getpass - phpMan

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File: libc.info,  Node: getpass,  Next: crypt,  Prev: Legal Problems,  Up: Cryptographic Functions

33.2 Reading Passwords
======================

When reading in a password, it is desirable to avoid displaying it on
the screen, to help keep it secret.  The following function handles this
in a convenient way.

 -- Function: char * getpass (const char *PROMPT)
     Preliminary: | MT-Unsafe term | AS-Unsafe heap lock corrupt |
     AC-Unsafe term lock corrupt | *Note POSIX Safety Concepts::.

     'getpass' outputs PROMPT, then reads a string in from the terminal
     without echoing it.  It tries to connect to the real terminal,
     '/dev/tty', if possible, to encourage users not to put plaintext
     passwords in files; otherwise, it uses 'stdin' and 'stderr'.
     'getpass' also disables the INTR, QUIT, and SUSP characters on the
     terminal using the 'ISIG' terminal attribute (*note Local Modes::).
     The terminal is flushed before and after 'getpass', so that
     characters of a mistyped password are not accidentally visible.

     In other C libraries, 'getpass' may only return the first
     'PASS_MAX' bytes of a password.  The GNU C Library has no limit, so
     'PASS_MAX' is undefined.

     The prototype for this function is in 'unistd.h'.  'PASS_MAX' would
     be defined in 'limits.h'.

   This precise set of operations may not suit all possible situations.
In this case, it is recommended that users write their own 'getpass'
substitute.  For instance, a very simple substitute is as follows:


     #include <termios.h>
     #include <stdio.h>

     ssize_t
     my_getpass (char **lineptr, size_t *n, FILE *stream)
     {
       struct termios old, new;
       int nread;

       /* Turn echoing off and fail if we can't. */
       if (tcgetattr (fileno (stream), &old) != 0)
         return -1;
       new = old;
       new.c_lflag &= ~ECHO;
       if (tcsetattr (fileno (stream), TCSAFLUSH, &new) != 0)
         return -1;

       /* Read the password. */
       nread = getline (lineptr, n, stream);

       /* Restore terminal. */
       (void) tcsetattr (fileno (stream), TCSAFLUSH, &old);

       return nread;
     }

   The substitute takes the same parameters as 'getline' (*note Line
Input::); the user must print any prompt desired.


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