Strace
strace is a diagnostic, debugging and instructional userspace utility for Linux. It is used to monitor and tamper with interactions between processes and the Linux kernel, which include system calls, signal deliveries, and changes of process state. The operation of strace is made possible by the kernel feature known as ptrace.
Some Unix-like systems provide other diagnostic tools similar to strace, such as truss.
History
Strace was originally written for SunOS by Paul Kranenburg in 1991, according to its copyright notice, and published early in 1992, in the volume three of comp.sources.sun. The initial README file contained the following:
is a system call tracer for Sun systems much like the Sun supplied program. is a useful utility to sort of debug programs for which no source is available which unfortunately includes almost all of the Sun supplied system software.
Later, Branko Lankester ported this version to Linux, releasing his version in November 1992 with the second release following in 1993. Richard Sladkey combined these separate versions of strace in 1993, and ported the program to SVR4 and Solaris in 1994, resulting in strace 3.0 that was announced in comp.sources.misc in mid-1994.
Beginning in 1996, strace was maintained by Wichert Akkerman. During his tenure, strace development migrated to CVS; ports to FreeBSD and many architectures on Linux were introduced. In 2002, the burden of strace maintainership was transferred to Roland McGrath. Since then, strace gained support for several new Linux architectures, bi-architecture support for some of them, and received numerous additions and improvements in syscalls decoders on Linux; strace development migrated to git during that period. Since 2009, strace is actively maintained by Dmitry Levin. strace gained support for AArch64, ARC, AVR32, Blackfin, Meta, Nios II, OpenSISC 1000, RISC-V, Tile/TileGx, Xtensa architectures since that time.
The last version of strace that had some code for non-Linux operating systems was 4.6, released in March 2011. In strace version 4.7, released in May 2012, all non-Linux code had been removed; since strace 4.13, the project follows Linux kernel's release schedule, and with the version 5.0, it follows Linux's versioning scheme as well.
In 2012 strace also gained support for path tracing and file descriptor path decoding. In August 2014, strace 4.9 was released, where support for stack traces printing was added. In December 2016, syscall fault injection feature was implemented.
Version history
Usage and features
The most common use is to start a program using strace, which prints a list of system calls made by the program. This is useful if the program continually crashes, or does not behave as expected; for example using strace may reveal that the program is attempting to access a file which does not exist or cannot be read.An alternative application is to use the flag to attach to a running process. This is useful if a process has stopped responding, and might reveal, for example, that the process is blocking whilst attempting to make a network connection.
Among other features, strace allows the following:
- Specifying a filter of syscall names that should be traced : by name, like ; using one of the predefined groups, like or ; or using regular expression syntax, like
-e trace=/clock_.*. - Specifying a list of paths to be traced.
- Specifying a list of file descriptors whose I/O should be dumped.
- Counting syscall execution time and count.
- Printing relative or absolute time stamps.
- Modifying return and error code of the specified syscalls, and inject signals upon their execution.
- Extracting information about file descriptors.
- Printing stack traces, including symbol demangling.
- Filtering by syscall return status.
As strace only details system calls, it cannot be used to detect as many problems as a code debugger such as GNU Debugger. It is, however, easier to use than a code debugger, and is a very useful tool for system administrators. It is also used by researchers to generate system call traces for later system call replay.
Examples
The following is an example of typical output of thestrace command:user@server:~$ strace ls
...
open = 3
fstat64 = 0
fcntl64 = 0x1
getdents64 = 496
getdents64 = 0
close = 0
fstat64 = 0
mmap2 = 0xb7f2c000
write(1, "autofs\nbackups\ncache\nflexlm\ngames"..., 86autofsA
The above fragment is only a small part of the output of strace when run on the 'ls' command. It shows that the current working directory is opened, inspected and its contents retrieved. The resulting list of file names is written to standard output.
Similar tools
Different operating systems feature other similar or related instrumentation tools, offering similar or more advanced features; some of the tools may use completely different work mechanisms, resulting in different feature sets or results. Such tools include the following:- Linux has ltrace that can trace library and system calls, xtrace that can trace X Window programs, SystemTap, perf, and trace-cmd and KernelShark that extend ftrace.
- AIX provides the command
- HP-UX offers the command
- Solaris / Illumos has truss and DTrace
- UnixWare provides the command
- FreeBSD provides the command, ktrace and DTrace
- NetBSD provides ktrace and DTrace
- OpenBSD uses ktrace and kdump
- macOS provides ktrace, DTrace and associated dtruss in 10.5 and later.
- MS Windows has a similar utility called StraceNT, written by Pankaj Garg, and a similar GUI-based utility called Process Monitor, developed by Sysinternals.