Find out if handle is seekable

ambrus has asked for the wisdom of the Perl Monks concerning the following question:

I am reading a text file from STDIN, and I'd like to optimize my program by seeking back in the input sometimes. However, the input can come from a pipe or even directly from terminal, in which case seeking back is not possible. I'd like my code to work in this case too, so I want to determine whether a handle can be seeked back.

I have two ideas on how to do this, but I'm not quite sure they'll always work.

Firstly

if (sysseek STDIN, 0, 1) {
    # run the optimized code
} else {
    # run the slow code that stores data in memory instead of seeking 
+back in the file
}
[download]

but linux manpage for lseek(2) reports that on some non-linux systems lseek can succeed on terminal devices.

The second is

if (-f STDIN || -b STDIN) {
    # fast code
} else {
    # slow code
}
[download]

but I'm not sure whether I can trust this either.

Please advice me, wise monks, on the proper way to find out whether it is possible to seek a handle back.

Comment on Find out if handle is seekable Select or Download Code

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Re: Find out if handle is seekable by Ben Win Lue (Friar) on Apr 08, 2005 at 08:33 UTC
`Posix::isatty` (in words: is_a_tty) could be worth a look. It returns true, if the filehandle is associated with a terminal.	[reply] [d/l]
Re^2: Find out if handle is seekable by ambrus (Abbot) on Apr 08, 2005 at 08:36 UTC
Isatty won't return true on a pipe or a named pipe (fifo) or a socket, so it's not what I want. By the way, isn't the -t filetest operator the same as isatty?	[reply]