Updating an existing file

Hello,

I have a text file and I need to change a part of it, but I'm not sure if that's possible.
Let's say the file looks like this:
a
b
c
d

I opened the file in the 'r+' more (update mode). I want to replace the 3rd char, 'c', with '3' but I don't know of a way to do it. I can use fseek(file,3,0) to get to the 3rd char in the file and I can write '3' there but I don't see a way to delete 'c'.

Any help would be greatly appriciated, thanks.

If you write '3' then it's as good as deleted.

If you write a space ' ' then again it's as good as deleted.

If you write '\0' it's ...

And don't forget the file position indicator begins at 0.

Thanks :)

>>fseek(file,3,0)
You can't use fseek in that manner unless you've opened the file in binary mode. The offset parameter must be either zero, or a value returned by ftell() when using text mode. This is because of the way in which end of line markers are used in different OS's.

The best way to change the text file, is to read it in, make the changes in memory and then write it all out again. Sometimes, you might want to write to a temporary file, then rename it to the same as the original file when you're ready.

Hammer.

Do you have documentation that supports the postulate that
fseek can only be used on a file opened in binary mode?

Else could you explain the reasoning behind the postulate?

Thanks in advance.

I opened the file and used fseek() like this:

Code:

---

f=fopen(file_m,"r+");
int x=fseek(f,3,0);

---

and it works fine

The doc is the C standard. Here's the extract:

Quote:

---

ISO/IEC 9899:1999 (E)

3 For a binary stream, the new position, measured in characters from the beginning of the
file, is obtained by adding offset to the position specified by whence. The specified
position is the beginning of the file if whence is SEEK_SET, the current value of the file
position indicator if SEEK_CUR, or end-of-file if SEEK_END. A binary stream need not
meaningfully support fseek calls with a whence value of SEEK_END.

4 For a text stream, either offset shall be zero, or offset shall be a value returned by
an earlier successful call to the ftell function on a stream associated with the same file
and whence shall be SEEK_SET.

---

The reason behind it is that some systems use 2 characters to mark end-of-line (CRLF) and some use one (LF). When running in text mode, CRLF in the file gets translated to one byte in the input stream to the program (\n). Therefore, fseeking to a specific byte offset in a text stream doesn't guarantee true offsets within the external file.

If you go against the rules, it may work for you, it may not. That's why its better to stick within the rules. :)

Lior.

Never assume just because something appears to work that it
is right or will always work under all circumstances.

C will most likely tear you apart in the not too distant future if you
view things that way. Respect C and it will respect you!

Hammer.

Okay, cool. I'll ponder that for a time.

Lior.

Am I assuming correctly that because you opened the file in text
mode and the edit operation was successful, that you ran the
executable on a *nix machine?

I'll also add that the code broke on this machine.

In linux you don't have to worry about it but it won't work in other machines. In Windows they use 2 characters return and line feed. In linux all files are opened in binary.

Quote:

---

Back in the dark ages BC (Before Computers), a magical device called the Teletype Modell 33 existed. This amazing machine contained a shift register made out of a motor, with a rotor, and a keyboard ROM consisting solely of levers and sprints. It caontained a keyboard, a printer, and a paper tape reader/punch. It could transmit messages over the phones using a modem at a rate of 10 characters a second.
The Teletype had a problem. It took two-tenths of a second to move the print head from the right side to the left. Two-tenths of a second is two characters times. If a second character came while the printhead was in the middle of a return, that character was lost.

---

Pratical C by Steve Oualline
So they made the return two characters as to not miss anything. Then when newer computers came out some(windows) of them decided to keep it that way.
Unix uses <LINE FEED>
MS-DOS/Windows uses <LINE FEED> <RETURN>
Apple uses <RETURN>

>Pratical C by Steve Oualline
Don't read that book, it's bad for your health. ;)

lol:)