Provided by: strace_4.15-2ubuntu3_amd64 bug

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

   Output format
       -a column   Align return values in a specific column (default column 40).

       -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.

       -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.

       -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.

       -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.

       -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.

       -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 protocol specific information associated with socket file descriptors.

   Statistics
       -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 , only aggregate totals for all traced processes are kept.

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

       -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.

       -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).

       -w          Summarise  the  time  difference  between the beginning and end of each system
                   call.  The default is to summarise the system time.

   Filtering
       -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, write, or
                   fault 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.

       -e fault=set[:error=errno][:when=expr]
                   Perform  a  syscall fault injection for the specified set of syscalls.  When a
                   fault is injected into a syscall invocation, the syscall number is replaced by
                   -1 which corresponds to an invalid syscall.

                   If  an  error  code is specified using a symbolic errno value like ENOSYS or a
                   numeric value within 1..4095 range, this  error  code  overrides  the  default
                   error  code  returned by the kernel, which is traditionally ENOSYS for invalid
                   syscall numbers on most architectures.

                   Unless a :when=expr subexpression is specified, a fault is injected into every
                   invocation of each syscall from the set.

                   The format of the subexpression is one of the following:

                     first
                       For  every syscall from the set, perform a syscall fault injection for the
                       syscall invocation number first only.

                     first+
                       For every syscall from the set, perform syscall fault injections  for  the
                       syscall invocation number first and all subsequent invocations.

                     first+step
                       For  every  syscall  from  the  set,  perform syscall fault injections for
                       syscall invocations number first, first+step, first+step+step, and so on.

                   For example, to fail each third and subsequent chdir syscalls with ENOENT, use
                   -e fault=chdir:error=ENOENT:when=3+.

                   The valid range for numbers first and step is 1..65535.

                   If  a  fault  expression contains multiple error= specifications, the last one
                   takes precedence.  Likewise, if a fault  expression  contains  multiple  when=
                   specifications, the last one takes precedence.

                   Accounting of syscalls that are subject to fault injection is done per syscall
                   and per tracee.

                   Specification of syscall fault injection can be combined  with  other  syscall
                   filtering options, for example, -P /dev/urandom -e fault=all:error=ENOENT.

       -P path     Trace  only  system  calls accessing path.  Multiple -P options can be used to
                   specify several paths.

       -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.

   Tracing
       -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.

       -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.

       -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.

       -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).

   Startup
       -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.

       -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 in addition to command (which is optional if at least one -p  option
                   is given).  -p "`pidof PROG`" syntax is supported.

       -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.

   Miscellaneous
       -d          Show some debugging output of strace itself on the standard error.

       -F          This option is now obsolete and it has the same functionality as -f.

       -h          Print the help summary.

       -V          Print the version number of strace.

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 without a command, the exit status of strace is zero unless no processes has
       been attached or 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.

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.

       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   general   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  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.

REPORTING BUGS

       Problems  with   strace   should   be   reported   to   the   strace   mailing   list   at
       <strace-devel@lists.sourceforge.net>.

SEE ALSO

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

                                            2010-03-30                                  STRACE(1)