KTRACE(2) FreeBSD System Calls Manual KTRACE(2)


ktraceprocess 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:
A component of the path prefix is not a directory.
A component of a pathname exceeded 255 characters, or an entire path name exceeded 1023 characters.
The named tracefile does not exist.
Search permission is denied for a component of the path prefix.
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.
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.


The ktrace() system call first appeared in 4.4BSD.
October 10, 2011 FreeBSD