Hi!

While compiling an example program in ioctl_tty(2) examples (now moving
to TCSETS(2const)), I saw several warnings:

tcgets.c:53:24: error: implicit conversion changes signedness: 'int' to 'tcflag_t' (aka 'unsigned int') [clang-diagnostic-sign-conversion,-warnings-as-errors]
        tio.c_cflag &= ~CBAUD;
                    ~~ ^~~~~~
tcgets.c:53:24: warning: use of a signed integer operand with a binary bitwise operator [hicpp-signed-bitwise]
        tio.c_cflag &= ~CBAUD;
                    ~~ ^~~~~~
tcgets.c:58:24: error: implicit conversion changes signedness: 'int' to 'tcflag_t' (aka 'unsigned int') [clang-diagnostic-sign-conversion,-warnings-as-errors]
        tio.c_cflag &= ~(CBAUD << IBSHIFT);
                    ~~ ^~~~~~~~~~~~~~~~~~~
tcgets.c:58:24: warning: use of a signed integer operand with a binary bitwise operator [hicpp-signed-bitwise]
        tio.c_cflag &= ~(CBAUD << IBSHIFT);
                    ~~ ^~~~~~~~~~~~~~~~~~~
tcgets.c:58:25: warning: use of a signed integer operand with a unary bitwise operator [hicpp-signed-bitwise]
        tio.c_cflag &= ~(CBAUD << IBSHIFT);
                       ~^~~~~~~~~~~~~~~~~~

Which seem reasonable warnings.  Bitwise operations on signed integers
is bad for ye health.  Let's see the definitions of struct termios:

	alx@debian:/usr/include$ grepc termios x86_64-linux-gnu/ asm-generic/
	x86_64-linux-gnu/bits/termios-struct.h:struct termios
	  {
	    tcflag_t c_iflag;		/* input mode flags */
	    tcflag_t c_oflag;		/* output mode flags */
	    tcflag_t c_cflag;		/* control mode flags */
	    tcflag_t c_lflag;		/* local mode flags */
	    cc_t c_line;			/* line discipline */
	    cc_t c_cc[NCCS];		/* control characters */
	    speed_t c_ispeed;		/* input speed */
	    speed_t c_ospeed;		/* output speed */
	#define _HAVE_STRUCT_TERMIOS_C_ISPEED 1
	#define _HAVE_STRUCT_TERMIOS_C_OSPEED 1
	  };
	asm-generic/termbits.h:struct termios {
		tcflag_t c_iflag;		/* input mode flags */
		tcflag_t c_oflag;		/* output mode flags */
		tcflag_t c_cflag;		/* control mode flags */
		tcflag_t c_lflag;		/* local mode flags */
		cc_t c_line;			/* line discipline */
		cc_t c_cc[NCCS];		/* control characters */
	};

Except for the speed members, they seem the same.  The types of the
members are correctly unsigned:

	alx@debian:/usr/include$ grepc tcflag_t x86_64-linux-gnu/ asm-generic/
	x86_64-linux-gnu/bits/termios.h:typedef unsigned int	tcflag_t;
	asm-generic/termbits.h:typedef unsigned int	tcflag_t;

So, all of the constants that are to be used in these members, which
will undergo bitwise operations, should use the U suffix.  Here's the
list for the UAPI:

	$ sed -n '/flag bits/,/^$/p' asm-generic/termbits.h
	/* c_iflag bits */
	#define IUCLC	0x0200
	#define IXON	0x0400
	#define IXOFF	0x1000
	#define IMAXBEL	0x2000
	#define IUTF8	0x4000

	/* c_oflag bits */
	#define OLCUC	0x00002
	#define ONLCR	0x00004
	#define NLDLY	0x00100
	#define   NL0	0x00000
	#define   NL1	0x00100
	#define CRDLY	0x00600
	#define   CR0	0x00000
	#define   CR1	0x00200
	#define   CR2	0x00400
	#define   CR3	0x00600
	#define TABDLY	0x01800
	#define   TAB0	0x00000
	#define   TAB1	0x00800
	#define   TAB2	0x01000
	#define   TAB3	0x01800
	#define   XTABS	0x01800
	#define BSDLY	0x02000
	#define   BS0	0x00000
	#define   BS1	0x02000
	#define VTDLY	0x04000
	#define   VT0	0x00000
	#define   VT1	0x04000
	#define FFDLY	0x08000
	#define   FF0	0x00000
	#define   FF1	0x08000

	/* c_lflag bits */
	#define ISIG	0x00001
	#define ICANON	0x00002
	#define XCASE	0x00004
	#define ECHO	0x00008
	#define ECHOE	0x00010
	#define ECHOK	0x00020
	#define ECHONL	0x00040
	#define NOFLSH	0x00080
	#define TOSTOP	0x00100
	#define ECHOCTL	0x00200
	#define ECHOPRT	0x00400
	#define ECHOKE	0x00800
	#define FLUSHO	0x01000
	#define PENDIN	0x04000
	#define IEXTEN	0x08000
	#define EXTPROC	0x10000

And glibc also (re)defines some of them, so those should also have the
U suffix.

Any opinions?

Have a lovely day!
Alex

-- 
<https://www.alejandro-colomar.es/>