If I open a file with mode a+ (for reading and writing), how do I set the stream back at the start of the file in D?
What I want to do is read the file, and than continuously keep updating it.
File.seek(0) should do it:
http://dlang.org/phobos/std_stdio.html#seek
works like C's fseek
Related
Well I was looking into ways to open a file in binary and I saw that you can open one for r+b and a+b but whats the difference because this is what it says for definition.
a+b :Open a file for both reading and writing in binary mode.
r+b: Open a file for both reading and writing in binary mode.
a+b
Open for reading and appending (writing at end of file). The file is created if it does not exist. The initial file position for reading is at the beginning of the file, but output is always appended to the end of the file.
r+b Open for reading and writing. The stream is positioned at the beginning of the file.
w+b Open for reading and writing. The file is created if it does not exist, otherwise it is truncated. The stream is positioned at the beginning of the file.
So, a+b will create the file if it does not exist, and it will append to the file when you write to it,
r+b will not create the file if it doesn't already exist (fopen() would fail), and writing to the file will start at the beginning of the file.
I am working on a sever/client applicataion. I want to maintain information of all active clients in a text file named "Information.txt".
I update this text file after every 3 seonds. So, I want the text file to clear all of its contents after every 3 seconds without deleting the file.
Is there any way to do it ? :(
I don't want to use freopen().
A problem with clearing the file periodically is that if your process crashes after the file has been cleared but before it has been written, you lose data: the old data is gone, but the new data is not there yet.
A common approach to this problem is to create a new file, writing it, and then moving the new file to replace the old one. This way you always have a file, and sometimes (for very brief periods of time) you have two files.
Try with
fopen(filename, flag)
Open your file with flag= "w" or "wb" and it will be cleared
Just open the file with fopen and setting the flag to w or w+ or wb
From fopen man page
w
Truncate file to zero length or create text file for writing. The
stream is positioned at the beginning of the file.
w+
Open for reading and writing. The file is created if it does not
exist, otherwise it is truncated. The stream is positioned at the
beginning of the file.
It might be a very dumb question but I am modifying someone else's code and it seems I need to read from a file that was opened in append mode. I tried to fseek to the beginning of the file but nothing is being read.
I know I can change the mode to rw but I wanted to know why fseek is not working. In the man page it does say write ignores fseek but nothing about read though.
There is just one pointer which initially is at the start of the file but when a write operation is attempted it is moved to the end of the file. You can reposition it using fseek or rewind anywhere in the file for reading, but writing operations will move it back to the end of file.
When you open in append mode, the file pointer is returned to the end of file before every write. You can reposition the pointer with fseek for reading, but as soon as you call a function that writes to the file, the pointer goes back to the end of file.
The answer at Does fseek() move the file pointer to the beginning of the file if it was opened in "a+b" mode? references the appropriate section of the C standard.
Use the "w+" mode if you would like to write to arbitrary places in file. An existing file will be overwritten.
If you would like to append to an existing file initially, but then fseek to arbitrary place, use "r+" followed by
fseek(f, 0, SEEK_END)
Hope it helps..
Consider the following scenario: I am opening a tar file (say abc.tar.gz), writing the data, and before closing the file descriptor, I am trying to extract the same file.
I am unable to do so. But if I extract the file after having closing the fd, it works fine.
I wonder what could be the reason.
All files has a position where data is read or written. After writing to the file, the position is at the end. Trying to read will attempt to read from that position. You have to change the position to the beginning of the file with a function like lseek.
Also, you did open the file in both read and write mode?
Edit
After reading your comments, I see you do not actually read the file from inside your program, but from an external program. Then it might be as simple as you not flushing the file to disk, which happens automatically when closing a file. You might want to check the fsync function for that, or possible the sync function.
If a file is opened using the following command:
FILE *f1=fopen("test.dat","a+");
The man page reads:
a+
Open for reading and appending (writing at end of file). The
file is created if it does not exist. The initial file position
for reading is at the beginning of the file, but output is
always appended to the end of the file.
So does f1 have 2 separate offset pointers, one for read & another for write?
No.
There is just one pointer which initially is at the start of the file but when a write operation is attempted it is moved to the end of the file. You can reposition it using fseek or rewind anywhere in the file for reading, but writing operations will move it back to the end of file.
No it has only one pointer.
You can never mix reading and writing operations on a FILE without calling fseek in between. It may work as you wish on some implementations, but a program that depends on this has undefined behavior. Thus the questions of having 2 positions is meaningless.