user can trace setuid or setgid programs.
49.Pp
50The
51.Fa tracefile
52argument
53gives the pathname of the file to be used for tracing.
54The file must exist and be a regular file writable by the calling process.
55All trace records are always appended to the file,
56so the file must be truncated to zero length to discard
57previous trace data.
58If tracing points are being disabled (see KTROP_CLEAR below),
59.Fa tracefile
60may be NULL.
61.Pp
62The
63.Fa ops
64argument specifies the requested ktrace operation.
65The defined operations are:
66.Bl -column KTRFLAG_DESCENDXXX -offset indent
67.It KTROP_SET Ta "Enable trace points specified in"
68.Fa trpoints .
69.It KTROP_CLEAR Ta "Disable trace points specified in"
70.Fa trpoints .
71.It KTROP_CLEARFILE Ta "Stop all tracing."
72.It KTRFLAG_DESCEND Ta "The tracing change should apply to the"
73specified process and all its current children.
74.El
75.Pp
76The
77.Fa trpoints
78argument specifies the trace points of interest.
79The defined trace points are:
80.Bl -column KTRFAC_PROCCTORXXX -offset indent
81.It KTRFAC_SYSCALL Ta "Trace system calls."
82.It KTRFAC_SYSRET Ta "Trace return values from system calls."
83.It KTRFAC_NAMEI Ta "Trace name lookup operations."
84.It KTRFAC_GENIO Ta "Trace all I/O (note that this option can"
85generate much output).
86.It KTRFAC_PSIG Ta "Trace posted signals."
87.It KTRFAC_CSW Ta "Trace context switch points."
88.It KTRFAC_USER Ta "Trace application-specific events."
89.It KTRFAC_STRUCT Ta "Trace certain data structures."
90.It KTRFAC_SYSCTL Ta "Trace sysctls."
91.It KTRFAC_PROCCTOR Ta "Trace process construction."
92.It KTRFAC_PROCDTOR Ta "Trace process destruction."
93.It KTRFAC_CAPFAIL Ta "Trace capability failures."
94.It KTRFAC_FAULT Ta "Trace page faults."
95.It KTRFAC_FAULTEND Ta "Trace the end of page faults."
96.It KTRFAC_STRUCT_ARRAY Ta "Trace arrays of certain data structures."
97.It KTRFAC_INHERIT Ta "Inherit tracing to future children."
98.El
99.Pp
100Each tracing event outputs a record composed of a generic header
101followed by a trace point specific structure.
102The generic header is:
103.Bd -literal
104struct ktr_header {
105	int		ktr_len;		/* length of buf */
106	short		ktr_type;		/* trace record type */
107	pid_t		ktr_pid;		/* process id */
108	char		ktr_comm[MAXCOMLEN+1];	/* command name */
109	struct timeval	ktr_time;		/* timestamp */
110	long		ktr_tid;		/* thread id */
111};
112.Ed
113.Pp
114The
115.Va ktr_len
116field specifies the length of the
117.Va ktr_type
118data that follows this header.
119The
120.Va ktr_pid
121and
122.Va ktr_comm
123fields specify the process and command generating the record.
124The
125.Va ktr_time
126field gives the time (with microsecond resolution)
127that the record was generated.
128The
129.Va ktr_tid
130field holds a thread id.
131.Pp
132The generic header is followed by
133.Va ktr_len
134bytes of a
135.Va ktr_type
136record.
137The type specific records are defined in the
138.In sys/ktrace.h
139include file.
140.Sh SYSCTL TUNABLES
141The following
142.Xr sysctl 8
143tunables influence the behaviour of
144.Fn ktrace :
145.Bl -tag -width indent
146.It Va kern.ktrace.genio_size
147bounds the amount of data a traced I/O request will log
148to the trace file.
149.It Va kern.ktrace.request_pool
150bounds the number of trace events being logged at a time.
151.El
152.Pp
153Sysctl tunables that control process debuggability (as determined by
154.Xr p_candebug 9 )
155also affect the operation of
156.Fn ktrace .
157.Sh RETURN VALUES
158.Rv -std ktrace
159.Sh ERRORS
160The
161.Fn ktrace
162system call
163will fail if:
164.Bl -tag -width Er
165.It Bq Er ENOTDIR
166A component of the path prefix is not a directory.
167.It Bq Er ENAMETOOLONG
168A component of a pathname exceeded 255 characters,
169or an entire path name exceeded 1023 characters.
170.It Bq Er ENOENT
171The named tracefile does not exist.
172.It Bq Er EACCES
173Search permission is denied for a component of the path prefix.
174.It Bq Er ELOOP
175Too many symbolic links were encountered in translating the pathname.
176.It Bq Er EIO
177An I/O error occurred while reading from or writing to the file system.
178.It Bq Er EINTEGRITY
179Corrupted data was detected while reading from the file system.
180.It Bq Er ENOSYS
181The kernel was not compiled with
182.Nm
183support.
184.El
185.Pp
186A thread may be unable to log one or more tracing events due to a
187temporary shortage of resources.
188This condition is remembered by the kernel, and the next tracing request
189that succeeds will have the flag
190.Li KTR_DROP
191set in its
192.Va ktr_type
193field.
194.Sh SEE ALSO
195.Xr kdump 1 ,
196.Xr ktrace 1 ,
197.Xr utrace 2 ,
198.Xr sysctl 8 ,
199.Xr p_candebug 9
200.Sh HISTORY
201The
202.Fn ktrace
203system call first appeared in
204.Bx 4.4 .
205