Reference

ktrace(2freebsd)

process tracing

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_SETEnable trace points specified in
KTROP_CLEARDisable trace points specified in
KTROP_CLEARFILEStop all tracing.
KTRFLAG_DESCENDThe tracing change should apply to the

The trpoints argument specifies the trace points of interest. The defined trace points are:

KTRFAC_SYSCALLTrace system calls.
KTRFAC_SYSRETTrace return values from system calls.
KTRFAC_NAMEITrace name lookup operations.
KTRFAC_GENIOTrace all I/O (note that this option can
KTRFAC_PSIGTrace posted signals.
KTRFAC_CSWTrace context switch points.
KTRFAC_USERTrace application-specific events.
KTRFAC_STRUCTTrace certain data structures.
KTRFAC_SYSCTLTrace sysctls.
KTRFAC_PROCCTORTrace process construction.
KTRFAC_PROCDTORTrace process destruction.
KTRFAC_CAPFAILTrace capability failures.
KTRFAC_FAULTTrace page faults.
KTRFAC_FAULTENDTrace the end of page faults.
KTRFAC_INHERITInherit 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.

History