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.

In reply to Find out if handle is seekable by ambrus

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