termio ioctl returns incorrect value for suspend character

[Steven Pemberton]

When calling ioctl to request some special key bindings, it seems to return a random value for the Suspend binding (see attached C file).

On my system, if I do a stty -a I get this:

$ stty -a
speed 38400 baud; rows 24; columns 80; line = 0;
intr = ^C; quit = ^\; erase = ^?; kill = ^U; eof = ^D; eol = M-^?; eol2 = M-^?;
swtch = M-^?; start = ^Q; stop = ^S; susp = ^Z; rprnt = ^R; werase = ^W;
lnext = ^V; flush = ^O; min = 1; time = 0;
-parenb -parodd cs8 hupcl -cstopb cread -clocal -crtscts
-ignbrk brkint -ignpar -parmrk -inpck -istrip -inlcr -igncr icrnl ixon -ixoff
-iuclc ixany imaxbel iutf8
opost -olcuc -ocrnl onlcr -onocr -onlret -ofill -ofdel nl0 cr0 tab0 bs0 vt0 ff0
isig icanon iexten echo echoe echok -echonl -noflsh -xcase -tostop -echoprt
echoctl echoke

If I run the enclosed C program, which does an ioctl for TCGETA:

       if (ioctl(0, TCGETA, (char*) &sgbuf) == 0) {

and then retrieves the key bindings for interrupt, erase, and suspend (^C, ^?, and ^Z in the above output). It returns Interrupt and Erase correctly, but it returns the wrong value for Suspend; each time I run the program it returns a different value, though it consistently returns the same (wrong) value within the program:

                                 if ((int) sgbuf.c_cc[VSUSP] != 0377) {
   printf("Suspend=%i\n", (int)sgbuf.c_cc[VSUSP]);
  }

The program calls the ioctl three times to see if it always returns the same value within the program. It does.

An example output:
Erase= 127, Interrupt=3, Suspend=31
Erase= 127, Interrupt=3, Suspend=31
Erase= 127, Interrupt=3, Suspend=31

but the next time I run it:

Erase= 127, Interrupt=3, Suspend=171
Erase= 127, Interrupt=3, Suspend=171
Erase= 127, Interrupt=3, Suspend=171

ProblemType: Bug
DistroRelease: Ubuntu 14.04
Package: gcc 4:4.8.2-1ubuntu6
ProcVersionSignature: Ubuntu 3.13.0-46.77-generic 3.13.11-ckt15
Uname: Linux 3.13.0-46-generic x86_64
ApportVersion: 2.14.1-0ubuntu3.7
Architecture: amd64
CurrentDesktop: Unity
Date: Thu Mar 5 14:41:28 2015
SourcePackage: gcc-defaults
UpgradeStatus: No upgrade log present (probably fresh install)

[Steven Pemberton (steven-pemberton)]

termios.h says there are 17 characters (0..16) in the c_cc buffer. If I print them all out (here I've run it three times), it seems like 4 characters change their value between calls: 8 (VSTART) 9 (VSTOP) 10 (VSUSP) 16 (VEOL2)

   0:3 1:28 2:127 3:21 4:4 5:0 6:1 7:255 8: 18 9:122 10:99 11:255 12:127 13:0 14:0 15:0 16:134
   0:3 1:28 2:127 3:21 4:4 5:0 6:1 7:255 8:215 9:152 10:145 11:255 12:127 13:0 14:0 15:0 16:129
   0:3 1:28 2:127 3:21 4:4 5:0 6:1 7:255 8: 10 9:97 10:251 11:255 12:127 13:0 14:0 15:0 16:239

I'll upload this program as termio2.c.

Finally the equivalent function from termios, namely tcgetattr, seems to give the right results, consistently over calls:

   0:3 1:28 2:127 3:21 4:4 5:0 6:1 7:255 8:17 9:19 10:26 11:255 12:18 13:15 14:23 15:22 16:255

I'll upload this as termio3.c

[Steven Pemberton (steven-pemberton)]

Since I ran into this bug again today, and since it hasn't been touched in the 5 years since I reported it, I thought I would just add the code to program around it, for those people who come here with the same problem (if there are such people).

replace
   #include <termio.h>
   struct termio sgbuf;
   ioctl(f, TCGETA, (char*)&sgbuf)

with
   #include <termios.h>
   struct termios sgbuf;
   tcgetattr(f, &sgbuf)