How to capture unbuffered output from stdout without modifying the program?
I'm writing a utility for running programs, and I need to capture unbuffered stdout and stderr from the programs. I need to:
- Capture stdout and stderr to separate files.
- Output needs to not be buffered (or be line buffered).
- Without modifying the source of the program being run.
The problem is, when piping output to a file, the stdout stream becomes block buffered rather than line buffered. If the program crashes, the output never gets flushed, and is blank. So I need to capture stdout without buffering (or with line buffering).
I think this can be done with pty's but I'm having difficulty finding any examples that do exactly what I want (most ignore stderr). In fact, I'm not sure I've found any pty examples in C at all; most use a higher-level interface like Python's pty and subprocess modules.
Can anyone help (with code snippets or links)? Any help would be appreciated.
EDIT: I think I've solved it. The following two links were pretty helpful.
- http://publib.boulder.ibm.com/infocenter/zos/v1r10/index.jsp?topic=/com.ibm.zos.r10.bpxbd00/posixopenpt.htm
- http://www.gidforums.com/t-3369.html
My code is avail开发者_开发问答able as a repository:
- https://bitbucket.org/elliottslaughter/pty
see man 7 pty
In particular:
Unix 98 pseudo-terminals
An unused Unix 98 pseudo-terminal master is opened by calling
posix_openpt(3)
. (This function opens the master clone device,/dev/ptmx
; seepts(4)
.) After performing any program-specific initializations, changing the ownership and permissions of the slave device usinggrantpt(3)
, and unlocking the slave usingunlockpt(3)
), the corresponding slave device can be opened by passing the name returned byptsname(3)
in a call toopen(2)
.
And now that you know the names of the library functions such a code will need to call, you can do two useful things:
- Look up their man pages
- Google for example code. Since you know what keywords to use with the search engine I suspect you will have much more luck hunting down examples.
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