.TH UART 3 
.SH NAME
ptrace \- process scheduling traces
.SH SYNOPSIS
.nf
.B bind -a #σ /dev

.B /dev/ptrace
.B /dev/ptracectl
.fi
.SH DESCRIPTION
.PP
The process trace device is a replacement for the trace facility in
.IR proc (3)
to report more information about process events.
.PP
The
.B ptracectl
admits the following requests:
.TP
.BI size " n
Set the size of the buffer to
.I n
entries. Before issuing this request tracing is not enabled.
.TP
.BI trace " pid value"
Set the trace on or off for the process with the given pid.
.PP
The
.B ptrace
file reports an integral number of events when read. To avoid
too much interference with scheduling, it does not block readers.
When no more events are available the read returns an EOF indication,
but the program is expected to poll the device for further events.
This behavior mimics what
.IR proc (3)
does with the trace interface.
.SH SOURCE
.B /sys/src/9/port/devptrace.c
.br
.B /sys/src/9k/port/devptrace.c
.SH "SEE ALSO
.IR ptrace (1),
.IR ptrace (2).
.SH BUGS
If the buffer size is too small, events will be discarded.
.PP
The reader must poll the trace device.