Provided by: strace_4.11-1ubuntu3_amd64 
NAME
strace - trace system calls and signals
SYNOPSIS
strace [-CdffhikqrtttTvVxxy] [-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]]...
[-uusername] command [args]
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 signal symbol and decoded siginfo structure. An excerpt from stracing and
interrupting the command "sleep 666" is:
sigsuspend([] <unfinished ...>
--- SIGINT {si_signo=SIGINT, si_code=SI_USER, si_pid=...} ---
+++ 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 ... ---
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
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 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 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.
-k Print the execution stack trace of the traced processes after each system call
(experimental). This option is available only if strace is built with libunwind.
-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.
-w Summarise the time difference between the beginning and end of each system call. The default
is to summarise the system time.
-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.
-y Print paths associated with file descriptor arguments.
-yy Print ip:port pairs associated with socket file descriptors.
-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
therefore 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
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 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) 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.
-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 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 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
paths.
-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. Note that parent-child relationship (signal stop notifications, getppid()
value, etc) between traced process and its parent are not preserved unless -D is used.
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.
Some system calls have different names in different architectures and personalities. In these cases,
system call filtering and printing uses the names that match corresponding __NR_* kernel macros of the
tracee's architecture and personality. There are two exceptions from this general rule:
arm_fadvise64_64(2) ARM syscall and xtensa_fadvise64_64(2) Xtensa syscall are filtered and printed as
fadvise64_64(2).
On some platforms a process that is attached to with the -p option may observe a spurious EINTR return
from the current system call that is not restartable. (Ideally, all system calls should be restarted on
strace attach, making the attach invisible to the traced process, but a few system calls aren't.
Arguably, every instance of such behavior is a kernel bug.) 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 runs slowly.
Traced processes which are descended from command may be left running after an interrupt signal (CTRL-C).
The -i option is weakly supported.
HISTORY
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.
PROBLEMS
Problems with strace should be reported to the strace mailing list at
<strace-devel@lists.sourceforge.net>.
2010-03-30 STRACE(1)