kemnitz@mitisft.Convergent.COM (Gregory Kemnitz) (04/29/89)
There is an ULTRIX (I think) function ftruncate(fd, length) int fd, length; that takes a file pointed at by fd and truncates the file to length bytes. The file descriptor may be only open for writing (which rules out dumping length bytes to a temporary file and copying them back, which involves read access). How would one implement such a function in System V (I have tried the obvious ways, ie fcntl(), etc). Do you have to write something to edit the inode to do this?? --Greg
bill@twwells.uucp (T. William Wells) (04/30/89)
In article <651@mitisft.Convergent.COM> kemnitz@mitisft.Convergent.COM (Gregory Kemnitz) writes:
: There is an ULTRIX (I think) function
:
: ftruncate(fd, length)
: int fd, length;
(Actually, BSD)
: that takes a file pointed at by fd and truncates the file to length bytes.
: The file descriptor may be only open for writing (which rules out dumping
: length bytes to a temporary file and copying them back, which involves
: read access). How would one implement such a function in System V (I have
: tried the obvious ways, ie fcntl(), etc). Do you have to write something
: to edit the inode to do this??
You don't do it with a standard system call. I once had occasion to
look long and hard for this and didn't find it.
---
Bill { uunet | novavax } !twwells!billdavidsen@steinmetz.ge.com (Wm. E. Davidsen Jr) (05/03/89)
There was a method in wizards a few years ago I believe. It had to do
with using O_TRUNC, which seems to truncate to the position of the
current file pointer. There was a trick with reopen or dup or ??? which
allowed opening of an already open file. Does this jog anyone's memory?
I think the question at that time was "how do I do ftruncate?"
--
bill davidsen (wedu@crd.GE.COM)
{uunet | philabs}!steinmetz!crdos1!davidsen
"Stupidity, like virtue, is its own reward" -meguy@auspex.auspex.com (Guy Harris) (05/03/89)
> There was a method in wizards a few years ago I believe. It had to do >with using O_TRUNC, which seems to truncate to the position of the >current file pointer. Not on S5R3.1, and not on 4.3BSD. O_TRUNC is used *only* on "open", and since the whole point of "open" is to *create* a file pointer, there *is* no current file pointer.... O_TRUNC isn't used on "fcntl". >There was a trick with reopen or dup or ??? which >allowed opening of an already open file. Well, "dup" will give you a second file descriptor that refers to an already-open file, but there's no trick that lets you open an already-open file with "open", unless you have something like the "/dev/fd" mechanism.
stevea@laidbak.UUCP (Steve Alexander) (05/04/89)
/*
* This works only on System V 3.1 and 3.2, and relies on an
* undocumented fcntl. Use this at your own risk. There are no guarantees
* that this will remain in V.4, especially since I could find no commands
* that use it.
*/
#include <sys/types.h>
#include <errno.h>
#include <fcntl.h>
#include <stdio.h>
extern void exit();
main(argc, argv)
int argc;
char **argv;
{
int f, i, r;
int trunclen, maxlen;
char buf[1024];
if (argc < 4)
error("usage: f file maxlen trunclen");
maxlen = atoi(argv[2]);
maxlen /= sizeof(buf);
trunclen = atoi(argv[3]);
f = open(argv[1], O_RDWR|O_CREAT, 0644);
if (f < 0)
error("open");
/* write maxlen * 1024 bytes */
for (i = 0; i < maxlen; i++)
if (write(f, buf, sizeof(buf)) < 0)
error("write");
/* truncate to trunclen */
r = ftruncate(f, trunclen);
if (r < 0)
error("ftruncate");
r = close(f);
if (r < 0)
error("close");
exit(0);
/*NOTREACHED*/
}
int
ftruncate(f, l)
int f;
int l;
{
struct flock fl;
fl.l_whence = 0;
fl.l_len = 0;
fl.l_start = l;
return fcntl(f, F_FREESP, &fl);
}
error(s)
char *s;
{
extern void perror();
if (errno)
perror(s);
else
(void)fprintf(stderr,"%s\n",s);
exit(1);
}
--
Steve Alexander, TCP/IP Development | stevea%laidbak@sun.com
Lachman Associates, Inc. | ...!sun!laidbak!steveaallbery@ncoast.ORG (Brandon S. Allbery) (05/06/89)
As quoted from <887@twwells.uucp> by bill@twwells.uucp (T. William Wells): +--------------- | In article <651@mitisft.Convergent.COM> kemnitz@mitisft.Convergent.COM (Gregory Kemnitz) writes: | : There is an ULTRIX (I think) function | : | : ftruncate(fd, length) | : int fd, length; | | (Actually, BSD) | | You don't do it with a standard system call. I once had occasion to | look long and hard for this and didn't find it. +--------------- Standard System V (except perhaps 3.2) doesn't have this. Xenix System V may use one of two methods: (1) zero-length write at point of truncation (2) a system call whose name I've forgotten which truncates the file at the current lseek() position SysV 3.2 probably uses the latter method. I can look up the syscall if you're desperate. ++Brandon -- Brandon S. Allbery, moderator of comp.sources.misc allbery@ncoast.org uunet!hal.cwru.edu!ncoast!allbery ncoast!allbery@hal.cwru.edu Send comp.sources.misc submissions to comp-sources-misc@<backbone> NCoast Public Access UN*X - (216) 781-6201, 300/1200/2400 baud, login: makeuser
allbery@ncoast.ORG (Brandon S. Allbery) (05/06/89)
As quoted from <13619@ncoast.ORG> by allbery@ncoast.ORG (Brandon S. Allbery): +--------------- | Standard System V (except perhaps 3.2) doesn't have this. Xenix System V | may use one of two methods: +--------------- I forgot to mention that different versions of Xenix use one or the other, never both. The zero-length write trick is pretty much obsolete. +--------------- | (1) zero-length write at point of truncation | (2) a system call whose name I've forgotten which truncates the file at | the current lseek() position +--------------- I remembered it: chsize(fd). It will also "grow" the file if you've lseek()'ed past EOF. (I don't know whether it uses "holes" where possible or always allocates disk space.) ++Brandon -- Brandon S. Allbery, moderator of comp.sources.misc allbery@ncoast.org uunet!hal.cwru.edu!ncoast!allbery ncoast!allbery@hal.cwru.edu Send comp.sources.misc submissions to comp-sources-misc@<backbone> NCoast Public Access UN*X - (216) 781-6201, 300/1200/2400 baud, login: makeuser
cs411s03@uhccux.uhcc.hawaii.edu (Cs411s03) (05/13/89)
The Xenix function chsize(S) can be found in /lib/libx.a under System V 3.2. It functions similarly to the BSD ftruncate() function: int chsize(fildes, size) int fildes; long size; The chsize function will change the file associated with fildes to be exactly size bytes in length ... cs411s03!uhccux