SYNOPSIS
strace -CdffhiqrtttTvVxxy -In -bexecve -eexpr... -acolumn -ofile -sstrsize -Ppath... -ppid... / -D [-Evar=val]...
-uusername command args
strace -cdf -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 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 pre-
serve 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 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 follow-
ing 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 ele-
ments. 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 prepar-
ing 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
EXAMPLES
Tracing particular system calls
strace -fe trace=system_call command command_params