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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
     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

     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_FAULT          Trace page faults.
           KTRFAC_FAULTEND       Trace the end of page faults.
           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
                        translating 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
     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 4.4BSD.

FreeBSD 11.1-RELEASE-p4         March 31, 2016         FreeBSD 11.1-RELEASE-p4
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