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Manual Pages  — KTRACE

NAME

ktrace – process tracing

CONTENTS

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 intrpoints.

KTROP_CLEAR
Disable trace points specified intrpoints.

KTROP_CLEARFILE
Stop all tracing.

KTRFLAG_DESCEND
The tracing change should apply to thespecified 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 cangenerate 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.
[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 BSD 4.4 .

KTRACE (2) March 31, 2016

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Please direct any comments about this manual page service to Ben Bullock.

This philosophy, in the hands of amateurs, leads to inexplicably mind-numbing botches like the existence of two programs, “head” and “tail,” which print the first part or the last part of a file, depending. Even though their operations are duals of one another, “head” and “tail” are different programs, written by different authors, and take different options!
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