strace - Unix, Linux Command

NAME

SYNOPSIS

strace -c [ -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 +++

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.

OPTIONS

SETUID INSTALLATION

SEE ALSO

• ltrace (1)

• time (1)

• ptrace (2)

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

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 tracing.

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

BUGS

PROBLEMS