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Man Pages

Manual Reference Pages  -  KTRACE (2)


ktrace - process tracing


Sysctl Tunables
Return Values
See Also


.Lb libc


.In sys/param.h
.In sys/time.h
.In sys/uio.h
.In sys/ktrace.h int ktrace const char *tracefile int ops int trpoints int pid


The 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.

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 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), 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
  specified 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
  generate 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_CAPFAIL      Trace capability failures.
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
.In 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.


.Rv -std ktrace


The ktrace system call will fail if:
  A component of the path prefix is not a directory.
  A component of a pathname exceeded 255 characters, or an entire path name exceeded 1023 characters.
  The named tracefile does not exist.
  Search permission is denied for a component of the path prefix.
  Too many symbolic links were encountered in translating the pathname.
[EIO] An I/O error occurred while reading from or writing to the file system.
  The kernel was not compiled with ktrace support.

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 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 BSD 4.4 .
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