Test for seekability?

[Steve]

It seems that the return values of fseek() and ftell() are undefined for
non-seekable devices. So is there any way to test whether a stream is
seekable?

 

[Irrwahn Grausewitz]

It seems that the return values of fseek() and ftell() are undefined for
non-seekable devices. So is there any way to test whether a stream is
seekable?

The return values of fseek and ftell are well defined:

fseek() returns nonzero for a request that cannot be satisfied.

ftell() function returns -1L on failure and stores an implementation-
defined positive value in errno.

You may use these to determine if you can set/get the file position
indicator for a stream.

Regards