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STRACE(1)                                                            STRACE(1)

       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]]...  [-uuser-
       name] 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

       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 com-
       mand ''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 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 par-
       ticular, 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 fol-
       lowing 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 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.

       -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 struc-
                   ture 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 there-
                   fore 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 for-
                   mat 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 obvi-
                   ous 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

       -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) sys-
                   tem 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 termi-
                   nated 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

       -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  set-
                   gid  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 com-

       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 transpar-
       ent to the invoking parent process.

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

       If strace is installed setuid to root then the invoking user will be able to attach to  and  trace  pro-
       cesses  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 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  pos-
       sible 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

       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  unpre-
       dictable 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 runs slowly.

       Traced  processes  which are descended from command may be left running after an interrupt signal (CTRL-

       The -i option is weakly supported.

       strace The original strace was written by Paul Kranenburg for SunOS and was inspired by its trace  util-
       ity.   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

       Problems  with  strace  should  be  reported  to  the strace mailing list at <strace-develATlists.source->.

                                  2010-03-30                         STRACE(1)