Overwrite the Contents of a File in C - c

Question

I am writing a code that forks multiple processes. They all share a file called "character." what I want to do is to have every process read the 'only character' in the file and then erase it by putting its own character so the other process can do the same. The file is the only way the processes can communicate each other. How can I erase the 'only character' in the file and put a new one in its place. I was advise to use freopen() (which closes the file and reopens it erasing its previous contents) but I am not sure if it is the best way to achieve this.

Answer 1

You should not have to reopen the file. That gains you nothing. If you're worried about each process buffering input or output, disable buffering if you want to use FILE *-based stdio functions.
But if I'm reading your question correctly (you want each process to replace the one character in the file when it's a specific value is held in the file, and that value changes for each process), this will do what you want, using POSIX open() pread(), and pwrite() (you're already using POSIX fork(), so using low-level POSIX IO makes things a lot simpler - note that pread() and pwrite() eliminate the need for seeking.)
I'll say this is what I think you're trying to do:
// header files and complete error checking is omitted for clarity
int fd = open( filename, O_RDWR );
// fork() here?
// loop until we read the char we want from the file
for ( ;; )
{
char data;
ssize_t result = pread( fd, &data, sizeof( data ), 0 );
// pread failed
if ( result != sizeof( data ) )
{
break;
}
// if data read matches this process's value, replace the value
// (replace 'a' with 'b', 'c', 'z' or '*' - whatever value you
// want the current process to wait for)
if ( data == 'a' )
{
data = 'b';
result = pwrite( fd, &data, sizeof( data ), 0 );
break;
}
}
close( fd );
For any decent number of processes, that's going to put a lot of stress on your filesystem.
If you really want to start with fopen() and use that family of calls, this might work depending on your implementation:
FILE *fp = fopen( filename, "rb+" );
// disable buffering
setbuf( fd, NULL );
// fork() here???
// loop until the desired char value is read from the file
for ( ;; )
{
char data;
// with fread(), we need to fseek()
fseek( fp, 0, SEEK_SET );
int result = fread( &data, 1, 1, fp );
if ( result != 1 )
{
break;
}
if ( data == 'a' )
{
data = 'b';
fseek( fp, 0, SEEK_SET );
fwrite( &data, 1, 1, fp );
break;
}
}
fclose( fp );
Again, that assumes I'm reading your question properly. Note that the POSIX rules John Bollinger mentioned in his comments regarding multiple handles don't apply - because the streams are explicitly not buffered.