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179

answers:

1

The following code outputs "Illegal seek":

#include <stdio.h>
#include <errno.h>
#include <string.h>

int main() {
    errno = 0;
    getchar();
    getchar();
    getchar();
    ftell( stdin );
    printf( "%s\n", strerror(errno) );
}

This occurs when I run "cat script | ./a.out" as well as when I just run "./a.out". The problem is with ftell, of course. My question is: why does this occur? I would think stdin can be seekable. fseek also causes the same error. If stdin is not seekable, is there some way I can do the same sort of thing?

Thank you for your replies.

+5  A: 

Fifos aren't seekable. They are simply a buffer. Once data has been read() from a fifo buffer, it can never be retrieved.

Note that if you ran your program:

./a.out < script

then standard input would be a file and not a fifo, so ftell() will then do what you expect.

geocar
Thanks so much!
Jim
+1 for an example that shows the difference between `cat file|./a.out` and `./a.out<file`.
R..