Provided by: strace_4.8-1ubuntu5_amd64 bug


       strace - trace system calls and signals


       strace  [-CdffhiqrtttTvVxxy] [-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]


       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''

       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
       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

       [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
       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.


       -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
           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  automatically
                   when output is redirected to a file and the command is run directly instead of

       -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.

       -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:


                   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

                   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

       -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

       -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

       -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
          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.


       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.

       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  feature,  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  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 system call.


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

       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.


       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 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  with   strace   should   be   reported   to   the   strace   mailing   list   at

                                            2010-03-30                                  STRACE(1)