Re: Using seek function to write out to top of file?

Replies are listed 'Best First'.
Re^2: Using seek function to write out to top of file? by o2bwise (Scribe) on May 27, 2005 at 21:43 UTC
Hi, Oh, tlm, I did not try your method, but (time permitting), I will give it a shot. I tried the following 8 cases. File t.txt: cat in the hat Case 1 (write,no + sign, use seek): `#!/usr/bin perl -w open (TEST,">t.txt); seek (TEST,0,0); print TEST "The\n"; close (TEST);` [download] Case 2 (write, + sign, use seek): `#!/usr/bin perl -w open (TEST,"+>t.txt); seek (TEST,0,0); print TEST "The\n"; close (TEST);` [download] Case 3 (append, no plus sign, use seek): `#!/usr/bin perl -w open (TEST,">>t.txt); seek (TEST,0,0); print TEST "The\n"; close (TEST);` [download] Case 4 (append, use + sign, use seek): `#!/usr/bin perl -w open (TEST,"+>>t.txt); seek (TEST,0,0); print TEST "The\n"; close (TEST);` [download] Case 5 (write, no plus sign, use sysseek): `#!/usr/bin perl -w open (TEST,">t.txt); sysseek (TEST,0,0); syswrite TEST, "The\n"; close (TEST);` [download] Case 6 (write, use + sign, use sysseek): `#!/usr/bin perl -w open (TEST,"+>t.txt); sysseek (TEST,0,0); syswrite TEST, "The\n"; close (TEST);` [download] <Case 7 (append, no + sign, use sysseek): `#!/usr/bin perl -w open (TEST,">>t.txt); sysseek (TEST,0,0); syswrite TEST, "The\n"; close (TEST);` [download] Case 8 (append, use + sign, use sysseek): `#!/usr/bin perl -w open (TEST,"+>>t.txt); sysseek (TEST,0,0); syswrite TEST, "The\n"; close (TEST);` [download] Results: Cases 1,2,5,6 t.txt The Cases 3,4,7,8: t.txt cat in the hat The Conclusion: The results are driven by one thing; whether opening the file to write (>) or to append (>>). Using + doesn't do anything and using sysseek gives the same result as seek. Whether going to the beginning of the file with seek or with sysseek (and using append), writing was performed AT THE END OF THE FILE. I'm kind of surprised. The responses I got had the expectation that writing would be done at the top of the file. I have never seen this to be the case. One quick question. Does opening to write (one > sign) ALWAYS cause the file to be emptied? This is what I have seen. I have never seen it have previous text. A reminder. This was for an on-line class, which requested this procedure. I find the class documentation to have had three mistakes (for which I am disappointed). 1. It assumed that opening with write (not append) might leave previous text. 2. It assumed you could write out at the top of a file (that has text in it from previously) using syseek at 0,0. 3. It also used flock on explicit file name and not filehandle. Thanks, monks, for your assistance. o2	[reply] [d/l] [select]
Re^3: Using seek function to write out to top of file? by Fletch (Bishop) on May 28, 2005 at 00:27 UTC
You need to read `perldoc -f open` and `perldoc perlopentut` as it explains all of this. Opening with any form of `>` clobbers the output, and any form of `>>` opens in append mode. If you want to update in place you need to use `+<`. -- We're looking for people in ATL	[reply] [d/l] [select]
Re^4: Using seek function to write out to top of file? by o2bwise (Scribe) on May 29, 2005 at 01:13 UTC
fletch, I finally tried out +<. That worked. I mean, I wouldn't have done it that way, but it worked with what the class asked for, for a procedure (using seek (filehandle,0,0) and truncate (filehandle, tell(filehandle)). This was a good learning experience, albeit a very narrow one. Thanks, o2	[reply]