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| <h4 class="subsection">17.4.8 Line Speed</h4> |
| |
| <p><a name="index-line-speed-1924"></a><a name="index-baud-rate-1925"></a><a name="index-terminal-line-speed-1926"></a><a name="index-terminal-line-speed-1927"></a> |
| The terminal line speed tells the computer how fast to read and write |
| data on the terminal. |
| |
| <p>If the terminal is connected to a real serial line, the terminal speed |
| you specify actually controls the line—if it doesn't match the |
| terminal's own idea of the speed, communication does not work. Real |
| serial ports accept only certain standard speeds. Also, particular |
| hardware may not support even all the standard speeds. Specifying a |
| speed of zero hangs up a dialup connection and turns off modem control |
| signals. |
| |
| <p>If the terminal is not a real serial line (for example, if it is a |
| network connection), then the line speed won't really affect data |
| transmission speed, but some programs will use it to determine the |
| amount of padding needed. It's best to specify a line speed value that |
| matches the actual speed of the actual terminal, but you can safely |
| experiment with different values to vary the amount of padding. |
| |
| <p>There are actually two line speeds for each terminal, one for input and |
| one for output. You can set them independently, but most often |
| terminals use the same speed for both directions. |
| |
| <p>The speed values are stored in the <code>struct termios</code> structure, but |
| don't try to access them in the <code>struct termios</code> structure |
| directly. Instead, you should use the following functions to read and |
| store them: |
| |
| <!-- termios.h --> |
| <!-- POSIX.1 --> |
| <div class="defun"> |
| — Function: speed_t <b>cfgetospeed</b> (<var>const struct termios *termios-p</var>)<var><a name="index-cfgetospeed-1928"></a></var><br> |
| <blockquote><p>This function returns the output line speed stored in the structure |
| <code>*</code><var>termios-p</var>. |
| </p></blockquote></div> |
| |
| <!-- termios.h --> |
| <!-- POSIX.1 --> |
| <div class="defun"> |
| — Function: speed_t <b>cfgetispeed</b> (<var>const struct termios *termios-p</var>)<var><a name="index-cfgetispeed-1929"></a></var><br> |
| <blockquote><p>This function returns the input line speed stored in the structure |
| <code>*</code><var>termios-p</var>. |
| </p></blockquote></div> |
| |
| <!-- termios.h --> |
| <!-- POSIX.1 --> |
| <div class="defun"> |
| — Function: int <b>cfsetospeed</b> (<var>struct termios *termios-p, speed_t speed</var>)<var><a name="index-cfsetospeed-1930"></a></var><br> |
| <blockquote><p>This function stores <var>speed</var> in <code>*</code><var>termios-p</var> as the output |
| speed. The normal return value is 0; a value of -1 |
| indicates an error. If <var>speed</var> is not a speed, <code>cfsetospeed</code> |
| returns -1. |
| </p></blockquote></div> |
| |
| <!-- termios.h --> |
| <!-- POSIX.1 --> |
| <div class="defun"> |
| — Function: int <b>cfsetispeed</b> (<var>struct termios *termios-p, speed_t speed</var>)<var><a name="index-cfsetispeed-1931"></a></var><br> |
| <blockquote><p>This function stores <var>speed</var> in <code>*</code><var>termios-p</var> as the input |
| speed. The normal return value is 0; a value of -1 |
| indicates an error. If <var>speed</var> is not a speed, <code>cfsetospeed</code> |
| returns -1. |
| </p></blockquote></div> |
| |
| <!-- termios.h --> |
| <!-- BSD --> |
| <div class="defun"> |
| — Function: int <b>cfsetspeed</b> (<var>struct termios *termios-p, speed_t speed</var>)<var><a name="index-cfsetspeed-1932"></a></var><br> |
| <blockquote><p>This function stores <var>speed</var> in <code>*</code><var>termios-p</var> as both the |
| input and output speeds. The normal return value is 0; a value |
| of -1 indicates an error. If <var>speed</var> is not a speed, |
| <code>cfsetspeed</code> returns -1. This function is an extension in |
| 4.4 BSD. |
| </p></blockquote></div> |
| |
| <!-- termios.h --> |
| <!-- POSIX.1 --> |
| <div class="defun"> |
| — Data Type: <b>speed_t</b><var><a name="index-speed_005ft-1933"></a></var><br> |
| <blockquote><p>The <code>speed_t</code> type is an unsigned integer data type used to |
| represent line speeds. |
| </p></blockquote></div> |
| |
| <p>The functions <code>cfsetospeed</code> and <code>cfsetispeed</code> report errors |
| only for speed values that the system simply cannot handle. If you |
| specify a speed value that is basically acceptable, then those functions |
| will succeed. But they do not check that a particular hardware device |
| can actually support the specified speeds—in fact, they don't know |
| which device you plan to set the speed for. If you use <code>tcsetattr</code> |
| to set the speed of a particular device to a value that it cannot |
| handle, <code>tcsetattr</code> returns -1. |
| |
| <p><strong>Portability note:</strong> In the GNU library, the functions above |
| accept speeds measured in bits per second as input, and return speed |
| values measured in bits per second. Other libraries require speeds to |
| be indicated by special codes. For POSIX.1 portability, you must use |
| one of the following symbols to represent the speed; their precise |
| numeric values are system-dependent, but each name has a fixed meaning: |
| <code>B110</code> stands for 110 bps, <code>B300</code> for 300 bps, and so on. |
| There is no portable way to represent any speed but these, but these are |
| the only speeds that typical serial lines can support. |
| |
| <!-- termios.h --> |
| <!-- POSIX.1 --> |
| <p><a name="index-B0-1934"></a><!-- termios.h --> |
| <!-- POSIX.1 --> |
| <a name="index-B50-1935"></a><!-- termios.h --> |
| <!-- POSIX.1 --> |
| <a name="index-B75-1936"></a><!-- termios.h --> |
| <!-- POSIX.1 --> |
| <a name="index-B110-1937"></a><!-- termios.h --> |
| <!-- POSIX.1 --> |
| <a name="index-B134-1938"></a><!-- termios.h --> |
| <!-- POSIX.1 --> |
| <a name="index-B150-1939"></a><!-- termios.h --> |
| <!-- POSIX.1 --> |
| <a name="index-B200-1940"></a><!-- termios.h --> |
| <!-- POSIX.1 --> |
| <a name="index-B300-1941"></a><!-- termios.h --> |
| <!-- POSIX.1 --> |
| <a name="index-B600-1942"></a><!-- termios.h --> |
| <!-- POSIX.1 --> |
| <a name="index-B1200-1943"></a><!-- termios.h --> |
| <!-- POSIX.1 --> |
| <a name="index-B1800-1944"></a><!-- termios.h --> |
| <!-- POSIX.1 --> |
| <a name="index-B2400-1945"></a><!-- termios.h --> |
| <!-- POSIX.1 --> |
| <a name="index-B4800-1946"></a><!-- termios.h --> |
| <!-- POSIX.1 --> |
| <a name="index-B9600-1947"></a><!-- termios.h --> |
| <!-- POSIX.1 --> |
| <a name="index-B19200-1948"></a><!-- termios.h --> |
| <!-- POSIX.1 --> |
| <a name="index-B38400-1949"></a><!-- termios.h --> |
| <!-- GNU --> |
| <a name="index-B57600-1950"></a><!-- termios.h --> |
| <!-- GNU --> |
| <a name="index-B115200-1951"></a><!-- termios.h --> |
| <!-- GNU --> |
| <a name="index-B230400-1952"></a><!-- termios.h --> |
| <!-- GNU --> |
| <a name="index-B460800-1953"></a> |
| <pre class="smallexample"> B0 B50 B75 B110 B134 B150 B200 |
| B300 B600 B1200 B1800 B2400 B4800 |
| B9600 B19200 B38400 B57600 B115200 |
| B230400 B460800 |
| </pre> |
| <p><a name="index-EXTA-1954"></a><a name="index-EXTB-1955"></a>BSD defines two additional speed symbols as aliases: <code>EXTA</code> is an |
| alias for <code>B19200</code> and <code>EXTB</code> is an alias for <code>B38400</code>. |
| These aliases are obsolete. |
| |
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