argdist.8

.TH argdist 8  "2016-02-11" "USER COMMANDS"
.SH NAME
argdist \- Trace a function and display a histogram or frequency count of its parameter values. Uses Linux eBPF/bcc.
.SH SYNOPSIS
.B argdist [-h] [-p PID] [-z STRING_SIZE] [-i INTERVAL] [-n COUNT] [-v] [-T TOP] [-H specifier [specifier ...]] [-C specifier [specifier ...]]
.SH DESCRIPTION
argdist attaches to function entry and exit points, collects specified parameter
values, and stores them in a histogram or a frequency collection that counts
the number of times a parameter value occurred. It can also filter parameter
values and instrument multiple entry points at once.

This currently only works on x86_64. Check for future versions.
.SH REQUIREMENTS
CONFIG_BPF and bcc.
.SH OPTIONS
.TP
\-h
Print usage message.
.TP
\-p PID
Trace only functions in the process PID.
.TP
\-z STRING_SIZE
When collecting string arguments (of type char*), collect up to STRING_SIZE 
characters. Longer strings will be truncated.
.TP
-i INTERVAL
Print the collected data every INTERVAL seconds. The default is 1 second.
.TP
-n NUMBER
Print the collected data COUNT times and then exit.
.TP
\-v
Display the generated BPF program, for debugging purposes.
.TP
\-T TOP
When collecting frequency counts, display only the top TOP entries.
.TP
-H SPECIFIER, -C SPECIFIER
One or more probe specifications that instruct argdist which functions to
probe, which parameters to collect, how to aggregate them, and whether to perform
any filtering. See SPECIFIER SYNTAX below.
.SH SPECIFIER SYNTAX
The general specifier syntax is as follows:

.B {p,r}:[library]:function(signature)[:type:expr[:filter]][;label]
.TP
.B {p,r}
Probe type \- "p" for function entry, "r" for function return;
\-H for histogram collection, \-C for frequency count.
Indicates where to place the probe and whether the probe should collect frequency
count information, or aggregate the collected values into a histogram. Counting 
probes will collect the number of times every parameter value was observed,
whereas histogram probes will collect the parameter values into a histogram.
Only integral types can be used with histogram probes; there is no such limitation
for counting probes.
.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.
.TP
.B function(signature)
The function to probe, and its signature.
The function name must match exactly for the probe to be placed. The signature,
on the other hand, is only required if you plan to collect parameter values 
based on that signature. For example, if you only want to collect the first
parameter, you don't have to specify the rest of the parameters in the signature.
.TP
.B [type]
The type of the expression to capture.
This is the type of the keys in the histogram or raw event collection that are
collected by the probes.
.TP
.B [expr]
The expression to capture.
These are the values that are assigned to the histogram or raw event collection.
You may use the parameters directly, or valid C expressions that involve the
parameters, such as "size % 10".
Return probes can use the argument values received by the
function when it was entered, through the $entry(paramname) special variable.
Return probes can also access the function's return value in $retval, and the
function's execution time in nanoseconds in $latency. Note that adding the
$latency or $entry(paramname) variables to the expression will introduce an
additional probe at the function's entry to collect this data, and therefore
introduce additional overhead.
.TP
.B [filter]
The filter applied to the captured data.
Only parameter values that pass the filter will be collected. This is any valid
C expression that refers to the parameter values, such as "fd == 1 && length > 16".
The $entry, $retval, and $latency variables can be used here as well, in return
probes.
.TP
.B [label]
The label that will be displayed when printing the probed values. By default,
this is the probe specifier. 
.SH EXAMPLES
.TP
Print a histogram of allocation sizes passed to kmalloc:
#
.B argdist.py -H 'p::__kmalloc(u64 size):u64:size'
.TP
Print a count of how many times process 1005 called malloc with an allocation size of 16 bytes:
#
.B argdist.py -p 1005 -C 'p:c:malloc(size_t size):size_t:size:size==16'
.TP
Snoop on all strings returned by gets():
#
.B argdist.py -C 'r:c:gets():char*:$retval'
.TP
Print a histogram of read sizes that were longer than 1ms:
#
.B argdist.py -H 'r::__vfs_read(void *file, void *buf, size_t count):size_t:$entry(count):$latency > 1000000'
.TP
Print frequency counts of how many times writes were issued to a particular file descriptor number, in process 1005:
#
.B argdist.py -p 1005 -C 'p:c:write(int fd):int:fd'
.TP
Print a histogram of error codes returned by read() in process 1005:
#
.B argdist.py -p 1005 -H 'r:c:read()'
.TP
Print a histogram of buffer sizes passed to write() across all processes, where the file descriptor was 1 (STDOUT):
#
.B argdist.py -H 'p:c:write(int fd, const void *buf, size_t count):size_t:count:fd==1'
.TP
Count fork() calls in libc across all processes, grouped by pid:
#
.B argdist.py -C 'p:c:fork():int:$PID;fork per process'
.TP
Print histograms of sleep() and nanosleep() parameter values:
#
.B argdist.py -H 'p:c:sleep(u32 seconds):u32:seconds' 'p:c:nanosleep(struct timespec { time_t tv_sec; long tv_nsec; } *req):long:req->tv_nsec'
.TP
Spy on writes to STDOUT performed by process 2780, up to a string size of 120 characters:
#
.B argdist.py -p 2780 -z 120 -C 'p:c:write(int fd, char* buf, size_t len):char*:buf:fd==1'
.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