Linux Commands Examples

A great documentation place for Linux commands


trace system calls and signals

see also : ltrace - time


strace [ -CdffhiqrtttTvxx ] [ -acolumn ] [ -eexpr ] ... [ -ofile ] [ -ppid ] ... [ -sstrsize ] [ -uusername ] [ -Evar=val ] ... [ -Evar ] ... [ command [ arg ... ] ]

strace -c [ -eexpr ] ... [ -Ooverhead ] [ -Ssortby ] [ command [ arg ... ] ]

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How do I strace the whole system?

It's not very feasible to "strace the whole system" from userspace. As I indicated in the previous question you asked, the best way is to use a kernel-mode tracing infrastructure such as kprobes, systemtap, or dtrace. Have you looked at any of these? Is there a reason why none of them will work for your use case?

The only way to truly reliably strace the entire system from userspace would be to start your trace with the init process... but I'm not sure that init or systemd would be very happy with you stracing it, since it does a lot of very low-level stuff that's pretty fragile and easy to break (and hard to inject wrapper commands around it too, I might add).

This is why the highest quality probing mechanisms have some type of kernel module, because the kernel "sees all". This is especially relevant since you are trying to monitor activity on character devices such as /dev/console and /dev/tty*, and the kernel has direct oversight over the calls to those devices since they are implemented in kernelspace.


Debugging connection timeouts using strace?

Some PIDs are not listed because they belong to threads. htop can show them if you press Shift+H (and optionally T for tree view), but lsof wants the PID of the main process. (All pthreads in a process share file descriptors, anyway.) You can also take a look in /proc/5546/fd/ and /proc/5546/task/.

EAGAIN is normal for non-blocking I/O; for example, it is returned by read() when there's no data to read. See "ERRORS" in read(2), write(2) and so on. Some of these fd's are likely connections to the X11 server – non-blocking I/O is used by the X11 client libraries.


Correlating strace output to source code function calls

The strace utility shows you system calls. Most compiled programs in Linux eventually link with the standard C library, referred to as "glibc" though the actual library file name is C language "system calls" such as "open", "read", "write" are in fact wrapper functions for the actual system calls that the glibc library executes. Sometimes the wrappers include a surprising amount of code that you usually don'y think about. Sometimes programmers use libraries with functions that make multiple glibc calls that do multiple system calls. Furthermore, if you see a specific "read" in the strace output, you don't have any way of connecting it with a specific "read" or other library function call in the source code. The result of this is that there is no general way to correlate strace output with specific lines of code in a source file.

I assume that when you state that you have the source code you mean that you can also compile it into a functioning executable program. If this is indeed the case, then your best bet is to instrument the code with printfs followed with fflush(stdout) and then run the program under strace. For the printfs you can try somethng like

printf(__FILE__ ", %s:%d Entered\n", __FUNCTION__, __LINE__), fflush(stdout);

at the start of every C function. You can define the above line as a preprocessor marcro that is conditionally defined as the above or as nothing, depending on another macro such as DEBUG, so that you can leave these macros in your code base and compile the code with or without DEBUG defined.

You will see the printf "write" system calls and their output interspersed among the system call that are reading the keystrokes. This should allow you to zero in on the source code functions that are reading the tty input. It might require some persistent effort.


What is a SIG_0 when looking at an strace

From man tgkill:

tgkill() sends the signal sig to the thread with the thread ID tid in the thread group tgid. (By contrast, kill(2) can only be used to send a signal to a process (i.e., thread group) as a whole, and the signal will be delivered to an arbitrary thread within that process.)

Which just leaves us the question of what signal 0 represents. The answer is, none at all:

If you have a process ID but aren't sure whether it's valid, you can use the most unlikely of candidates to test it: the kill command. If you don't see any reference to this on the kill(1) man page, check the info pages. The man/info page states that signal 0 is special and that the exit code from kill tells whether a signal could be sent to the specified process (or processes).

The tgkill calls, then, are testing for the existence of various threads within whatever process you're monitoring via strace. The return value of 0 indicates that the threads so tested do exist; the question to answer now is, why is the process looping over the test? (I assume that's what it's doing, at any rate; presumably if it ever did anything else that you saw, you'd have mentioned it in your question.)

strace -o out.txt recursive.o
example added by an anonymous user
strace -o out.txt recursive.o
example added by an anonymous user
strace -f -o./dbunit_strace.log -s 1000 pake dbunit
strace -f -c -s 1000 pake dbunit

Linux equivalent to Mac OS X's fs_usage

Install sysstat and use the command

sar -B 1


Will strace watch system calls recursively on child processes of the main process being observed?

Yes, but you need to add the -f flag to watch for forks.


How does 'strace' work?

You have pretty much got the answer right in your original question.

The default behaviour of strace is to report what system calls are being performed for that process, it will also report signals called and what handler handled that signal.

In your example the syscalls being called are at the start of the line: fcntl(), getdents64() close() etc.

The arguments that were passed to those syscalls are shown - often truncated, as the raw data can be kilo/mega/giga Bytes in size) fstat(3, {st_mode=S_IFDIR|0755, st_size=4096, ...}) (the ... donates the truncation). What those value represents varies from syscall to syscall but man will show what they represent (provided you have the basic set of man pages related to the sys calls installed on your system). man 2 fstat will show the programmers manual page for fstat and details the arguments.

And the result of that syscall getdents64(3, /* 22 entries */, 32768) = 1008 1008 in this case. The value of of this result code will be documentated in the man(1) page of the specific syscall being called in the same way arguments are detailed. Note in this case the /* 22 entries */ is another truncation operation by strace, this can be expanded upon with -e abbr=none.

Note that strace is not a universal command, some releases of unix use truss, ktrace and dump to perform similar debugging on a process. You should also take a look at the man page for strace, it comes with some extended options (maybe this gets close your bonus question) such as -i (print the instruction pointer) -v (verbosity) -a (change the location - column - of the result part to see more of the arguments passed to the command) and expression filtering -e.

edit added some notes about other forms argument truncation.


Why do strace/truss sometimes 'fix' stuck processes?

May be it is a bug either in kernel or in program you are tracing?

The program may have incorrectly implemented event loop that is waits for wrong thigs, but waits for other things after EINTR.


for(;;) {
  if(FD_SET(...i...)) {
    write(...j...); // Naive blocking write

It will work in trivial test, but the whole program may block if any write blocks.

Suspending/resuming the program aborts blocking write and causes the main loop to continue.


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 redirection:

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 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.



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.


Like -c but also print regular output while processes are running.


Show some debugging output of strace itself on the standard error.


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 parent 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.


If the -o filename option is in effect, each processes trace is written to where pid is the numeric process id of each process. This is incompatible with -c, since no per-process counts are kept.


This option is now obsolete and it has the same functionality as -f.


Print the help summary.


Print the instruction pointer at the time of the system call.


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.


Print a relative timestamp upon entry to each system call. This records the time difference between the beginning of successive system calls.


Prefix each line of the trace with the time of day.


If given twice, the time printed will include the microseconds.


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.


Show the time spent in system calls. This records the time difference between the beginning and the end of each system call.


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.


Print the version number of strace.


Print all non-ASCII strings in hexadecimal string format.


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:


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 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 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.


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.

When using -p, the exit status of strace is zero unless there was an unexpected error in doing the tracing.


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.


Problems with strace should be reported via the Debian Bug Tracking System, or to the strace mailing list at <strace-devel[:at:]lists.sourceforge[:dot:]net>.

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.


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).


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 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.

see also

ltrace , time , ptrace, proc

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