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| <h3 class="section">32.2 Reading Passwords</h3> |
| |
| <p>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. |
| |
| <!-- unistd.h --> |
| <!-- BSD --> |
| <div class="defun"> |
| — Function: char * <b>getpass</b> (<var>const char *prompt</var>)<var><a name="index-getpass-3676"></a></var><br> |
| <blockquote> |
| <p><code>getpass</code> outputs <var>prompt</var>, then reads a string in from the |
| terminal without echoing it. It tries to connect to the real terminal, |
| <samp><span class="file">/dev/tty</span></samp>, if possible, to encourage users not to put plaintext |
| passwords in files; otherwise, it uses <code>stdin</code> and <code>stderr</code>. |
| <code>getpass</code> also disables the INTR, QUIT, and SUSP characters on the |
| terminal using the <code>ISIG</code> terminal attribute (see <a href="Local-Modes.html#Local-Modes">Local Modes</a>). |
| The terminal is flushed before and after <code>getpass</code>, so that |
| characters of a mistyped password are not accidentally visible. |
| |
| <p>In other C libraries, <code>getpass</code> may only return the first |
| <code>PASS_MAX</code> bytes of a password. The GNU C library has no limit, so |
| <code>PASS_MAX</code> is undefined. |
| |
| <p>The prototype for this function is in <samp><span class="file">unistd.h</span></samp>. <code>PASS_MAX</code> |
| would be defined in <samp><span class="file">limits.h</span></samp>. |
| </p></blockquote></div> |
| |
| <p>This precise set of operations may not suit all possible situations. In |
| this case, it is recommended that users write their own <code>getpass</code> |
| substitute. For instance, a very simple substitute is as follows: |
| |
| <pre class="smallexample"> #include <termios.h> |
| #include <stdio.h> |
| |
| ssize_t |
| my_getpass (char **lineptr, size_t *n, FILE *stream) |
| { |
| struct termios old, new; |
| int nread; |
| |
| /* <span class="roman">Turn echoing off and fail if we can't.</span> */ |
| if (tcgetattr (fileno (stream), &old) != 0) |
| return -1; |
| new = old; |
| new.c_lflag &= ~ECHO; |
| if (tcsetattr (fileno (stream), TCSAFLUSH, &new) != 0) |
| return -1; |
| |
| /* <span class="roman">Read the password.</span> */ |
| nread = getline (lineptr, n, stream); |
| |
| /* <span class="roman">Restore terminal.</span> */ |
| (void) tcsetattr (fileno (stream), TCSAFLUSH, &old); |
| |
| return nread; |
| } |
| </pre> |
| <p>The substitute takes the same parameters as <code>getline</code> |
| (see <a href="Line-Input.html#Line-Input">Line Input</a>); the user must print any prompt desired. |
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