Question

I have written a password program in c. and I saved the password in a text file using some encryptiion and retrieving the old password from a file using decryption techniques So that user cannot see the password.Yes ofcourse this might not be the efficient way to do because there is a chance for user to delete the text file.

But still I want to lock (protect) a file using my c password code.I have searched on google I found perl can be used to lock a file.The question is that possible in c? Do we have any function like flock in perl? or suggest me some functions that can be helpful to develop a file lock function in c.becuase People may say we have c# and lots of stuff out to lock a file and why your sticking to c? but I like challenging the things.So please let me know If theres something I need to learn.Thank you in advance.

Answer 1

You can, on a POSIX system, use fcntl() along with the F_SETLK value for the command argument, and then set F_WRLCK (i.e, and exclusive lock) in the struct flock data-member l_type.

So for instance (add your own error-detection mechanisms):

#include <fcntl.h>
#include <unistd.h>

struct flock fl;
int file_descriptor;
int file_length;

fd = open("my_password_file", O_RDWR);

//... get the length of the file and set the variable file_length

fl.l_type = F_WRLCK;
fl.l_whence = SEEK_SET;
fl.l_start = 0;
fl.l_len = file_length;

//set the exclusive-lock on the file
fcntl(fd, F_SETLK, &fl);