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STRACE(1)               FreeBSD General Commands Manual              STRACE(1)

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)

NAME | SYNOPSIS | DESCRIPTION | OPTIONS | DIAGNOSTICS | SETUID INSTALLATION | SEE ALSO | NOTES | BUGS | HISTORY | BUGS | PROBLEMS

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