I want to open a file for write and I have been given the file descriptor for that file.
I know that fdopen can be used to write to the file as follows:
FILE * fp;
fp = fdopen(filedes, "a+");
fwrite("\n", sizeof(char), 1, fp);
Is there any other way to achive the same, i.e., write to a file given a file descriptor?
Thanks
Is there any other way to achive the same, i.e., write to a file given
a file descriptor
You can write directly using the system call write(2).
write(fd, "\n", 1);
You can write a buffer of data to a file descriptor with the write system call.
Related
Implement the Caesar Cipher algorithm to encrypt and decrypt a file contents using C language. The cipher basic all use algorithm . Your program should have two C files named encrypt.c and decrypt.c that contains encrypt() and decrypt() functions correspondently for the purpose. In the encryption.c file, use the main() function to take input from a “input.txt” file and store the encrypted message to “enc_msg.txt” file. In the decryption.c file, use the main() function to take input from a “enc_msg.txt” file and store the decrypted message to “dec_msg.txt” file and print the decrypted message in console output as well. The key is 3.
Thanks
Create two .c files encrypt.c and decrypt.c
Create sample data input.txt file
Run your encrypt program to create output file enc_msg.txt from input.txt file
Run your decrypt program to create output file dec_msg.txt from input enc_msg.txt file
So you need to create 3 files encrypt.c decrypt.c and input.txt
And running your programs will generate two more files enc_msg.txt and dec_msg.txt
I have a 5GB 1 liner file with JSON data and each line starts from this pattern "{"created". I need to be able to use Unix commands on my Mac to convert this monster of a 1 liner into as many lines as it deserves. Any commands?
ASCII English text, with very long lines, with no line terminators
If you have enough memory you can open the file once with the TextWrangler application (the free BBEdit cousin) and use regular search/replace on the whole file. Use \r in replace to add a return. Will be very slow at opening the file, may even hang if low on memory, but in the end it may probably work. No scripting, no commands,.. etc.. I did this with big SQL files and sometimes it did the job.
You have to replace your line-start string with the same string with \n or \r or \r\n in front of it.
Unclear how it can be a “one liner” file but then each line starts with "{"created", but perhaps python -mjson.tool can help you get started:
cat your_source_file.json | python -mjson.tool > nicely_formatted_file.json
Piping raw JSON through ``python -mjson.tool` will cleanly format the JSON to be more human readable. More info here.
OS X ships with both flex and bison, you can use those to write a parser for your data.
You can use PHP as a shell command (if PHP is installed), just save a text file with name "myscript" and appropriate code (I cannot test code now, but the idea is as follows)
UNTESTED CODE
#!/usr/bin/php
<?php
$REPLACE_STRING='{"created'; // anything you like
// open input file with fopen() in read mode
$inFp=fopen('big_in_file.txt', "r");
// open output file with fopen() in write mode
$outFp=fopen('big_out_file.txt', "w+");
// while not end of file
while (!feof($inFp)) {
// read file chunks here with fread() in variable $chunk
$chunk = fread($inFp, 8192);
// do a $chunk=str_replace($REPLACE_STRING,"\r".$REPLACE_STRING; // to add returns
// (or use \r\n for windows end of lines)
$chunk=str_replace($REPLACE_STRING,"\r".$REPLACE_STRING,$chunk);
// problem: if chunk contains half the string at the end
// easily solved if $REPLACE_STRING is a one char like '{'
// otherwise test for fist char { in the end of $chunk
// remove final part and save it in a var for nest iteration
// write $chunk to output file
fwrite($outFp, $chunk);
// End while
}
?>
After you save it you must make it executable whith sudo chmod a+x ./myscript
and then launch it as ./myscript in terminal
After this, the myscript file is a full unix command
I would like to determine real file extension.
example :
file = "test.fakeExt"
// but the real extention is .exe // for security reason I wish to avoid using it!
How can I do that?
If you want to determine the extension you could use findmimefromdata.
It looks at the first part of the file to determine what type of file it is.
FindMimeFromData function
Sample code
The first two bytes of an .exe file are allways 'MZ'.
So you could read the binary file, and see if the first two bytes are MZ, then you know it's an .exe file...
I wrote a script that executes commands in parallel. I let them all write an entry to the same log file. It does not matter if the order is wrong or entries are interleaved, but i noticed that some entries are missing. I should probably lock the file before writing, however, is it true that if multiple processes try to write to a file simultaneously, it will result in missing entries?
Yes, if different processes independently open and write to the same file, it may result in overlapping writes and missing data. This happens because each process will get its own file pointer, that advances only by local writes.
Instead of locking, a better option might be to open the log file once in an ancestor of all worker processes, have it inherited across fork(), and used by them for logging. This means that there will be a single shared file pointer, that advances when any of the processes writes a new entry.
In a script you should use ">> file" (double greater than) to append output to that file. The interpreter will open the destination in "append" mode. If your program also wants to append, follow the directives below:
Open a text file in "append" mode ("a+") and give preference to printing only full lines (don't do multiple 'print' followed by a final 'println', but print the entire line with a single 'println').
The fopen documentation states this:
DESCRIPTION
The fopen() function opens the file whose pathname is the
string pointed to by filename, and associates a stream with
it.
The argument mode points to a string beginning with one of
the following sequences:
r or rb Open file for reading.
w or wb Truncate to zero length or create file
for writing.
a or ab Append; open or create file for writing
at end-of-file.
r+ or rb+ or r+b Open file for update (reading and writ-
ing).
w+ or wb+ or w+b Truncate to zero length or create file
for update.
a+ or ab+ or a+b Append; open or create file for update,
writing at end-of-file.
The character b has no effect, but is allowed for ISO C
standard conformance (see standards(5)). Opening a file with
read mode (r as the first character in the mode argument)
fails if the file does not exist or cannot be read.
Opening a file with append mode (a as the first character in
the mode argument) causes all subsequent writes to the file
to be forced to the then current end-of-file, regardless of
intervening calls to fseek(3C). If two separate processes
open the same file for append, each process may write freely
to the file without fear of destroying output being written
by the other. The output from the two processes will be
intermixed in the file in the order in which it is written.
It is because of this intermixing that you want to give preference to
using only 'println' (or its equivalent).
I'm using the SD.h library to write onto an SD card with the Arduino Uno. I need to write out in a file a template string with some placeholder replaced by certain values, in the way of printf behaves. I would use the fprintf function, but when I tried this:
File dataFile = SD.open("myfile.txt", FILE_WRITE);
fprintf(dataFile, "mynumber: %d\n", 100);
I got this error:
cannot convert 'File*' to '__file*' for argument '1' to 'int
fprintf(__file*, const char*, ...)'
How can I manage this?
printf() makes your executable object ~1000 bytes larger, so you may not want to use it if size is a problem.
The fprintf is not intended to use with the SD.h so I think
The simple solution that comes into my mind is using sprintf to format your text then write it to the file with the println function
File dataFile = SD.open("myfile.txt", FILE_WRITE);
char text[100];
sprintf(text,"My number: %d",yournumber);
dataFile.println(text);