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Linux 2.6 - man page for strace (linux section 1)

STRACE(1)										STRACE(1)

       strace - trace system calls and signals

       strace  [  -CdffhiqrtttTvxx  ]  [  -acolumn  ] [ -eexpr ] ...  [ -ofile ] [ -ppid ] ...	[
       -sstrsize ] [ -uusername ] [ -Evar=val ] ...  [ -Evar ] ...  [ command [ arg ...  ] ]

       strace -c [ -eexpr ] ...  [ -Ooverhead ] [ -Ssortby ] [ command [ arg ...  ] ]

       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 pro-
       grams  for  which  the source is not readily available since they do not need to be recom-
       piled 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  paren-
       theses 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 +++

       If a system call is being executed and meanwhile another one is being called from  a  dif-
       ferent  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 (Alarm clock) @ 0(0) ---
       rt_sigreturn(0xe)		       = 0
       read(0, ""..., 1)		       = 0

       Arguments are printed in symbolic form with a passion.  This example shows the shell  per-
       forming ``>>xyzzy'' output redirection:

       open("xyzzy", O_WRONLY|O_APPEND|O_CREAT, 0666) = 3

       Here  the  three  argument form 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 pre-
       ferred.	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 follow-
       ing  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 com-
       mand ``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 sep-
       arated 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.

       -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  pro-
		   cesses are kept.

       -C	   Like -c but also print regular output while processes are running.

       -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) system call.

		   On non-Linux platforms 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.  On Linux the child is
		   traced from its first instruction with no delay.  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).

		   On  SunOS  4.x  the tracing of vforks is accomplished with some dynamic
		   linking trickery.

       -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.

       -q	   Suppress messages about attaching, detaching etc.  This  happens  auto-
		   matically  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

       -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

       -T	   Show the time spent in system calls. This records the  time	difference
		   between the beginning 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 format.

       -a column   Align return values in a specific 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:


		   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 val-
		   ues.  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 exam-
		   ple,  trace=open,close,read,write means to only trace those four system
		   calls.  Be careful when making inferences about the user/kernel  bound-
		   ary	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 use-
		   ful 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 abbrev=set
		   Abbreviate the output from printing each member  of	large  structures.
		   The	 default   is  abbrev=all.   The  -v  option  has  the	effect	of

       -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.

       -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  out-
		   put 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 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  considered strings and are always printed in

       -S sortby   Sort the output of the histogram printed by the -c option by the speci-
		   fied  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 effec-
		   tive 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.

       When  command  exits, strace exits with the same exit status.  If command is termi-
       nated 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.

       When  using  -p,  the  exit status of strace is zero unless there was an unexpected
       error in doing the tracing.

       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 fea-
       ture, please remember to install a non-setuid version of strace for ordinary lusers
       to use.

       ltrace(1), time(1), ptrace(2), proc(5)

       It  is  a pity that so much tracing clutter is produced by systems employing shared

       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  charac-
       teristics  of  the system call interface and are accounted for by C library wrapper

       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 system call.

       Programs  that  use  the  setuid bit do not have effective user ID privileges while
       being traced.

       A traced process ignores SIGSTOP except on 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 process is forbidden.

       The -i option is weakly supported.

       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 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  sec-
       ond  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.

       The SIGTRAP signal is used internally by the kernel implementation of  system  call
       tracing.  When a traced process receives a SIGTRAP signal not associated with trac-
       ing, strace will not report that signal correctly.  This  signal  is  not  normally
       used by programs, but could be via a hard-coded break instruction or via kill(2).

       Problems  with  strace should be reported via the Debian Bug Tracking System, or to
       the strace mailing list at <strace-devel@lists.sourceforge.net>.

					    2010-03-30					STRACE(1)

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