cfmakeraw / tcdrain / tcflow / tcflush / tcsendbreak (3) - glibc

Split from the chapter glibc: Low-Level Terminal Interface into the per-function layout; see the method page and the glibc ledger for coverage status.

Declaration and gates

#include <termios.h>

void cfmakeraw(struct termios *termios_p);              /* __USE_MISC */

int tcsendbreak(int fd, int duration);
int tcdrain(int fd);                 /* no __THROW: cancellation point */
int tcflush(int fd, int queue_selector);
int tcflow(int fd, int action);

cfmakeraw sits in the __USE_MISC block of /usr/include/termios.h (lines 74-77; man gate _DEFAULT_SOURCE since glibc 2.19). tcsendbreak/tcflush/tcflow are unconditional (lines 80-94) and tcdrain is declared without __THROW (line 86) because it is a cancellation point.

Semantics

cfmakeraw turns a termios structure into BSD “raw mode” by clearing IGNBRK|BRKINT|PARMRK|ISTRIP|INLCR|IGNCR|ICRNL|IXON on c_iflag, OPOST on c_oflag, ECHO|ECHONL|ICANON|ISIG|IEXTEN on c_lflag, and CSIZE|PARENB while setting CS8 on c_cflag (manual, Raw Mode; the code is quoted verbatim there). tcdrain blocks until all output written to fd has been transmitted. tcsendbreak transmits a break for duration units (zero means about 0.25-0.5 s). tcflush discards queued input and/or output per TCIFLUSH, TCOFLUSH, TCIOFLUSH. tcflow suspends or resumes output (TCOOFF, TCOON) or sends stop/start input control characters (TCIOFF, TCION).

Gotchas and portability

• All five return 0 on success, -1 with errno on failure (termios(3) RETURN VALUE).
• tcdrain’s cancellation-point status matters for pthreads code: a canceled thread can abort mid-drain.
• cfmakeraw only edits the structure; the change reaches the device only via tcsetattr, which can partially succeed (see termios(3)).
• musl 1.2.5: all five present (musl src/termios/cfmakeraw.c, tcdrain.c, tcflow.c, tcflush.c, tcsendbreak.c).

References

• glibc manual: terminal.texi, nodes Raw Mode, Line Control, Noncanonical Input
• man3: termios(3) (covers all five)
• headers: /usr/include/termios.h
• symbols: nm -D --with-symbol-versions libc.so.6