NAME¶
strace - trace system calls and signals
SYNOPSIS¶
strace [
-CdffhikqrtttTvVxxy] [
-In] [
-bexecve] [
-eexpr]... [
-acolumn] [
-ofile] [
-sstrsize] [
-Ppath]...
-ppid... / [
-D] [
-Evar[=
val]]...
[
-uusername]
command [
args]
strace -c[
df] [
-In] [
-bexecve] [
-eexpr]... [
-Ooverhead]
[
-Ssortby]
-ppid... / [
-D] [
-Evar[=
val]]... [
-uusername]
command [
args]
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 diagnostic, instructional, and debugging tool. System
administrators, diagnosticians and trouble-shooters will find it invaluable
for solving problems with programs for which the source is not readily
available 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, followed 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 signal symbol and decoded siginfo structure. An excerpt
from stracing and interrupting the command "sleep 666" is:
sigsuspend([] <unfinished ...>
--- SIGINT {si_signo=SIGINT, si_code=SI_USER, si_pid=...} ---
+++ killed by SIGINT +++
If a system call is being executed and meanwhile another one is being called
from a different thread/process then
strace will try to preserve the
order of those events and mark the ongoing call as being
unfinished.
When the call returns it will be marked as
resumed.
[pid 28772] select(4, [3], NULL, NULL, NULL <unfinished ...>
[pid 28779] clock_gettime(CLOCK_REALTIME, {1130322148, 939977000}) = 0
[pid 28772] <... select resumed> ) = 1 (in [3])
Interruption of a (restartable) system call by a signal delivery is processed
differently as kernel terminates the system call and also arranges its
immediate reexecution after the signal handler completes.
read(0, 0x7ffff72cf5cf, 1) = ? ERESTARTSYS (To be restarted)
--- SIGALRM ... ---
rt_sigreturn(0xe) = 0
read(0, "", 1) = 0
Arguments are printed in symbolic form with a passion. This example shows the
shell performing ">>xyzzy" output redirection:
open("xyzzy", O_WRONLY|O_APPEND|O_CREAT, 0666) = 3
Here the third argument of open is decoded by breaking down the flag argument
into its three bitwise-OR constituents and printing the mode value in octal by
tradition. 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 formatted 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 are
not modified if the system call fails, arguments may not always be
dereferenced. For example, retrying the "ls -l" example with a
non-existent 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 command:
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. On Linux, this attempts to show system time (CPU time spent
running in the kernel) independent of wall clock time. If -c is
used with -f or -F (below), only aggregate totals for all
traced processes are kept.
- -C
- Like -c but also print regular output while processes are
running.
- -D
- Run tracer process as a detached grandchild, not as parent of the tracee.
This reduces the visible effect of strace by keeping the tracee a
direct child of the calling process.
- -d
- Show some debugging output of strace itself on the standard
error.
- -f
- Trace child processes as they are created by currently traced processes as
a result of the fork(2), vfork(2) and clone(2) system
calls. Note that -p PID -f will attach all threads of
process PID if it is multi-threaded, not only thread with thread_id =
PID.
- -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. This is incompatible with -c, since no per-process
counts are kept.
- -F
- This option is now obsolete and it has the same functionality as
-f.
- -h
- Print the help summary.
- -i
- Print the instruction pointer at the time of the system call.
- -k
- Print the execution stack trace of the traced processes after each system
call (experimental).
- -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.
- -qq
- If given twice, suppress messages about process exit status.
- -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 beginning and the end of each system call.
- -w
- Summarise the time difference between the beginning and end of each system
call. The default is to summarise the system time.
- -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 format.
- -y
- Print paths associated with file descriptor arguments.
- -a column
- Align return values in a specific column (default column 40).
- -b syscall
- If specified syscall is reached, detach from traced process. Currently,
only execve syscall is supported. This option is useful if you want
to trace multi-threaded process and therefore require -f, but don't want
to trace its (potentially very complex) children.
- -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,
verbose, raw, signal, read, or write
and value is a qualifier-dependent symbol or number. The default
qualifier is trace. Using an exclamation mark negates the set of
values. For example, -e open means literally
-e trace=open which in turn means trace only
the open system call. By contrast,
-e trace=!open means to trace every system
call except open. In addition, the special values all and
none have the obvious meanings.
- Note that some shells use the exclamation point for history 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 referencing.
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 management. 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 trace=desc
- Trace all file descriptor related system calls.
- -e trace=memory
- Trace all memory mapping 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 specified 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 decoding 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(2) 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(2) system call which is
controlled by the option -e trace=write.
- -I interruptible
- When strace can be interrupted by signals (such as pressing ^C). 1: no
signals are blocked; 2: fatal signals are blocked while decoding syscall
(default); 3: fatal signals are always blocked (default if '-o FILE
PROG'); 4: fatal signals and SIGTSTP (^Z) are always blocked (useful to
make strace -o FILE PROG not stop on ^Z).
- -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 without affecting the redirections of executed
programs.
- -O overhead
- Set the overhead for tracing system calls to overhead 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 accuracy of the heuristic can be gauged by timing a
given program run without tracing (using time(1)) and comparing the
accumulated 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 many
processes. -p "`pidof PROG`" syntax is supported.
- -P path
- Trace only system calls accessing path. Multiple -P options
can be used to specify several paths.
- -s strsize
- Specify the maximum string size to print (the default is 32). Note that
filenames are not considered strings and are always printed in full.
- -S sortby
- Sort the output of the histogram printed by the -c option by the
specified criterion. Legal values are time, calls,
name, and nothing (default is time).
- -u username
- Run command with the user ID, group ID, and supplementary groups of
username. This option is only useful 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.
- -E var=val
- Run command with var=val in its list of environment
variables.
- -E var
- Remove var from the inherited list of environment variables before
passing it on to the command.
DIAGNOSTICS¶
When
command exits,
strace exits with the same exit status. If
command is terminated by a signal,
strace terminates itself with
the same signal, so that
strace can be used as a wrapper process
transparent to the invoking parent process. Note that parent-child
relationship (signal stop notifications, getppid() value, etc) between traced
process and its parent are not preserved unless
-D is used.
When using
-p, the exit status of
strace is zero unless there was
an unexpected error in doing the tracing.
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
privileges. 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¶
ltrace(1),
time(1),
ptrace(2),
proc(5)
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 deductive
inferences about process behavior using inputs and outputs as propositions.
In some cases, a system call will differ from the documented 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 is attached to with the
-p option may
observe a spurious EINTR return from the current system call that is not
restartable. (Ideally, all system calls should be restarted on strace attach,
making the attach invisible to the traced process, but a few system calls
aren't. Arguably, every instance of such behavior is a kernel bug.) This may
have an unpredictable effect on the process if the process takes no action to
restart the system call.
BUGS¶
Programs that use the
setuid bit do not have effective user
ID privileges while being traced.
A traced process runs slowly.
Traced processes which are descended from
command may be left running
after an interrupt signal (
CTRL-C).
The
-i option is weakly supported.
HISTORY¶
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
support. 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(1) 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
strace mailing list
at <strace-devel@lists.sourceforge.net>.