man/man8/trace.8

.TH trace 8  "2016-02-18" "USER COMMANDS"
.SH NAME
trace \- Trace a function and print its arguments or return value, optionally evaluating a filter. Uses Linux eBPF/bcc.
.SH SYNOPSIS
.B trace [-h] [-p PID] [-v] [-Z STRING_SIZE] [-S] [-M MAX_EVENTS] [-o] probe [probe ...]
.SH DESCRIPTION
trace probes functions you specify and displays trace messages if a particular
condition is met. You can control the message format to display function 
arguments and return values. 

Since this uses BPF, only the root user can use this tool.
.SH REQUIREMENTS
CONFIG_BPF and bcc.
.SH OPTIONS
.TP
\-h
Print usage message.
.TP
\-p PID
Trace only functions in the process PID.
.TP
\-v
Display the generated BPF program, for debugging purposes.
.TP
\-z STRING_SIZE
When collecting string arguments (of type char*), collect up to STRING_SIZE 
characters. Longer strings will be truncated.
.TP
\-S
If set, trace messages from trace's own process. By default, this is off to
avoid tracing storms -- for example, if you trace the write system call, and
consider that trace is writing to the standard output.
.TP
\-M MAX_EVENTS
Print up to MAX_EVENTS trace messages and then exit.
.TP
\-o
Print times relative to the beginning of the trace (offsets), in seconds. The
default is to print absolute time.
.TP
probe [probe ...]
One or more probes that attach to functions, filter conditions, and print
information. See PROBE SYNTAX below.
.SH PROBE SYNTAX
The general probe syntax is as follows:

.B [{p,r}]:[library]:function [(predicate)] ["format string"[, arguments]]

.B {t:category:event,u:library:probe} [(predicate)] ["format string"[, arguments]]
.TP
.B {[{p,r}],t,u}
Probe type \- "p" for function entry, "r" for function return, "t" for kernel
tracepoint, "u" for USDT probe. The default probe type is "p".
.TP
.B [library]
Library containing the probe.
Specify the full path to the .so or executable file where the function to probe
resides. Alternatively, you can specify just the lib name: for example, "c"
refers to libc. If no library name is specified, the kernel is assumed. Also,
you can specify an executable name (without a full path) if it is in the PATH.
For example, "bash".
.TP
.B category
The tracepoint category. For example, "sched" or "irq".
.TP
.B function
The function to probe.
.TP
.B event
The tracepoint event. For example, "block_rq_complete".
.TP
.B probe
The USDT probe name. For example, "pthread_create".
.TP
.B [(predicate)]
The filter applied to the captured data. Only if the filter evaluates as true,
the trace message will be printed. The filter can use any valid C expression
that refers to the argument values: arg1, arg2, etc., or to the return value
retval in a return probe. If necessary, use C cast operators to coerce the
arguments to the desired type. For example, if arg1 is of type int, use the
expression ((int)arg1 < 0) to trace only invocations where arg1 is negative.
Note that only arg1-arg6 are supported, and only if the function is using the
standard x86_64 convention where the first six arguments are in the RDI, RSI, 
RDX, RCX, R8, R9 registers. If no predicate is specified, all function 
invocations are traced.
.TP
.B ["format string"[, arguments]]
A printf-style format string that will be used for the trace message. You can
use the following format specifiers: %s, %d, %u, %lld, %llu, %hd, %hu, %c,
%x, %llx -- with the same semantics as printf's. Make sure to pass the exact
number of arguments as there are placeholders in the format string. The
format specifier replacements may be any C expressions, and may refer to the
same special keywords as in the predicate (arg1, arg2, etc.).

In tracepoints, both the predicate and the arguments may refer to the tracepoint
format structure, which is stored in the special "tp" variable. For example, the
block:block_rq_complete tracepoint can print or filter by tp.nr_sector. To 
discover the format of your tracepoint, use the tplist tool. Note that you can
also use the members of the "tp" struct directly, e.g "nr_sector" instead of
"tp.nr_sector".

In USDT probes, the arg1, ..., argN variables refer to the probe's arguments.
To determine which arguments your probe has, use the tplist tool.

The predicate expression and the format specifier replacements for printing
may also use the following special keywords: $pid, $tgid to refer to the 
current process' pid and tgid; $uid, $gid to refer to the current user's
uid and gid; $cpu to refer to the current processor number.
.SH EXAMPLES
.TP
Trace all invocations of the open system call with the name of the file being opened:
#
.B trace '::do_sys_open """%s"", arg2'
.TP
Trace all invocations of the read system call where the number of bytes requested is greater than 20,000:
#
.B trace '::sys_read (arg3 > 20000) """read %d bytes"", arg3'
.TP
Trace all malloc calls and print the size of the requested allocation:
#
.B trace ':c:malloc """size = %d"", arg1'
.TP
Trace returns from the readline function in bash and print the return value as a string:
#
.B trace 'r:bash:readline """%s"", retval' 
.TP
Trace the block:block_rq_complete tracepoint and print the number of sectors completed:
#
.B trace 't:block:block_rq_complete """%d sectors"", nr_sector'
.TP
Trace the pthread_create USDT probe from the pthread library and print the address of the thread's start function:
#
.B trace 'u:pthread:pthread_create """start addr = %llx"", arg3'
.SH SOURCE
This is from bcc.
.IP
https://github.com/iovisor/bcc
.PP
Also look in the bcc distribution for a companion _examples.txt file containing
example usage, output, and commentary for this tool.
.SH OS
Linux
.SH STABILITY
Unstable - in development.
.SH AUTHOR
Sasha Goldshtein