| .\" Copyright (c) 1991, 1992 Paul Kranenburg <pk@cs.few.eur.nl> |
| .\" Copyright (c) 1993 Branko Lankester <branko@hacktic.nl> |
| .\" Copyright (c) 1993, 1994, 1995, 1996 Rick Sladkey <jrs@world.std.com> |
| .\" All rights reserved. |
| .\" |
| .\" Redistribution and use in source and binary forms, with or without |
| .\" modification, are permitted provided that the following conditions |
| .\" are met: |
| .\" 1. Redistributions of source code must retain the above copyright |
| .\" notice, this list of conditions and the following disclaimer. |
| .\" 2. Redistributions in binary form must reproduce the above copyright |
| .\" notice, this list of conditions and the following disclaimer in the |
| .\" documentation and/or other materials provided with the distribution. |
| .\" 3. The name of the author may not be used to endorse or promote products |
| .\" derived from this software without specific prior written permission. |
| .\" |
| .\" THIS SOFTWARE IS PROVIDED BY THE AUTHOR ``AS IS'' AND ANY EXPRESS OR |
| .\" IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES |
| .\" OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE DISCLAIMED. |
| .\" IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR ANY DIRECT, INDIRECT, |
| .\" INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT |
| .\" NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, |
| .\" DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY |
| .\" THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT |
| .\" (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF |
| .\" THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE. |
| .\" |
| .\" $Id$ |
| .\" |
| .de CW |
| .sp |
| .nf |
| .ft CW |
| .. |
| .de CE |
| .ft R |
| .fi |
| .sp |
| .. |
| .TH STRACE 1 "2010-03-30" |
| .SH NAME |
| strace \- trace system calls and signals |
| .SH SYNOPSIS |
| .B strace |
| [ |
| .B \-CdffhiqrtttTvxx |
| ] |
| [ |
| .BI \-a column |
| ] |
| [ |
| .BI \-e expr |
| ] |
| \&... |
| [ |
| .BI \-o file |
| ] |
| [ |
| .BI \-p pid |
| ] |
| \&... |
| [ |
| .BI \-s strsize |
| ] |
| [ |
| .BI \-u username |
| ] |
| [ |
| .BI \-E var=val |
| ] |
| \&... |
| [ |
| .BI \-E var |
| ] |
| \&... |
| [ |
| .I command |
| [ |
| .I arg |
| \&... |
| ] |
| ] |
| .sp |
| .B strace |
| .B \-c |
| [ |
| .BI \-e expr |
| ] |
| \&... |
| [ |
| .BI \-O overhead |
| ] |
| [ |
| .BI \-S sortby |
| ] |
| [ |
| .I command |
| [ |
| .I arg |
| \&... |
| ] |
| ] |
| .SH DESCRIPTION |
| .IX "strace command" "" "\fLstrace\fR command" |
| .LP |
| In the simplest case |
| .B strace |
| runs the specified |
| .I 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 |
| .B \-o |
| option. |
| .LP |
| .B 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. |
| .LP |
| 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: |
| .CW |
| open("/dev/null", O_RDONLY) = 3 |
| .CE |
| Errors (typically a return value of \-1) have the errno symbol |
| and error string appended. |
| .CW |
| open("/foo/bar", O_RDONLY) = -1 ENOENT (No such file or directory) |
| .CE |
| Signals are printed as a signal symbol and a signal string. |
| An excerpt from stracing and interrupting the command ``sleep 666'' is: |
| .CW |
| sigsuspend([] <unfinished ...> |
| --- SIGINT (Interrupt) --- |
| +++ killed by SIGINT +++ |
| .CE |
| If a system call is being executed and meanwhile another one is being called |
| from a different thread/process then |
| .B strace |
| will try to preserve the order of those events and mark the ongoing call as |
| being |
| .IR unfinished . |
| When the call returns it will be marked as |
| .IR resumed . |
| .CW |
| [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]) |
| .CE |
| 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. |
| .CW |
| read(0, 0x7ffff72cf5cf, 1) = ? ERESTARTSYS (To be restarted) |
| --- SIGALRM (Alarm clock) @ 0 (0) --- |
| rt_sigreturn(0xe) = 0 |
| read(0, ""..., 1) = 0 |
| .CE |
| Arguments are printed in symbolic form with a passion. |
| This example shows the shell performing ``>>xyzzy'' output redirection: |
| .CW |
| open("xyzzy", O_WRONLY|O_APPEND|O_CREAT, 0666) = 3 |
| .CE |
| 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, |
| .B strace |
| output has proven to be more readable than the source. |
| .LP |
| 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: |
| .CW |
| lstat("/dev/null", {st_mode=S_IFCHR|0666, st_rdev=makedev(1, 3), ...}) = 0 |
| .CE |
| 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: |
| .CW |
| lstat("/foo/bar", 0xb004) = -1 ENOENT (No such file or directory) |
| .CE |
| In this case the porch light is on but nobody is home. |
| .LP |
| 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 |
| .I 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 |
| .B getpwuid |
| library routine is reading the password file: |
| .CW |
| read(3, "root::0:0:System Administrator:/"..., 1024) = 422 |
| .CE |
| 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: |
| .CW |
| getgroups(32, [100, 0]) = 2 |
| .CE |
| 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: |
| .CW |
| sigprocmask(SIG_BLOCK, [CHLD TTOU], []) = 0 |
| .CE |
| 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: |
| .CW |
| sigprocmask(SIG_UNBLOCK, ~[], NULL) = 0 |
| .CE |
| Here the second argument represents the full set of all signals. |
| .SH OPTIONS |
| .TP 12 |
| .TP |
| .B \-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 |
| .B \-c |
| is used with |
| .B \-f |
| or |
| .B \-F |
| (below), only aggregate totals for all traced processes are kept. |
| .TP |
| .B \-C |
| Like |
| .B \-c |
| but also print regular output while processes are running. |
| .TP |
| .B \-d |
| Show some debugging output of |
| .B strace |
| itself on the standard error. |
| .TP |
| .B \-f |
| Trace child processes as they are created by currently traced |
| processes as a result of the |
| .BR fork (2) |
| system call. |
| .IP |
| On non-Linux platforms the new process is |
| attached to as soon as its pid is known (through the return value of |
| .BR fork (2) |
| in the parent process). This means that such children may run |
| uncontrolled for a while (especially in the case of a |
| .BR vfork (2)), |
| until the parent is scheduled again to complete its |
| .RB ( v ) fork (2) |
| call. On Linux the child is traced from its first instruction with no delay. |
| If the parent process decides to |
| .BR 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). |
| .IP |
| On SunOS 4.x the tracing of |
| .BR vfork s |
| is accomplished with some dynamic linking trickery. |
| .TP |
| .B \-ff |
| If the |
| .B \-o |
| .I filename |
| option is in effect, each processes trace is written to |
| .I filename.pid |
| where pid is the numeric process id of each process. |
| This is incompatible with |
| .BR \-c , |
| since no per-process counts are kept. |
| .TP |
| .B \-F |
| This option is now obsolete and it has the same functionality as |
| .BR \-f . |
| .TP |
| .B \-h |
| Print the help summary. |
| .TP |
| .B \-i |
| Print the instruction pointer at the time of the system call. |
| .TP |
| .B \-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. |
| .TP |
| .B \-r |
| Print a relative timestamp upon entry to each system call. This |
| records the time difference between the beginning of successive |
| system calls. |
| .TP |
| .B \-t |
| Prefix each line of the trace with the time of day. |
| .TP |
| .B \-tt |
| If given twice, the time printed will include the microseconds. |
| .TP |
| .B \-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. |
| .TP |
| .B \-T |
| Show the time spent in system calls. This records the time |
| difference between the beginning and the end of each system call. |
| .TP |
| .B \-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. |
| .TP |
| .B \-V |
| Print the version number of |
| .BR strace . |
| .TP |
| .B \-x |
| Print all non-ASCII strings in hexadecimal string format. |
| .TP |
| .B \-xx |
| Print all strings in hexadecimal string format. |
| .TP |
| .BI "\-a " column |
| Align return values in a specific column (default column 40). |
| .TP |
| .BI "\-e " expr |
| A qualifying expression which modifies which events to trace |
| or how to trace them. The format of the expression is: |
| .RS 15 |
| .IP |
| [\fIqualifier\fB=\fR][\fB!\fR]\fIvalue1\fR[\fB,\fIvalue2\fR]... |
| .RE |
| .IP |
| where |
| .I qualifier |
| is one of |
| .BR trace , |
| .BR abbrev , |
| .BR verbose , |
| .BR raw , |
| .BR signal , |
| .BR read , |
| or |
| .B write |
| and |
| .I value |
| is a qualifier-dependent symbol or number. The default |
| qualifier is |
| .BR trace . |
| Using an exclamation mark negates the set of values. For example, |
| .BR \-e "\ " open |
| means literally |
| .BR \-e "\ " trace = open |
| which in turn means trace only the |
| .B open |
| system call. By contrast, |
| .BR \-e "\ " trace "=!" open |
| means to trace every system call except |
| .BR open . |
| In addition, the special values |
| .B all |
| and |
| .B none |
| have the obvious meanings. |
| .IP |
| 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. |
| .TP |
| \fB\-e\ trace\fR=\fIset\fR |
| Trace only the specified set of system calls. The |
| .B \-c |
| option is useful for determining which system calls might be useful |
| to trace. For example, |
| .BR 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 |
| .BR trace = all . |
| .TP |
| \fB\-e\ trace\fR=\fIfile\fR |
| Trace all system calls which take a file name as an argument. You |
| can think of this as an abbreviation for |
| .BR "\-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 |
| .B lstat |
| in the list. Betchya woulda forgot that one. |
| .TP |
| \fB\-e\ trace\fR=\fIprocess\fR |
| Trace all system calls which involve process management. This |
| is useful for watching the fork, wait, and exec steps of a process. |
| .TP |
| \fB\-e\ trace\fR=\fInetwork\fR |
| Trace all the network related system calls. |
| .TP |
| \fB\-e\ trace\fR=\fIsignal\fR |
| Trace all signal related system calls. |
| .TP |
| \fB\-e\ trace\fR=\fIipc\fR |
| Trace all IPC related system calls. |
| .TP |
| \fB\-e\ trace\fR=\fIdesc\fR |
| Trace all file descriptor related system calls. |
| .TP |
| \fB\-e\ abbrev\fR=\fIset\fR |
| Abbreviate the output from printing each member of large structures. |
| The default is |
| .BR abbrev = all . |
| The |
| .B \-v |
| option has the effect of |
| .BR abbrev = none . |
| .TP |
| \fB\-e\ verbose\fR=\fIset\fR |
| Dereference structures for the specified set of system calls. The |
| default is |
| .BR verbose = all . |
| .TP |
| \fB\-e\ raw\fR=\fIset\fR |
| 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. |
| .TP |
| \fB\-e\ signal\fR=\fIset\fR |
| Trace only the specified subset of signals. The default is |
| .BR signal = all . |
| For example, |
| .B signal "=!" SIGIO |
| (or |
| .BR signal "=!" io ) |
| causes SIGIO signals not to be traced. |
| .TP |
| \fB\-e\ read\fR=\fIset\fR |
| 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 |
| .I 3 |
| and |
| .I 5 |
| use |
| \fB\-e\ read\fR=\fI3\fR,\fI5\fR. |
| Note that this is independent from the normal tracing of the |
| .BR read (2) |
| system call which is controlled by the option |
| .BR -e "\ " trace = read . |
| .TP |
| \fB\-e\ write\fR=\fIset\fR |
| 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 |
| .I 3 |
| and |
| .I 5 |
| use |
| \fB\-e\ write\fR=\fI3\fR,\fI5\fR. |
| Note that this is independent from the normal tracing of the |
| .BR write (2) |
| system call which is controlled by the option |
| .BR -e "\ " trace = write . |
| .TP |
| .BI "\-o " filename |
| Write the trace output to the file |
| .I filename |
| rather than to stderr. |
| Use |
| .I filename.pid |
| if |
| .B \-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. |
| .TP |
| .BI "\-O " overhead |
| Set the overhead for tracing system calls to |
| .I 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 |
| .B \-c |
| option. The accuracy of the heuristic can be gauged by timing a given |
| program run without tracing (using |
| .BR time (1)) |
| and comparing the accumulated |
| system call time to the total produced using |
| .BR \-c . |
| .TP |
| .BI "\-p " pid |
| Attach to the process with the process |
| .SM ID |
| .I pid |
| and begin tracing. |
| The trace may be terminated |
| at any time by a keyboard interrupt signal (\c |
| .SM CTRL\s0-C). |
| .B strace |
| will respond by detaching itself from the traced process(es) |
| leaving it (them) to continue running. |
| Multiple |
| .B \-p |
| options can be used to attach to up to 32 processes in addition to |
| .I command |
| (which is optional if at least one |
| .B \-p |
| option is given). |
| .TP |
| .BI "\-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. |
| .TP |
| .BI "\-S " sortby |
| Sort the output of the histogram printed by the |
| .B \-c |
| option by the specified criterion. Legal values are |
| .BR time , |
| .BR calls , |
| .BR name , |
| and |
| .B nothing |
| (default is |
| .BR time ). |
| .TP |
| .BI "\-u " username |
| Run command with the user \s-1ID\s0, group \s-2ID\s0, and |
| supplementary groups of |
| .IR 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. |
| .TP |
| \fB\-E\ \fIvar\fR=\fIval\fR |
| Run command with |
| .IR var = val |
| in its list of environment variables. |
| .TP |
| .BI "\-E " var |
| Remove |
| .IR var |
| from the inherited list of environment variables before passing it on to |
| the command. |
| .SH DIAGNOSTICS |
| When |
| .I command |
| exits, |
| .B strace |
| exits with the same exit status. |
| If |
| .I command |
| is terminated by a signal, |
| .B strace |
| terminates itself with the same signal, so that |
| .B strace |
| can be used as a wrapper process transparent to the invoking parent process. |
| .LP |
| When using |
| .BR \-p , |
| the exit status of |
| .B strace |
| is zero unless there was an unexpected error in doing the tracing. |
| .SH "SETUID INSTALLATION" |
| If |
| .B 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 |
| .B 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 |
| .B strace |
| with mode `rwsr-xr--', user |
| .B root |
| and group |
| .BR trace , |
| where members of the |
| .B trace |
| group are trusted users. |
| If you do use this feature, please remember to install |
| a non-setuid version of |
| .B strace |
| for ordinary lusers to use. |
| .SH "SEE ALSO" |
| .BR ltrace (1), |
| .BR time (1), |
| .BR ptrace (2), |
| .BR proc (5) |
| .SH NOTES |
| It is a pity that so much tracing clutter is produced by systems |
| employing shared libraries. |
| .LP |
| 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. |
| .LP |
| 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 |
| .BR time (2) |
| system call does not take an argument and the |
| .B stat |
| function is called |
| .B 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. |
| .LP |
| On some platforms a process that has a system call trace applied |
| to it with the |
| .B \-p |
| option will receive a |
| .BR \s-1SIGSTOP\s0 . |
| 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. |
| .SH BUGS |
| Programs that use the |
| .I setuid |
| bit do not have |
| effective user |
| .SM ID |
| privileges while being traced. |
| .LP |
| A traced process ignores |
| .SM SIGSTOP |
| except on SVR4 platforms. |
| .LP |
| A traced process which tries to block SIGTRAP will be sent a SIGSTOP |
| in an attempt to force continuation of tracing. |
| .LP |
| A traced process runs slowly. |
| .LP |
| Traced processes which are descended from |
| .I command |
| may be left running after an interrupt signal (\c |
| .SM CTRL\s0-C). |
| .LP |
| On Linux, exciting as it would be, tracing the init process is forbidden. |
| .LP |
| The |
| .B \-i |
| option is weakly supported. |
| .SH HISTORY |
| .B strace |
| The original |
| .B strace |
| was written by Paul Kranenburg |
| for SunOS and was inspired by its trace utility. |
| The SunOS version of |
| .B strace |
| was ported to Linux and enhanced |
| by Branko Lankester, who also wrote the Linux kernel support. |
| Even though Paul released |
| .B strace |
| 2.5 in 1992, |
| Branko's work was based on Paul's |
| .B strace |
| 1.5 release from 1991. |
| In 1993, Rick Sladkey merged |
| .B strace |
| 2.5 for SunOS and the second release of |
| .B strace |
| for Linux, added many of the features of |
| .BR truss (1) |
| from SVR4, and produced an |
| .B strace |
| that worked on both platforms. In 1994 Rick ported |
| .B strace |
| to SVR4 and Solaris and wrote the |
| automatic configuration support. In 1995 he ported |
| .B strace |
| to Irix |
| and tired of writing about himself in the third person. |
| .SH 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 |
| .BR kill (2). |
| .SH PROBLEMS |
| Problems with |
| .B strace |
| should be reported via the Debian Bug Tracking System, |
| or to the |
| .B strace |
| mailing list at <strace\-devel@lists.sourceforge.net>. |