Thread: Setting position indicators in Streams

Hello all, I need to be able to seek through a stream. This is a non-file stream, created by a pipe between my program and a subprogram, more specifically, created by the "popen()" function. Seeking position markers in a file stream is pretty easy, but it does not seem to be as straightforward with this kind of stream.
So far, I have tried the rewind() and fseek() functions, but these do not work (the first character scanned after a call to rewind() should be the first character in the stream again, but instead results in a EOF (and I know the stream was not empty)).
Is there something else I need to be doing, or are these types of streams (ones created by popen()) un-seekable?
Thanks for any help.

Here is a sample of the code I am using:

Code:

int
main(void)
{
	int num_lines;
	FILE *streamp;

	streamp = popen(COMMAND, "r");

       //pass stream onto function
	num_lines = get_num_lines(streamp);

       //prints "10" in the specific case I am using, so I know stream is not empty.
       printf("%d", num_lines);

       //RESET marker to beginning of stream (I had hoped)
       rewind(streamp);

       //instead this indicated that the marker was still at the end of the stream
	if( fgetc(streamp) == EOF)
             printf("END");
}

int
get_num_lines(FILE *streamp)
{
	int num_lines = 0;
	char ch, prev = NEWLINE;

	while((ch = fgetc(streamp)) != EOF)
	{
		if(ch == NEWLINE)
		{
			++num_lines;
		}

		prev = ch;
	}

	if(prev != NEWLINE)
	{
		++num_lines;
	}

	return(num_lines);
}