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From: acl3k@pine.cs.Virginia.EDU (Allan Christian Long)
Newsgroups: comp.sys.sgi
Subject: Re: ioctl
Message-ID: <1991May23.191033.20183@murdoch.acc.Virginia.EDU>
Date: 23 May 91 19:10:33 GMT
References: <1991May22.143844.6858@cs.dal.ca> <1991May22.161432.25064@murdoch.acc.Virginia.EDU> <105737@sgi.sgi.com>
Sender: usenet@murdoch.acc.Virginia.EDU
Organization: University of Virginia Computer Science Department
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In article <105737@sgi.sgi.com> rpw3@sgi.com (Rob Warnock) writes:
>In article <1991May22.161432.25064@murdoch.acc.Virginia.EDU>
>acl3k@jade.cs.Virginia.EDU (Allan Christian Long) writes:
>+---------------
>| The problem is that the prototype for ioctl is
>| 	int ioctl (int fildes, int request, ...);
>| That says what the first and second parameters should be, but I need to
>| find out about the rest, which are dependent upon what request is.  The
>| entry for FIONREAD in ioctl.h is
>| 	#define FIONREAD _IOR(f, 127, int)      /* get # bytes to read */
>| I tried changing charToRead to an int, but that didn't help.  I need
>| documentation for FIONREAD, more than what it says in the header file.
>+---------------
>
>The third arg for FIONREAD is the *address* of an integer, that is:
>
>	int num_bytes_avail_to_read;
>
>	err = ioctl(file_descriptor, FIONREAD, &num_bytes_avail_to_read);

I tried a call exactly like this, but I always got error 22 (invalid
argument).  

[stuff about how # of bytes can change right after the ioctl call and
how you can use read() since it returns the number of bytes actually read]

If I were writing a program from scratch, this would probably be fine.
I may not have mentioned, tho, that what I'm doing is porting a tracker
library from SunOS to IRIX (a library that I didn't write).  I COULD use
read, but it would require some rewiring of the library that would be
somewhat difficult.  What I'd really like to have is a way to tell how
many bytes there are to be read without actually reading them.
	Thanks to everyone who has responded.  I'm still open to suggestions.
If I don't get any soon, I'll probably go ahead and rewrite part of the
library to use read() and/or select().

- Chris


A. Chris Long, Jr.			acl3k@virginia.edu
If only we could overcome our tools...