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 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_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.

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 translating the pathname.

       [EIO]              An I/O error occurred while reading from or writing to the file system.

       [EINTEGRITY]       Corrupted data was detected while reading from 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.

SEE ALSO

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

HISTORY

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

Debian                                           March 30, 2020                                        KTRACE(2)