1STRACE(1) General Commands Manual STRACE(1)
2
6 strace - trace system calls and signals
7
9 strace [-CdffhiqrtttTvVxxy] [-In] [-bexecve] [-eexpr]... [-acolumn]
10 [-ofile] [-sstrsize] [-Ppath]... -ppid... / [-D] [-Evar[=val]]...
11 [-uusername] command [args]
12 strace -c[df] [-In] [-bexecve] [-eexpr]... [-Ooverhead] [-Ssortby]
14 -ppid... / [-D] [-Evar[=val]]... [-uusername] command [args]
15
17 In the simplest case strace runs the specified command until it exits.
18 It intercepts and records the system calls which are called by a
19 process and the signals which are received by a process. The name of
20 each system call, its arguments and its return value are printed on
21 standard error or to the file specified with the -o option.
22 strace is a useful diagnostic, instructional, and debugging tool. Sys‐
24 tem administrators, diagnosticians and trouble-shooters will find it
25 invaluable for solving problems with programs for which the source is
26 not readily available since they do not need to be recompiled in order
27 to trace them. Students, hackers and the overly-curious will find that
28 a great deal can be learned about a system and its system calls by
29 tracing even ordinary programs. And programmers will find that since
30 system calls and signals are events that happen at the user/kernel
31 interface, a close examination of this boundary is very useful for bug
32 isolation, sanity checking and attempting to capture race conditions.
33 Each line in the trace contains the system call name, followed by its
35 arguments in parentheses and its return value. An example from strac‐
36 ing the command ``cat /dev/null'' is:
37 open("/dev/null", O_RDONLY) = 3
39 Errors (typically a return value of -1) have the errno symbol and error
41 string appended.
42 open("/foo/bar", O_RDONLY) = -1 ENOENT (No such file or directory)
44 Signals are printed as a signal symbol and a signal string. An excerpt
46 from stracing and interrupting the command ``sleep 666'' is:
47 sigsuspend([] <unfinished ...>
49 --- SIGINT (Interrupt) ---
50 +++ killed by SIGINT +++
51 If a system call is being executed and meanwhile another one is being
53 called from a different thread/process then strace will try to preserve
54 the order of those events and mark the ongoing call as being unfin‐
55 ished. When the call returns it will be marked as resumed.
56 [pid 28772] select(4, [3], NULL, NULL, NULL <unfinished ...>
58 [pid 28779] clock_gettime(CLOCK_REALTIME, {1130322148, 939977000}) = 0
59 [pid 28772] <... select resumed> ) = 1 (in [3])
60 Interruption of a (restartable) system call by a signal delivery is
62 processed differently as kernel terminates the system call and also
63 arranges its immediate reexecution after the signal handler completes.
64 read(0, 0x7ffff72cf5cf, 1) = ? ERESTARTSYS (To be restarted)
66 --- SIGALRM (Alarm clock) @ 0 (0) ---
67 rt_sigreturn(0xe) = 0
68 read(0, ""..., 1) = 0
69 Arguments are printed in symbolic form with a passion. This example
71 shows the shell performing ``>>xyzzy'' output redirection:
72 open("xyzzy", O_WRONLY|O_APPEND|O_CREAT, 0666) = 3
74 Here the third argument of open is decoded by breaking down the flag
76 argument into its three bitwise-OR constituents and printing the mode
77 value in octal by tradition. Where traditional or native usage differs
78 from ANSI or POSIX, the latter forms are preferred. In some cases,
79 strace output has proven to be more readable than the source.
80 Structure pointers are dereferenced and the members are displayed as
82 appropriate. In all cases arguments are formatted in the most C-like
83 fashion possible. For example, the essence of the command ``ls -l
84 /dev/null'' is captured as:
85 lstat("/dev/null", {st_mode=S_IFCHR|0666, st_rdev=makedev(1, 3), ...}) = 0
87 Notice how the `struct stat' argument is dereferenced and how each mem‐
89 ber is displayed symbolically. In particular, observe how the st_mode
90 member is carefully decoded into a bitwise-OR of symbolic and numeric
91 values. Also notice in this example that the first argument to lstat
92 is an input to the system call and the second argument is an output.
93 Since output arguments are not modified if the system call fails, argu‐
94 ments may not always be dereferenced. For example, retrying the ``ls
95 -l'' example with a non-existent file produces the following line:
96 lstat("/foo/bar", 0xb004) = -1 ENOENT (No such file or directory)
98 In this case the porch light is on but nobody is home.
100 Character pointers are dereferenced and printed as C strings. Non-
102 printing characters in strings are normally represented by ordinary C
103 escape codes. Only the first strsize (32 by default) bytes of strings
104 are printed; longer strings have an ellipsis appended following the
105 closing quote. Here is a line from ``ls -l'' where the getpwuid
106 library routine is reading the password file:
107 read(3, "root::0:0:System Administrator:/"..., 1024) = 422
109 While structures are annotated using curly braces, simple pointers and
111 arrays are printed using square brackets with commas separating ele‐
112 ments. Here is an example from the command ``id'' on a system with
113 supplementary group ids:
114 getgroups(32, [100, 0]) = 2
116 On the other hand, bit-sets are also shown using square brackets but
118 set elements are separated only by a space. Here is the shell prepar‐
119 ing to execute an external command:
120 sigprocmask(SIG_BLOCK, [CHLD TTOU], []) = 0
122 Here the second argument is a bit-set of two signals, SIGCHLD and SIGT‐
124 TOU. In some cases the bit-set is so full that printing out the unset
125 elements is more valuable. In that case, the bit-set is prefixed by a
126 tilde like this:
127 sigprocmask(SIG_UNBLOCK, ~[], NULL) = 0
129 Here the second argument represents the full set of all signals.
131
133 -c Count time, calls, and errors for each system call
134 and report a summary on program exit. On Linux,
135 this attempts to show system time (CPU time spent
136 running in the kernel) independent of wall clock
137 time. If -c is used with -f or -F (below), only
138 aggregate totals for all traced processes are kept.
139 -C Like -c but also print regular output while pro‐
141 cesses are running.
142 -D Run tracer process as a detached grandchild, not as
144 parent of the tracee. This reduces the visible
145 effect of strace by keeping the tracee a direct
146 child of the calling process.
147 -d Show some debugging output of strace itself on the
149 standard error.
150 -f Trace child processes as they are created by cur‐
152 rently traced processes as a result of the fork(2),
153 vfork(2) and clone(2) system calls. Note that -p PID
154 -f will attach all threads of process PID if it is
155 multi-threaded, not only thread with thread_id =
156 PID.
157 -ff If the -o filename option is in effect, each pro‐
159 cesses trace is written to filename.pid where pid is
160 the numeric process id of each process. This is
161 incompatible with -c, since no per-process counts
162 are kept.
163 -F This option is now obsolete and it has the same
165 functionality as -f.
166 -h Print the help summary.
168 -i Print the instruction pointer at the time of the
170 system call.
171 -q Suppress messages about attaching, detaching etc.
173 This happens automatically when output is redirected
174 to a file and the command is run directly instead of
175 attaching.
176 -qq If given twice, suppress messages about process exit
178 status.
179 -r Print a relative timestamp upon entry to each system
181 call. This records the time difference between the
182 beginning of successive system calls.
183 -t Prefix each line of the trace with the time of day.
185 -tt If given twice, the time printed will include the
187 microseconds.
188 -ttt If given thrice, the time printed will include the
190 microseconds and the leading portion will be printed
191 as the number of seconds since the epoch.
192 -T Show the time spent in system calls. This records
194 the time difference between the beginning and the
195 end of each system call.
196 -v Print unabbreviated versions of environment, stat,
198 termios, etc. calls. These structures are very
199 common in calls and so the default behavior displays
200 a reasonable subset of structure members. Use this
201 option to get all of the gory details.
202 -V Print the version number of strace.
204 -x Print all non-ASCII strings in hexadecimal string
206 format.
207 -xx Print all strings in hexadecimal string format.
209 -y Print paths associated with file descriptor argu‐
211 ments.
212 -a column Align return values in a specific column (default
214 column 40).
215 -b syscall If specified syscall is reached, detach from traced
217 process. Currently, only execve syscall is sup‐
218 ported. This option is useful if you want to trace
219 multi-threaded process and therefore require -f, but
220 don't want to trace its (potentially very complex)
221 children.
222 -e expr A qualifying expression which modifies which events
224 to trace or how to trace them. The format of the
225 expression is:
226 [qualifier=][!]value1[,value2]...
228 where qualifier is one of trace, abbrev, verbose,
230 raw, signal, read, or write and value is a quali‐
231 fier-dependent symbol or number. The default quali‐
232 fier is trace. Using an exclamation mark negates
233 the set of values. For example, -e open means lit‐
234 erally -e trace=open which in turn means trace only
235 the open system call. By contrast, -e trace=!open
236 means to trace every system call except open. In
237 addition, the special values all and none have the
238 obvious meanings.
239 Note that some shells use the exclamation point for
241 history expansion even inside quoted arguments. If
242 so, you must escape the exclamation point with a
243 backslash.
244 -e trace=set
246 Trace only the specified set of system calls. The
247 -c option is useful for determining which system
248 calls might be useful to trace. For example,
249 trace=open,close,read,write means to only trace
250 those four system calls. Be careful when making
251 inferences about the user/kernel boundary if only a
252 subset of system calls are being monitored. The
253 default is trace=all.
254 -e trace=file
256 Trace all system calls which take a file name as an
257 argument. You can think of this as an abbreviation
258 for -e trace=open,stat,chmod,unlink,... which is
259 useful to seeing what files the process is referenc‐
260 ing. Furthermore, using the abbreviation will
261 ensure that you don't accidentally forget to include
262 a call like lstat in the list. Betchya woulda for‐
263 got that one.
264 -e trace=process
266 Trace all system calls which involve process manage‐
267 ment. This is useful for watching the fork, wait,
268 and exec steps of a process.
269 -e trace=network
271 Trace all the network related system calls.
272 -e trace=signal
274 Trace all signal related system calls.
275 -e trace=ipc
277 Trace all IPC related system calls.
278 -e trace=desc
280 Trace all file descriptor related system calls.
281 -e trace=memory
283 Trace all memory mapping related system calls.
284 -e abbrev=set
286 Abbreviate the output from printing each member of
287 large structures. The default is abbrev=all. The
288 -v option has the effect of abbrev=none.
289 -e verbose=set
291 Dereference structures for the specified set of sys‐
292 tem calls. The default is verbose=all.
293 -e raw=set Print raw, undecoded arguments for the specified set
295 of system calls. This option has the effect of
296 causing all arguments to be printed in hexadecimal.
297 This is mostly useful if you don't trust the decod‐
298 ing or you need to know the actual numeric value of
299 an argument.
300 -e signal=set
302 Trace only the specified subset of signals. The
303 default is signal=all. For example, signal =! SIGIO
304 (or signal=!io) causes SIGIO signals not to be
305 traced.
306 -e read=set Perform a full hexadecimal and ASCII dump of all the
308 data read from file descriptors listed in the speci‐
309 fied set. For example, to see all input activity on
310 file descriptors 3 and 5 use -e read=3,5. Note that
311 this is independent from the normal tracing of the
312 read(2) system call which is controlled by the
313 option -e trace=read.
314 -e write=set
316 Perform a full hexadecimal and ASCII dump of all the
317 data written to file descriptors listed in the spec‐
318 ified set. For example, to see all output activity
319 on file descriptors 3 and 5 use -e write=3,5. Note
320 that this is independent from the normal tracing of
321 the write(2) system call which is controlled by the
322 option -e trace=write.
323 -I interruptible
325 When strace can be interrupted by signals (such as
326 pressing ^C). 1: no signals are blocked; 2: fatal
327 signals are blocked while decoding syscall
328 (default); 3: fatal signals are always blocked
329 (default if '-o FILE PROG'); 4: fatal signals and
330 SIGTSTP (^Z) are always blocked (useful to make
331 strace -o FILE PROG not stop on ^Z).
332 -o filename Write the trace output to the file filename rather
334 than to stderr. Use filename.pid if -ff is used.
335 If the argument begins with `|' or with `!' then the
336 rest of the argument is treated as a command and all
337 output is piped to it. This is convenient for pip‐
338 ing the debugging output to a program without
339 affecting the redirections of executed programs.
340 -O overhead Set the overhead for tracing system calls to over‐
342 head microseconds. This is useful for overriding
343 the default heuristic for guessing how much time is
344 spent in mere measuring when timing system calls
345 using the -c option. The accuracy of the heuristic
346 can be gauged by timing a given program run without
347 tracing (using time(1)) and comparing the accumu‐
348 lated system call time to the total produced using
349 -c.
350 -p pid Attach to the process with the process ID pid and
352 begin tracing. The trace may be terminated at any
353 time by a keyboard interrupt signal (CTRL-C).
354 strace will respond by detaching itself from the
355 traced process(es) leaving it (them) to continue
356 running. Multiple -p options can be used to attach
357 to many processes. -p "`pidof PROG`" syntax is sup‐
358 ported.
359 -P path Trace only system calls accessing path. Multiple -P
361 options can be used to specify several paths.
362 -s strsize Specify the maximum string size to print (the
364 default is 32). Note that filenames are not consid‐
365 ered strings and are always printed in full.
366 -S sortby Sort the output of the histogram printed by the -c
368 option by the specified criterion. Legal values are
369 time, calls, name, and nothing (default is time).
370 -u username Run command with the user ID, group ID, and supple‐
372 mentary groups of username. This option is only
373 useful when running as root and enables the correct
374 execution of setuid and/or setgid binaries. Unless
375 this option is used setuid and setgid programs are
376 executed without effective privileges.
377 -E var=val Run command with var=val in its list of environment
379 variables.
380 -E var Remove var from the inherited list of environment
382 variables before passing it on to the command.
383
385 When command exits, strace exits with the same exit status. If
386 command is terminated by a signal, strace terminates itself with
387 the same signal, so that strace can be used as a wrapper process
388 transparent to the invoking parent process.
389 When using -p, the exit status of strace is zero unless there
391 was an unexpected error in doing the tracing.
392
394 If strace is installed setuid to root then the invoking user
395 will be able to attach to and trace processes owned by any user.
396 In addition setuid and setgid programs will be executed and
397 traced with the correct effective privileges. Since only users
398 trusted with full root privileges should be allowed to do these
399 things, it only makes sense to install strace as setuid to root
400 when the users who can execute it are restricted to those users
401 who have this trust. For example, it makes sense to install a
402 special version of strace with mode `rwsr-xr--', user root and
403 group trace, where members of the trace group are trusted users.
404 If you do use this feature, please remember to install a non-
405 setuid version of strace for ordinary lusers to use.
406
408 ltrace(1), time(1), ptrace(2), proc(5)
409
411 It is a pity that so much tracing clutter is produced by systems
412 employing shared libraries.
413 It is instructive to think about system call inputs and outputs
415 as data-flow across the user/kernel boundary. Because user-
416 space and kernel-space are separate and address-protected, it is
417 sometimes possible to make deductive inferences about process
418 behavior using inputs and outputs as propositions.
419 In some cases, a system call will differ from the documented
421 behavior or have a different name. For example, on System V-
422 derived systems the true time(2) system call does not take an
423 argument and the stat function is called xstat and takes an
424 extra leading argument. These discrepancies are normal but
425 idiosyncratic characteristics of the system call interface and
426 are accounted for by C library wrapper functions.
427 On some platforms a process that has a system call trace applied
429 to it with the -p option will receive a SIGSTOP. This signal
430 may interrupt a system call that is not restartable. This may
431 have an unpredictable effect on the process if the process takes
432 no action to restart the system call.
433
435 Programs that use the setuid bit do not have effective user ID
436 privileges while being traced.
437 A traced process runs slowly.
439 Traced processes which are descended from command may be left
441 running after an interrupt signal (CTRL-C).
442 The -i option is weakly supported.
444
446 strace The original strace was written by Paul Kranenburg for
447 SunOS and was inspired by its trace utility. The SunOS version
448 of strace was ported to Linux and enhanced by Branko Lankester,
449 who also wrote the Linux kernel support. Even though Paul
450 released strace 2.5 in 1992, Branko's work was based on Paul's
451 strace 1.5 release from 1991. In 1993, Rick Sladkey merged
452 strace 2.5 for SunOS and the second release of strace for Linux,
453 added many of the features of truss(1) from SVR4, and produced
454 an strace that worked on both platforms. In 1994 Rick ported
455 strace to SVR4 and Solaris and wrote the automatic configuration
456 support. In 1995 he ported strace to Irix and tired of writing
457 about himself in the third person.
458
460 Problems with strace should be reported to the strace mailing
461 list at <strace-devel@lists.sourceforge.net>.
462 2010-03-30 STRACE(1)