I made a C program that reads a string from a .txt file, then it encrypts the string, and finally it writes the string in the same file.
The thing is that if I use fopen("D:\\Prueba.txt","w+"), the program doesn't work, it prints garbage like this )PHI N.
I've debugged and I know the error is there in that line, because if I use fopen("D:\\Prueba.txt","r+"), the program works, and it writes what it should.
But I want to use w+ because it will rewrite what the .txt file had. Why is w+ not working?
If you're opening with w+ to first read the content, that's not going to work. From C11:
w+: truncate to zero length or create text file for update.
What's probably happening is that you read data from the now empty file but don't correctly check that it worked. That would explain the weird "content" you see of )PHI N.
One solution is to open the file as with r, open another file with w, and transfer the contents, encrypting them as part of that process. Then close both, delete the original, and rename the new one to the original name. This will allow you to process arbitrarily-sized files since you process them a bit at a time.
If you don't want to use a temporary file, and you're sure you can store the entire content in memory, you could open it r+, get the content, the reopen it with a new mode, such as with:
FILE *readFh = fopen( "myfile.txt", "r+");
// Read in content, massage as needed.
FILE *writeFh = frepoen( NULL, "w+", readFh);
// Provided that worked, you should now have an empty file to write to.
// Write back your massaged data.
Related
Here's my issue: I have a file with the highscore written in it (just the first line, no nicknames, just the highscore), I need to read that line and compare it with the actual score obtained in the game session, if the score is higher, overwrite the file with the new value, but if I try to read it I get a null value... Seems like i'm not reading it the right way. What's wrong with my code?
Thanks for the help!
local path = system.pathForFile( "data.sav", system.DocumentsDirectory )
local file = io.open( path, "w+" )
highscore_letta = file:read("*n")
print(highscore_letta)
if (_G.player_score > tonumber(highscore_letta)) then
file:write(_G.player_score)
end
io.close( file )
I had this problem myself. I found out that if you open a file in "w+" mode, the current contents are deleted, so that you can write new contents. So to read and write you have to open the file twice. First, you open the file in "rb" mode and get the file contents, then close it. Then you reopen it in "wb" mode, write the new number, and close it.
In Windows, you need "b" in the file mode. Otherwise, the strings that you are reading and writing may be modified in unexpected ways: for instance, a newline ("\n") may be replaced with carriage return–newline ("\r\n").
The file modes that Lua supports are borrowed from the C language. (I found a description on page 305 of what I guess is a draft of the C specification.) I think the Lua manual sort of assumes that you will know what these modes mean, as an experienced C programmer would, but to me it wasn't at all obvious.
Thus to read a number and then write a new one:
local filepath = "path/to/file"
-- Create a file handle that will allow you to read the current contents.
local read_file = io.open(filepath, "rb")
number = read_file:read "*n" -- Read one number. In Lua 5.3, use "n"; the asterisk is not needed.
read_file:close() -- Close the file handle.
local new_number = 0 -- Replace this with the number you actually want to write.
-- Create a file handle that allows you to write new contents to the file,
-- while deleting the current contents.
write_file = io.open(filepath, "wb")
write_file:write(new_number) -- Overwrite the entire contents of the file.
write_file:flush() -- Make sure the new contents are actually saved.
write_file:close() -- Close the file handle.
I created a script to do these operations automatically, as they're somewhat annoying to type every time.
The mode "r+" or "r+b" is supposed to allow you to read and write, but I couldn't get it to work when the original contents are longer than the new contents. If the original contents are "abcd", four bytes, and the new contents are "efg", three bytes, and you write at offset 0 in the file, the file will now have "efgd": the last byte of the original contents is not deleted.
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.
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.
I need to add a string before the 45th byte in an existing file. I tried using fseek as shown below.
int main()
{
FILE *fp;
char str[] = "test";
fp = fopen(FILEPATH,"a");
fseek(fp,-45, SEEK_END);
fprintf(fp,"%s",str);
fclose(fp);
return(0);
}
I expected that this code will add "test" before the 45th char from EOF, instead, it just appends "test" to the EOF.
Please help me to find the solution.
This is continuation of my previous question
Append item to a file before last line in c
Open it with mode r+ (if it already exists) or a+ (if it doesn't exist and you want to create it). Since you're seeking to 45 bytes before the end of file, I'm assuming it already exists.
fp = fopen(FILEPATH,"r+");
The rest of your code is fine. Also note that this will not insert the text, but will overwrite whatever is currently at that position in the file.
ie, if your file looks like this:
xxxxxxxxxxxxxxxxxxxxxxxxxx
xxxxxxxxxxxxxxxxxxxxxxxxxx
Then after running this code, it will look like this:
xxxxxxxtestxxxxxxxxxxxxxxx
xxxxxxxxxxxxxxxxxxxxxxxxxx
If you really want to insert and not overwrite, then you need to read all the text from SEEK_END-45 to EOF into memory, write test and then write the text back
Don't open it as append (a) if you plan to write at arbitrary positions; it will force all writes to the end of the file. You can use r+ to read or write anywhere.
To avoid platform-specific configurations, always explicitely indicate the binary or text mode in your fopen() call.
This will save you hours of desperations if you port your code one day.
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.