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    Command:

    strace

    
    
    

    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 pro-
           cess 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 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 unfin-
           ished.  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])
    
           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 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
           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 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
           -C          Like  -c  but  also  print regular output while pro-
                       cesses are running.
    
           -d          Show some debugging output of strace itself  on  the
                       standard error.
    
           -f          Trace  child  processes  as they are created by cur-
                       rently 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  pro-
                       cesses 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.
    
                       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  quali-
                       fier-dependent symbol or number.  The default quali-
                       fier is trace.  Using an  exclamation  mark  negates
                       the  set of values.  For example, -e open means lit-
                       erally -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 referenc-
                       ing.   Furthermore,  using  the  abbreviation   will
                       ensure that you don't accidentally forget to include
    
           -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 sys-
                       tem 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 decod-
                       ing 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 speci-
                       fied 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 spec-
                       ified 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  pip-
                       ing  the  debugging  output  to  a  program  without
                       affecting the redirections of executed programs.
                       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 consid-
                       ered 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 supple-
                       mentary 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)
           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 implementa-
           tion 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).
    
    
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