Using fseek with a file pointer that points to stdin

Yes, it's perfectly normal that fseek won't work on stdin -- it'll normally only work on a disk file, or something reasonably similar.

Though it's really a POSIX thing, you can typically use if (isatty(fileno(myFile))) to get at least a pretty good idea of whether seeking will work in a particular file. In some cases, isatty and/or fileno will have a leading underscore (e.g., IIRC the versions provided with Microsoft's compilers do).

Here is the relevant entry in the ANSI standard concerning the fseek function:

For a text stream, either offset shall be zero, or offset shall be a value returned by an earlier successful call to the ftell function on a stream associated with the same file and whence shall be SEEK_SET

So, possible but with some limitations

Fseek() is based on lseek(), and the lseek man page discusses possible errors, including:

 [ESPIPE]           Fildes is associated with a pipe, socket, or FIFO.

If stdin is connected to a pseudo tty, I believe it will have socket behavior.