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     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 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 <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 error.


     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.