OpenGrok

..

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 .CE 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) .CE 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 +++ .CE 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 .CE 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 .CE 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) .CE 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 .CE 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 .CE 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 .CE 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 .CE Here the second argument represents the full set of all signals.

12

-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.

-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. 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. 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).

-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.

-F Attempt to follow vfork s. (On SunOS 4.x, this is accomplished with some dynamic linking trickery.) Otherwise, vfork s will not be followed even if -f has been given.

-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 redirected 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: