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.
Related
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
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.
All I can find using fopen() and fwrite() in C is to delete all contents and start writing again or append to the end of the file. What if I need to go to some offset in the file and overwrite a few bytes?
Is that possible with some function?
You can open the file with the "rb+" option and then use fseek with SEEK_SET to go to a specific location. Therb+ opens it for both reading and writing as a binary file (the file must exist in order for it to succeed - it will not create a new file).
I have a small code block that should append text to the beg of a file. However it still only adds to the end of the file. I thought the rewind set the pointer to the front of the file, thus when I added the text using fprintf it should add to the front. How can I change this?
fp = fopen("Data.txt", "a");
rewind(fp);
fprintf(fp, "%s\n", text);
fclose(fp);
Text is a char array to be added at the front of the file
1) Don't open in append mode.
When you open in append mode, all writes go to the end of the file, regardless of the seek position.
http://www.opengroup.org/onlinepubs/009695399/functions/fopen.html
Opening a file with append mode (a as
the first character in the mode
argument) shall cause all subsequent
writes to the file to be forced to the
then current end-of-file, regardless
of intervening calls to fseek().
2) Opening without "a" still won't do what you want. It's not possible to insert into a file using the ANSI/POSIX file operations, because given the way most file systems store their data, insert is not a simple operation.
You need either to open a new file, write your new data, then append the old file afterwards, or else you need to mess around shuffling data forward in blocks. Either option is very inefficient for large files, compared with appending at the end, not to mention error-prone if you need the program or the machine to be able to unexpectedly die without corrupting data. So if this is a log file or similar, it's probably worth redesigning so that you can write new data to the end, and then reverse it all when you prepare a report from the log.
You can replace data in a file, but you can't prepend or insert it anywhere but at the very end of the file.
Just create a new file with your data and then append the old data in this file you created.