how to decode strace dump string

neo created at4 months ago view count: 55
 printf "%b" "\10\0\0\0\3COMMIT"


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 passion. This example shows the shell performing ">>xyzzy" output redirection:

open("xyzzy", O_WRONLY|O_APPEND|O_CREAT, 0666) = 3

Here, the second and the third argument of open(2) are decoded by breaking down the flag argument into its three bitwise-OR constituents and printing the mode value in octal by tradition. Where the traditional or native usage differs from ANSI or POSIX, the latter forms are preferred. In some cases, strace output is proven to be more readable than the source.

Structure pointers are dereferenced and the members are displayed as appropriate. In most 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(0x1, 0x3), ...}) = 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(2) 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.

Syscalls unknown to strace are printed raw, with the unknown system call number printed in hexadecimal form and prefixed with "syscall_":

syscall_0xbad(0x1, 0x2, 0x3, 0x4, 0x5, 0x6) = -1 ENOSYS (Function not implemented)

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(3) 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(1) 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.  


Print ARGUMENT(s) according to FORMAT, or execute according to OPTION:

display this help and exit
output version information and exit

FORMAT controls the output as in C printf. Interpreted sequences are:

double quote
alert (BEL)
produce no further output
form feed
new line
carriage return
horizontal tab
vertical tab
byte with octal value NNN (1 to 3 digits)
byte with hexadecimal value HH (1 to 2 digits)
Unicode (ISO/IEC 10646) character with hex value HHHH (4 digits)
Unicode character with hex value HHHHHHHH (8 digits)
a single %
ARGUMENT as a string with '\' escapes interpreted, except that octal escapes are of the form \0 or \0NNN
ARGUMENT is printed in a format that can be reused as shell input, escaping non-printable characters with the proposed POSIX $'' syntax.

and all C format specifications ending with one of diouxXfeEgGcs, with ARGUMENTs converted to proper type first. Variable widths are handled.

NOTE: your shell may have its own version of printf, which usually supersedes the version described here. Please refer to your shell's documentation for details about the options it supports.  



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