Provided by: strace_4.5.20-2.3ubuntu1_amd64 bug

NAME

       strace - trace system calls and signals

SYNOPSIS

       strace  [  -CdffhiqrtttTvxx  ]  [  -acolumn  ] [ -eexpr ] ...  [ -ofile ] [ -ppid ] ...  [
       -sstrsize ] [ -uusername ] [ -Evar=val ] ...  [ -Evar ] ...  [ 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 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 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
       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
       performing ``>>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
       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          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  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 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:

                             [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 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.

       -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 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 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.

       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 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.

BUGS

       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.

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 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.

BUGS

       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 tracing, 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

       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)