strace - Online in the Cloud

PROGRAM:

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.

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