Computer Science
STRACE(1) STRACE(1)
NAME
strace - trace system calls and signals
SYNOPSIS
strace [ -dffhiqrtttTvxx ] [ -acolumn ] [ -eexpr ] ... [
-ofile ] [ -ppid ] ... [ -sstrsize ] [ -uusername ] [
command [ arg ... ] ]
strace -c [ -eexpr ] ... [ -Ooverhead ] [ -Ssortby ] [
command [ arg ... ] ]
DESCRIPTION
In the simplest case strace runs the specified command
until it exits. It intercepts and records the system
calls which are called by a process and the signals which
are received by a process. The name of each system call,
its arguments and its return value are printed on standard
error or to the file specified with the -o option.
strace is a useful diagnositic, instructional, and debug-
ging tool. System adminstrators, diagnosticians and trou-
ble-shooters will find it invaluable for solving problems
with programs for which the source is not readily avail-
able since they do not need to be recompiled in order to
trace them. Students, hackers and the overly-curious will
find that a great deal can be learned about a system and
its system calls by tracing even ordinary programs. And
programmers will find that since system calls and signals
are events that happen at the user/kernel interface, a
close examination of this boundary is very useful for bug
isolation, sanity checking and attempting to capture race
conditions.
Each line in the trace contains the system call name, fol-
lowed by its arguments in parentheses and its return
value. An example from stracing the command ``cat
/dev/null'' is:
open("/dev/null", O_RDONLY) = 3
Errors (typically a return value of -1) have the errno
symbol and error string appended.
open("/foo/bar", O_RDONLY) = -1 ENOENT (No such file or directory)
Signals are printed as a signal symbol and a signal
string. An excerpt from stracing and interrupting the
command ``sleep 666'' is:
sigsuspend([] <unfinished ...>
--- SIGINT (Interrupt) ---
+++ killed by SIGINT +++
Arguments are printed in symbolic form with a passion.
This example shows the shell peforming ``>>xyzzy'' output
redirection:
open("xyzzy", O_WRONLY|O_APPEND|O_CREAT, 0666) = 3
Here the three argument form of open is decoded by break-
ing down the flag argument into its three bitwise-OR con-
stituents and printing the mode value in octal by tradi-
tion. Where traditional or native usage differs from ANSI
or POSIX, the latter forms are preferred. In some cases,
strace output has proven to be more readable than the
source.
Structure pointers are dereferenced and the members are
displayed as appropriate. In all cases arguments are for-
matted in the most C-like fashion possible. For example,
the essence of the command ``ls -l /dev/null'' is captured
as:
lstat("/dev/null", {st_mode=S_IFCHR|0666, st_rdev=makedev(1, 3), ...}) = 0
Notice how the `struct stat' argument is dereferenced and
how each member is displayed symbolically. In particular,
observe how the st_mode member is carefully decoded into a
bitwise-OR of symbolic and numeric values. Also notice in
this example that the first argument to lstat is an input
to the system call and the second argument is an output.
Since output arguments not modified if the system call
fails, arguments may not always be dereferenced. For
example, retrying the ``ls -l'' example with a non-exis-
tent file produces the following line:
lstat("/foo/bar", 0xb004) = -1 ENOENT (No such file or directory)
In this case the porch light is on but nobody is home.
Character pointers are dereferenced and printed as C
strings. Non-printing characters in strings are normally
represented by ordinary C escape codes. Only the first
strsize (32 by default) bytes of strings are printed;
longer strings have an ellipsis appended following the
closing quote. Here is a line from ``ls -l'' where the
getpwuid library routine is reading the password file:
read(3, "root::0:0:System Administrator:/"..., 1024) = 422
While structures are annotated using curly braces, simple
pointers and arrays are printed using square brackets with
commas separating elements. Here is an example from the
command ``id'' on a system with supplementary group ids:
getgroups(32, [100, 0]) = 2
On the other hand, bit-sets are also shown using square
brackets but set elements are separated only by a space.
Here is the shell preparing to execute an external com-
mand:
sigprocmask(SIG_BLOCK, [CHLD TTOU], []) = 0
Here the second argument is a bit-set of two signals,
SIGCHLD and SIGTTOU. In some cases the bit-set is so full
that printing out the unset elements is more valuable. In
that case, the bit-set is prefixed by a tilde like this:
sigprocmask(SIG_UNBLOCK, ~[], NULL) = 0
Here the second argument represents the full set of all
signals.
OPTIONS
-c Count time, calls, and errors for each system
call and report a summary on program exit.
-d Show some debugging output of strace itself on
stderr .
-f Trace child processes as they are created by
currently traced processes as a result of the
fork(2) system call. The new process is
attached to as soon as its pid is known
(through the return value of fork(2) in the
parent process). This means that such children
may run uncontrolled for a while (especially
in the case of a vfork(2)), until the parent
is scheduled again to complete its (v)fork(2)
call. If the parent process decides to
wait(2) for a child that is currently being
traced, it is suspended until an appropriate
child process either terminates or incurs a
signal that would cause it to terminate (as
determined from the child's current signal
disposition).
-ff If the -o filename option is in effect, each
processes trace is written to filename.pid
where pid is the numeric process id of each
process.
-F On SunOS 4.x, this option has the effect of
attempting to follow vforks by performing some
dynamic linking trickery. Otherwise, vforks
will not be followed even if -f has been
given.
-h Print the help summary.
-i Print the instruction pointer at the time of
the system call.
-q Suppress messages about attaching, detaching
etc. This happens automatically when output
is redirected to a file and the command is run
directly instead of attaching.
-r Print a relative timestamp upon entry to each
system call. This records the time difference
between the beginning of successive system
calls.
-t Prefix each line of the trace with the time of
day.
-tt If given twice, the time printed will include
the microseconds.
-ttt If given thrice, the time printed will include
the microseconds and the leading portion will
be printed as the number of seconds since the
epoch.
-T Show the time spent in system calls. This
records the time difference between the begin-
ning and the end of each system call.
-v Print unabbreviated versions of environment,
stat, termios, etc. calls. These structures
are very common in calls and so the default
behavior displays a reasonable subset of
structure members. Use this option to get all
of the gory details.
-V Print the version number of strace.
-x Print all non-ascii strings in hexadecimal
string format.
-xx Print all strings in hexadecimal string for-
mat.
-a column Align return values in a secific column
(default column 40).
-e expr A qualifying expression which modifies which
events to trace or how to trace them. The
format of the expression is:
[qualifier=][!]value1[,value2]...
where qualifier is one of trace, abbrev, ver-
bose, raw, signal, read, or write and value is
a qualifier-dependent symbol or number. The
default qualifier is trace. Using an exclama-
tion mark negates the set of values. For
example -eopen means literally -e trace=open
which in turn means trace only the open system
call. By contrast, -etrace=!open means to
trace every system call except open. In addi-
tion the special values all and none have the
obvious meanings.
Note that some shells use the exclamation point for his-
tory expansion; even inside quoted arguments. If so, you
must escape the exclamation point with a backslash.
-e trace=set
Trace only the specified set of system calls. The
-c option is useful for determining which system
calls might be useful to trace. For example,
trace=open,close,read,write means to only trace
those four system calls. Be careful when making
inferences about the user/kernel boundary if only a
subset of system calls are being monitored. The
default is trace=all.
-e trace=file
Trace all system calls which take a file name as an
argument. You can think of this as an abbreviation
for -e trace=open,stat,chmod,unlink,... which is
useful to seeing what files the process is refer-
encing. Furthermore, using the abbreviation will
ensure that you don't accidentally forget to
include a call like lstat in the list. Betchya
woulda forgot that one.
-e trace=process
Trace all system calls which involve process man-
agement. This is useful for watching the fork,
wait, and exec steps of a process.
-e trace=network
Trace all the network related system calls.
-e trace=signal
Trace all signal related system calls.
-e trace=ipc
Trace all IPC related system calls.
-e abbrev=set
Abbreviate the output from printing each member of
large structures. The default is abbrev=all. The
-v option has the effect of abbrev=none.
-e verbose=set
Dereference structures for the specified set of
system calls. The default is verbose=all.
-e raw=set
Print raw, undecoded arguments for the specifed set
of system calls. This option has the effect of
causing all arguments to be printed in hexadecimal.
This is mostly useful if you don't trust the decod-
ing or you need to know the actual numeric value of
an argument.
-e signal=set
Trace only the specified subset of signals. The
default is signal=all. For example signal=!SIGIO
(or signal=!io) causes SIGIO signals not to be
traced.
-e read=set
Perform a full hexadecimal and ascii dump of all
the data read from file descriptors listed in the
specified set. For example, to see all input
activity on file descriptors 3 and 5 use -e
read=3,5. Note that this is independent from the
normal tracing of the read system call which is
controlled by the option -e trace=read.
-e write=set
Perform a full hexadecimal and ascii dump of all
the data written to file descriptors listed in the
specified set. For example, to see all output
activity on file descriptors 3 and 5 use -e
write=3,5. Note that this is independent from the
normal tracing of the write system call which is
controlled by the option -e trace=write.
-o filename
Write the trace output to the file filename rather
than to stderr. Use filename.pid if -ff is used.
If the argument begins with `|' or with `!' then
the rest of the argument is treated as a command
and all output is piped to it. This is convenient
for piping the debugging output to a program with-
out affecting the redirections of executed pro-
grams.
-O overhead
Set the overhead for tracing system calls to over-
head microseconds. This is useful for overriding
the default heuristic for guessing how much time is
spent in mere measuring when timing system calls
using the -c option. The acuracy of the heuristic
can be gauged by timing a given program run without
tracing (using time(1)) and comparing the accumu-
lated system call time to the total produced using
-c .
-p pid Attach to the process with the process ID pid and
begin tracing. The trace may be terminated at any
time by a keyboard interrupt signal (CTRL-C).
strace will respond by detaching itself from the
traced process(es) leaving it (them) to continue
running. Multiple -p options can be used to attach
to up to 32 processes in addition to command (which
is optional if at least one -p option is given).
-s strsize
Specify the maximum string size to print (the
default is 32). Note that filenames are not con-
sidered strings and are always printed in full.
-S sortby
Sort the output of the histogram printed by the -c
option by the specified critereon. Legal values
are time, calls, name, and nothing (default time).
-u username
Run command with the userid, groupid and supplemen-
tary groups of username. This option is only use-
ful when running as root and enables the correct
execution of setuid and/or setgid binaries. Unless
this option is used setuid and setgid programs are
executed without effective privileges.
SETUID INSTALLATION
If strace is installed setuid to root then the invoking
user will be able to attach to and trace processes owned
by any user. In addition setuid and setgid programs will
be executed and traced with the correct effective privi-
leges. Since only users trusted with full root privileges
should be allowed to do these things, it only makes sense
to install strace as setuid to root when the users who can
execute it are restricted to those users who have this
trust. For example, it makes sense to install a special
version of strace with mode `rwsr-xr--', user root and
group trace, where members of the trace group are trusted
users. If you do use this feature, please remember to
install a non-setuid version of strace for ordinary lusers
to use.
SEE ALSO
ptrace(2), proc(4), time(1), trace(1), truss(1)
NOTES
It is a pity that so much tracing clutter is produced by
systems employing shared libraries.
It is instructive to think about system call inputs and
outputs as data-flow across the user/kernel boundary.
Because user-space and kernel-space are separate and
address-protected, it is sometimes possible to make deduc-
tive inferences about process behavior using inputs and
outputs as propositions.
In some cases, a system call will differ from the docu-
mented behavior or have a different name. For example, on
System V derived systems the true time(2) system call does
not take an argument and the stat function is called xstat
and takes an extra leading argument. These discrepancies
are normal but idiosyncratic characteristics of the system
call interface and are accounted for by C library wrapper
functions.
On some platforms a process that has a system call trace
applied to it with the -p option will receive a SIGSTOP.
This signal may interrupt a system call that is not
restartable. This may have an unpredictable effect on the
process if the process takes no action to restart the sys-
tem call.
BUGS
Programs that use the setuid bit do not have effective
user ID privileges while being traced.
A traced process ignores SIGSTOP except of SVR4 platforms.
A traced process which tries to block SIGTRAP will be sent
a SIGSTOP in an attempt to force continuation of tracing.
A traced process runs slowly.
Traced processes which are descended from command may be
left running after an interrupt signal (CTRL-C).
On Linux, exciting as it would be, tracing the init pro-
cess is forbidden.
The -i option is weakly supported.
HISTORY
strace The original strace was written by Paul Kranenburg
for SunOS and was inspired by its trace utility. The
SunOS version of strace was ported to Linux and enhanced
by Branko Lankester, who also wrote the Linux kernel sup-
port. Even though Paul released strace 2.5 in 1992,
Branko's work was based on Paul's strace 1.5 release from
1991. In 1993, Rick Sladkey merged strace 2.5 for SunOS
and the second release of strace for Linux, added many of
the features of truss from SVR4, and produced an strace
that worked on both platforms. In 1994 Rick ported strace
to SVR4 and Solaris and wrote the automatic configuration
support. In 1995 he ported strace to Irix and tired of
writing about himself in the third person.
PROBLEMS
Problems with strace should be reported to the current
strace maintainer, Rick Sladkey, at <jrs@world.std.com>.
96/02/13 1
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