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Conference turris::c_plus_plus

Title:	C++
Notice:	Read 1.* and use keywords (e.g. SHOW KEY/FULL KIT_CXX_VAX_VMS)
Moderator:	DECCXX::AMARTIN

Created:	Fri Nov 06 1987
Last Modified:	Thu Jun 05 1997
Last Successful Update:	Fri Jun 06 1997
Number of topics:	3604
Total number of notes:	18242

3489.0. "how about a "truncate at current position" manipulator for fstream?" by CUJO::SAMPSON () Thu Mar 13 1997 01:06

	Hello,

	A customer made a suggestion today that seems reasonable.
He was looking for a way to open a stream file, read the contents,
add, remove, and/or modify information in memory, overwrite the
file with the entire modified contents, and truncate the file at
its new size.  I suggested possible methods that would use ACP QIO,
RMS, or DEC C RTL calls, but he wanted to stay with the C++ class
library.

	He would like to have a function or manipulator (or whatever)
for the fstream class, that would truncate the file at the current
position.  There is an available option (called ios::trunc, I think)
that allows an existing file's contents to be truncated away when
first opened, but this would not allow him to read the file first
without closing it.  He wants to open the file once for read and write
access, using it as a lock against other accessors.

	Anyway, he decided that it's no big deal if the file isn't
truncated precisely at the end of the valid contents, since he can
always add a byte count at the start, and/or a special end-of-data
indicator at the end.  In principle, though, it would be useful to
have such a tool available.  What do you think of the idea?

	Thanks,
	Bob Sampson

T.R	Title	User	Personal Name	Date	Lines
3489.1		TLE::D_SMITH	Duane Smith -- DEC C RTL	`Thu Mar 13 1997 06:41`	4
	In C, the functions are called "truncate" and "ftruncate". They were added in OpenVMS V7. Duane
3489.2	that's good; how to use them?	CUJO::SAMPSON		`Thu Mar 13 1997 20:36`	1
	Oh! Can they be applied to an open fstream object? If so, how?
3489.3	answer to .2	DECC::J_WARD		`Fri Mar 14 1997 12:50`	4
	You can always get access to the underlying C file descriptor for an fstream by calling rdbuf()->fd(). Then use the file descriptor for the ftruncate() call.
3489.4	oops, I was thinking trunc-at-x, not trunc-here	SPECXN::DERAMO	Dan D'Eramo	`Fri Mar 14 1997 15:42`	50
	// for discussion purposes only ... #include <iostream.hxx> #include <fstream.hxx> #include <unistd.h> class Trunc { // Sample usage: ofstrm << Trunc(100); public: int offset; Trunc() :offset(0) {} Trunc(int o) :offset(o) {} }; ofstream &operator<<(ofstream &ofs, const Trunc &t) { filebuf fbp = ofs.rdbuf(); if (fbp == 0) { / error handling ... / } int fd = fbp->fd(); if (fd < 0) { / error handling ... / } // anything else you need/want to do first, such as // flushing the buffer ... int result = ftruncate(fd, t.offset); if (result != 0) { / error handling ... */ } // put the ofstream and filebuf objects back into a // consistent state with respect to the now truncated // file ... // Must return the ofstream argument. return ofs; } Three problems ... First is the missing code fragments. Second, as is a Trunc object can't encode "truncate at current position", it can only encode an offset. Third, Trunc objects don't play well with cascading <<'s. For example, const char done[] = "\nDone\n"; ofstrm << done << Trunc(currpos + strlen(done)); This is (ofstrm << done) << Trunc(...) and the type of the left hand side argument is ostream, not ofstream, but the only defined << with a Trunc on the right hand side takes an ofstream on the left hand side. Dan
3489.5	find current position with ftell()?	CUJO::SAMPSON		`Fri Mar 14 1997 21:45`	2
	Okay, cool, thanks! So, the answer is: the needed tools are available in OpenVMS V7.0; after that, it's just a SMOP...
3489.6	ftruncate() opens and closes the file	DECC::J_WARD		`Mon Mar 17 1997 09:10`	10
	From the ftruncate() man page: path Specifies the name of a file that is opened, truncated, and then closed. The path parameter must point to a pathname which names ... So if what the user wants is to be able to truncate the file without also opening and closing it, then I don't think this command will help.
3489.7		SPECXN::DERAMO	Dan D'Eramo	`Mon Mar 17 1997 10:48`	40
	But the path argument is passed to truncate(). ftruncate() takes a file descriptor to an already opened file. How much information does an ofstream and/or filebuf object cache about the state of the file that will be messed up by calling ftruncate() with the file descriptor? Dan truncate(2) truncate(2) NAME truncate, ftruncate - Changes file length SYNOPSIS #include <sys/types.h> int truncate ( const char *path, off_t length ); int ftruncate ( int filedes, off_t length ); PARAMETERS path Specifies the name of a file that is opened, truncated, and then closed. The path parameter must point to a pathname which names a regular file for which the calling process has write permis- sion. If the path parameter refers to a symbolic link, the length of the file pointed to by the symbolic link is truncated. filedes Specifies the descriptor of a file that must be open for writing. length Specifies the new length of the file in bytes.
3489.8	might work for this customer	CUJO::SAMPSON		`Mon Mar 17 1997 21:08`	7
	> How much information does an ofstream and/or filebuf object > cache about the state of the file that will be messed up by > calling ftruncate() with the file descriptor? Hmmm. Would it work out okay if the file were closed immediately after the ftruncate() call? Perhaps by next calling fclose(), then close on the fstream?