strace Man page

Resume Wikipedia de Strace

strace est un outil de débogage sous Linux pour surveiller les appels système utilisés par un programme, et tous les signaux qu’il reçoit, similaire à l’outil “truss” sur les autres systèmes Unix. Il a été rendu possible grâce à une fonctionnalité du noyau Linux appelée ptrace.
Un outil similaire est fourni par Cygwin.

Resume Wikipedia de Strace

strace est un outil de débogage sous Linux pour surveiller les appels système utilisés par un programme, et tous les signaux qu’il reçoit, similaire à l’outil “truss” sur les autres systèmes Unix. Il a été rendu possible grâce à une fonctionnalité du noyau Linux appelée ptrace.
Un outil similaire est fourni par Cygwin.

STRACE(1) General Commands Manual STRACE(1)

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. Sys‐
tem 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 strac‐
ing 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([]
— 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 unfin‐
ished. When the call returns it will be marked as resumed.

[pid 28772] select(4, [3], NULL, NULL, NULL
[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 mem‐
ber 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, argu‐
ments 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 ele‐
ments. Here is an example from the command “id” on a system with sup‐
plementary 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 prepar‐
ing to execute an external command:

sigprocmask(SIG_BLOCK, [CHLD TTOU], []) = 0

Here the second argument is a bit-set of two signals, SIGCHLD and SIGT‐
TOU. 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 functional‐
ity 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). This option is
available only if strace is built with libunwind.

-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 sta‐
tus.

-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 microsec‐
onds.

-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 sys‐
tem 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.

-yy Print ip:port pairs associated with socket file descrip‐
tors.

-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, sig‐
nal, read, or write and value is a qualifier-dependent sym‐
bol 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 argu‐
ment. 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 acciden‐
tally 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 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 pro‐
grams.

-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 mea‐
suring 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) 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 key‐
board 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 run‐
ning 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 privi‐
leges.

-E var=val Run command with var=val in its list of environment vari‐
ables.

-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 sig‐
nal, 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 privi‐
leges 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, time, 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 ker‐
nel-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 system call does not take an argument and the stat func‐
tion is called xstat and takes an extra leading argument. These dis‐
crepancies are normal but idiosyncratic characteristics of the system
call interface and are accounted for by C library wrapper functions.

Some system calls have different names in different architectures and
personalities. In these cases, system call filtering and printing uses
the names that match corresponding __NR_* kernel macros of the tracee’s
architecture and personality. There are two exceptions from this gen‐
eral rule: arm_fadvise64_64(2) ARM syscall and xtensa_fadvise64_64(2)
Xtensa syscall are filtered and printed as fadvise64_64(2).

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 privi‐
leges 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 from SVR4, and pro‐
duced an strace that worked on both platforms. In 1994 Rick ported
strace to SVR4 and Solaris and wrote the automatic configuration sup‐
port. In 1995 he ported strace to Irix and tired of writing about him‐
self in the third person.

PROBLEMS
Problems with strace should be reported to the strace mailing list at
.

2010-03-30 STRACE(1)