#include <termios.h> speed_t cfgetispeed(const struct termios *termios_p);
The cfgetospeed(), cfgetispeed(), cfsetospeed(), and cfsetispeed() functions do not take arguments as numbers, but rather as symbolic names. There are two reasons for this:
There is nothing to prevent an implementation accepting as an extension a number (such as 126), and since the encoding of the Bxxx symbols is not specified, this can be done to avoid introducing ambiguity.
Setting the input baud rate to zero was a mechanism to allow for split baud rates. Clarifications in this volume of POSIX.1-2017 have made it possible to determine whether split rates are supported and to support them without having to treat zero as a special case. Since this functionality is also confusing, it has been declared obsolescent. The 0 argument referred to is the literal constant 0, not the symbolic constant B0. This volume of POSIX.1-2017 does not preclude B0 from being defined as the value 0; in fact, implementations would likely benefit from the two being equivalent. This volume of POSIX.1-2017 does not fully specify whether the previous cfsetispeed() value is retained after a tcgetattr() as the actual value or as zero. Therefore, conforming applications should always set both the input speed and output speed when setting either.
In historical implementations, the baud rate information is traditionally kept in c_cflag. Applications should be written to presume that this might be the case (and thus not blindly copy c_cflag), but not to rely on it in case it is in some other field of the structure. Setting the c_cflag field absolutely after setting a baud rate is a non-portable action because of this. In general, the unused parts of the flag fields might be used by the implementation and should not be blindly copied from the descriptions of one terminal device to another.
The Base Definitions volume of POSIX.1-2017, Chapter 11, General Terminal Interface, <termios.h>
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