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STRACE(1) STRACE(1)
NAME
strace - trace system calls and signals
SYNOPSIS
strace [ -CdDffhiqrtttTvxx ] [ -acolumn ] [ -eexpr ] ... [ -ofile ] [ -ppid ] ... [ -sstrsize ] [ -uusername ] [
-Evar=val ] ... [ -Evar ] ... [ command [ arg ... ] ]
strace -c [ -D ] [ -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 redi-
rection:
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 repre-
sented 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 com-
mas 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 (Not available on SVR4 and FreeBSD.) 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)
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 par-
ent 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 redi-
rected 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 quali-
fier-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 abbrevia-
tion for -e trace=open,stat,chmod,unlink,... which is useful to seeing what files the process is ref-
erencing. 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 termi-
nates 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 privi-
leges. 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 trac-
ing.
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 sup-
port. 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-develATlists.net>.
2010-03-30 STRACE(1)
