I have a function that takes in a file pointer (FILE * file) and copies everything in that file. Now, is it possible to erase everything inside this file?
I do not have the file name so I can not use fopen(filename, "w") again.
I have stdio.h and string.h included.
You can use the ftruncate() system call to empty a file using its file descriptor, e.g.
ftruncate(fileno(fh), 0);
You will probably want to follow that up with a call to rewind(fh) so that any further writes to the file are made at the beginning, rather than at the previous offset.
Related
I am just creating a basic file handling program.
the code is this:
#include <stdio.h>
int main()
{
FILE *p;
p=fopen("D:\\TENLINES.TXT","r");
if(p==0)
{
printf("Error",);
}
fclose(p);
}
This is giving Error, I cannot create files tried reinstalling the compiler and using different locations and names for files but no success.
I am using Windows 7 and compiler is Dev C++ version 5
Change the mode argument in fopen(const char *filename, const char *mode) from:
p=fopen("D:\\TENLINES.TXT","r");//this will not _create_ a file
if(p==0) // ^
To this:
p=fopen("D:\\TENLINES.TXT","w");//this will create a file for writing.
if(p==NULL) // ^ //If the file already exists, it will write over
//existing data.
If you want to add content to an existing file, you can use "a+" for the open mode.
See fopen() (for more open modes, and additional information about the fopen family of functions)
According to tutorial, fopen returns NULL when error occurs. Therefore, you should check if p equals NULL.
Also, in printf("Error",);, omit the comma after string.
Yes you should open the file in write mode.
Which creates the file . Read mode is only to read content
or else you can use "r+" for both read and write.
You should be able to open the file, but you need to make it first. Make a txt document with the name res.txt. It should be able to write your result into the text document.
<?php
$result = $variable1 . $variable2 "=" .$res ."";
echo $result;
$myfile = fopen("res.txt", "a+") or die("nope");
fwrite($myfile, $result);
fclose($myfile)
?>
fopen()
Syntax:
FILE *fp;
fp=fopen(“data.txt”,”r”);
if(fp!=NULL){
//file operations
}
It is necessary to write FILE in the uppercase. The function fopen() will open a file “data.txt”
in read mode.
The fopen() performs the following important task.
It searches the disk for opening the file.
In case the file exists, it loads the file from the disk into memory. If the file is found with huge contents then it loads the file part by part.
If the file does not exist this function returns a NULL. NULL is a macro defined character in the header file “stdio.h”. This indicates that it is unable to open file. There may be following reasons for failure of fopen() functions.
a.When the file is in protected or hidden mode.
b.The file may be used by another program.
It locates a character pointer, which points the first cha
racter of the file. Whenever a file is
opened the character pointer points to the first character 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.
How do I close a file and remove it?
I have the following code:
FILE *filePtr = fopen("fileName", "w");
...
Now I want to close filePtr and remove the file "fileName".
Should I:
fclose(filePtr);
remove("fileName");
Or:
remove("fileName");
fclose(filePtr);
Does it matter which I do first?
Thanks!!
That is OS-dependent. On *nix, deleting an open file leaves it open and the data on disk, but removes the filename from the filesystem, and actually deletes the file on close; some other operating systems may not let you delete an open file at all. Therefore the former is recommended for maximum portability.
It makes more sense to fclose and then unlink.
As man unlink(2) says (for Unix systems) :
The unlink() function removes the link
named by path from its directory and
decrements the link count of the file
which was referenced by the link. If
that decrement reduces the link count
of the file to zero, and no process
has the file open, then all resources
associated with the file are
reclaimed. If one or more process
have the file open when the last link
is removed, the link is removed, but
the removal of the file is delayed
until all references to it have been
closed.
So the order doesn't matter at all.
You do not need to fopen a file to remove it. But, in linux, if you remove an fopened file, it will be deleted only after closing it. You can still read/write to it.
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).