Input and Output concurrently

autumngardens · 07-10-2008, 08:28 PM

Hi,

I would like to know if it is possible to read a file and write to the same file at the same time.
For example, if I have

Code:

#include <fstream>
// and other headers


using namespace std;

string temp_string;
char result;


fstream studfile;
studfile.open("student.dat", ios::in | ios::out); // Is this correct?

while (!studfile.eof() ) {
     getline ( studfile, temp_string);
     studfile << "Results: " << result << endl;
}

Is there any way to manipulate a .dat file while reading it?
I've checked quite a couple of sites, but there are only solutions for binary files..

Thanks! =)

cpjust · 07-10-2008, 09:42 PM

It should be possible. Are you having problems?
Do you mean doing both reading & writing in a single thread, or trying to read and write at exactly the same time? If it's the latter, then you should be using some kind of mutex to serialize either reading or writing but not both.

**Salem** · 07-12-2008, 12:12 AM

If you're reading and writing fixed length records, yes.
If you're reading a text file and trying to make each line longer, NO!

Oh, and read the FAQ on why your use of eof() is bad.

Jackie Chan · 07-12-2008, 12:49 AM

here it should be enough testing as:

Code:

while (studfile)
{
    //...
}

but why studfile.eof() is bad ?
and is my suggestion better ?

**Salem** · 07-12-2008, 02:18 AM

> but why studfile.eof() is bad ?
Hello! - Read the FAQ!

Jackie Chan · 07-14-2008, 12:43 AM

kindly can you point me to the FAQ? because the FAQ of this board only mentions that C File I/O feof() is bad. it says nothing about C++ eof().

**Salem** · 07-14-2008, 10:52 AM

http://faq.cprogramming.com/cgi-bin/...&id=1043284351

07-10-2008, 08:28 PM	#1
autumngardens Registered User Join Date: Jul 2008 Posts: 1	Input and Output concurrently Hi, I would like to know if it is possible to read a file and write to the same file at the same time. For example, if I have Code: #include <fstream> // and other headers using namespace std; string temp_string; char result; fstream studfile; studfile.open("student.dat", ios::in \| ios::out); // Is this correct? while (!studfile.eof() ) { getline ( studfile, temp_string); studfile << "Results: " << result << endl; } Is there any way to manipulate a .dat file while reading it? I've checked quite a couple of sites, but there are only solutions for binary files.. Thanks! =)
autumngardens is offline

07-10-2008, 09:42 PM	#2
cpjust and the hat of sweating Join Date: Aug 2007 Location: Toronto, ON Posts: 3,120	It should be possible. Are you having problems? Do you mean doing both reading & writing in a single thread, or trying to read and write at exactly the same time? If it's the latter, then you should be using some kind of mutex to serialize either reading or writing but not both.
cpjust is offline

07-12-2008, 12:12 AM	#3
Salem and the hat of vanishing Join Date: Aug 2001 Location: The edge of the known universe Posts: 21,214	If you're reading and writing fixed length records, yes. If you're reading a text file and trying to make each line longer, NO! Oh, and read the FAQ on why your use of eof() is bad. __________________ If you dance barefoot on the broken glass of undefined behaviour, you've got to expect the occasional cut. Up to 8Mb PlusNet broadband from only £5.99 a month!
Salem is offline

07-12-2008, 12:49 AM	#4
Jackie Chan Big & Little Wong Tin-Bar Join Date: May 2008 Location: Hong Kong Posts: 23	here it should be enough testing as: Code: while (studfile) { //... } but why studfile.eof() is bad ? and is my suggestion better ?
Jackie Chan is offline

07-12-2008, 02:18 AM	#5
Salem and the hat of vanishing Join Date: Aug 2001 Location: The edge of the known universe Posts: 21,214	> but why studfile.eof() is bad ? Hello! - Read the FAQ! __________________ If you dance barefoot on the broken glass of undefined behaviour, you've got to expect the occasional cut. Up to 8Mb PlusNet broadband from only £5.99 a month!
Salem is offline

07-14-2008, 12:43 AM	#6
Jackie Chan Big & Little Wong Tin-Bar Join Date: May 2008 Location: Hong Kong Posts: 23	kindly can you point me to the FAQ? because the FAQ of this board only mentions that C File I/O feof() is bad. it says nothing about C++ eof().
Jackie Chan is offline

07-14-2008, 10:52 AM	#7
Salem and the hat of vanishing Join Date: Aug 2001 Location: The edge of the known universe Posts: 21,214	http://faq.cprogramming.com/cgi-bin/...&id=1043284351 __________________ If you dance barefoot on the broken glass of undefined behaviour, you've got to expect the occasional cut. Up to 8Mb PlusNet broadband from only £5.99 a month!
Salem is offline

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