Why is Linux's pty driver replacing VEOFs with NULs? - c

It seems that the pty driver on Linux is replacing VEOF characters (^D, \4) with NUL bytes (\0) in the data already written from the master side if the terminal settings are changed with tcsetattr(TCSANOW) to non-canonical mode before reading it on the slave side.
Why is this happening? Does it have any justification or it's simply a bug?
Is there any way to avoid it? -- other than waiting for input from the slave on the master side before writing anything, which is not practical, because on the slave side there may be another program -- the routine of setting the terminal into raw mode which I've simplified here is usually what any shell with line-editing capabilities does.
While having eg \r replaced by \n could be expected (because the ICRNL flag was already applied), I cannot make any rationale for those NUL bytes appearing out of nowhere.
Test case below: it will print foo\x00\x00\x00\x00 on Linux, but foo\x04\x04\x04\x04 on *BSD and foo on Solaris.
#define _XOPEN_SOURCE 600
#include <unistd.h>
#include <fcntl.h>
#include <stdlib.h>
#include <ctype.h>
#include <stdio.h>
#include <termios.h>
#include <err.h>
#ifdef __sun
#include <stropts.h>
#define push_streams(fd)\
if(ioctl(fd, I_PUSH, "ptem")) err(1, "ioctl(I_PUSH, ptem)");\
if(ioctl(fd, I_PUSH, "ldterm")) err(1, "ioctl(I_PUSH, ldterm)");
#else
#define push_streams(sd) /* no need */
#endif
int main(void){
int mt, st; char *sname;
/* openpty()-like boilerplate */
if((mt = posix_openpt(O_RDWR|O_NOCTTY)) == -1) err(1, "posix_openpt");
if(grantpt(mt)) err(1, "grantpt");
if(unlockpt(mt)) err(1, "unlockpt");
if(!(sname = ptsname(mt))) err(1, "ptsname");
if((st = open(sname, O_RDWR|O_NOCTTY)) == -1) err(1, "open %s", sname);
push_streams(st);
/* master */ {
char test[] = "foo\4\4\4\4";
if(write(mt, test, sizeof test - 1) < sizeof test - 1)
err(1, "write");
}
/* slave */ {
unsigned char buf[512]; int i, r;
struct termios ts;
usleep(1000);
if(tcgetattr(st, &ts)) err(1, "tcgetattr");
ts.c_lflag &= ~ICANON;
if(tcsetattr(st, TCSANOW, &ts)) err(1, "tcsetattr");
if((r = read(st, buf, sizeof buf)) < 0)
err(1, "read");
for(i = 0; i < r; i++)
if(isprint(buf[i])) putchar(buf[i]);
else printf("\\x%02x", buf[i]);
putchar('\n');
}
return 0;
}

This conversion is done by the line discipline driver when the data is written by the master, not when the slave reads the data. The relevant code is:
https://elixir.bootlin.com/linux/latest/source/drivers/tty/n_tty.c#L1344

Related

`ioctl` returns -1 when trying to TUNSETIFF a TUN device

I've been following the official documentation on creating a TUN device. There's very little documentation, but what even is there does not seem to work. Here's what I have so far:
#include <linux/if.h>
#include <linux/if_tun.h>
#include <sys/fcntl.h>
#include <sys/stat.h>
#include <sys/ioctl.h>
#include <unistd.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <fcntl.h>
int tun_alloc(char *dev)
{
struct ifreq ifr;
int fd, err;
if( (fd = open("/dev/net/tun", O_RDWR)) < 0 )
return 7; // tun_alloc_old(dev); // this does not happen, linux docs don't say what tun_alloc_old is anyway
memset(&ifr, 0, sizeof(ifr));
/* Flags: IFF_TUN - TUN device (no Ethernet headers)
* IFF_TAP - TAP device
*
* IFF_NO_PI - Do not provide packet information
*/
ifr.ifr_flags = IFF_TUN;
if( *dev ) {
printf("setting ifr.ifr_name to \"%s\"\n", dev);
strncpy(ifr.ifr_name, dev, IFNAMSIZ);
}
printf("ifr.ifr_name is \"%s\"\n", ifr.ifr_name);
if( (err = ioctl(fd, TUNSETIFF, (void *) &ifr)) < 0 ){
perror("ERR");
close(fd);
printf("GOT ERROR: %d\n", err);
return err;
}
printf("past TUNSETIFF error checking\n");
strcpy(dev, ifr.ifr_name);
return fd;
}
int main() {
char name[128];
strcpy(&name, "tun23");
int tun = tun_alloc(name);
printf("tun_alloc: %s id: %d\n", name, tun);
if(tun == -1) return -1;
while(1) {
printf("in loop\n");
char buf[128];
ssize_t readAmount = read(tun, buf, 128);
printf("finished read\n");
if(readAmount == -1) {
printf("read 0 bytes\n");
continue;
}
printf("Read %d: ", readAmount);
for(int i = 0; i < 128; i++) {
printf("%hhx", buf[i]);
}
printf("\n");
}
return tun;
}
Compiler step is gcc -g3 test.c && a.out.
When ran as non-root, I get this output
setting ifr.ifr_name to "tun23"
ifr.ifr_name is "tun23"
ERR: Operation not permitted
GOT ERROR: -1
tun_alloc: tun23 id: -1
And as root, it successfully gets into the loop, but then the call to read seems to block.
setting ifr.ifr_name to "tun23"
ifr.ifr_name is "tun23"
past TUNSETIFF error checking
tun_alloc: tun23 id: 3
in loop
(In the event of an XY problem, the reason I'm doing this is to try and make a very simple vpn-like application)
The issue is that while it's blocking in this loop, no TUN devices are created. I am looking in /dev/. I'm not sure how I would give it any bytes to read.
Edit: added a proper perror call and the respective output Operation not permitted, valgrind output, and a couple print statements. Going to try and debug the strcpy error.
Edit: fixed the strcpy error, was pretty simple. Seems to fail in non-root due to a lack of permission to create TUN devices.

TunTap Device receiving ping

I am trying to get familiar with tuntap devices. I have read the following article:
https://backreference.org/2010/03/26/tuntap-interface-tutorial/
but somehow the code from the article doesn't work.
I have this code:
#include <sys/socket.h> //well get our socket
#include <sys/ioctl.h> //thats our input output control
#include <sys/time.h>
#include <fcntl.h>
#include <asm/types.h> //these are data types liked signed unsingend
#include <math.h>
#include <string.h>
#include <stdio.h>
#include <stdlib.h>
#include <unistd.h> //read write close and stuff
#include <signal.h> //different signals
#include <linux/if_packet.h> //interface for packets
#include <linux/if_ether.h> //interface or ethernet frames
#include <linux/if_arp.h> //interface for arp
#include <linux/if.h>
#include <linux/if_tun.h>
#include <arpa/inet.h>
int tun_alloc(char *dev, int flags) {
struct ifreq ifr;
int fd, err;
char *clonedev = "/dev/net/tun";
/* Arguments taken by the function:
*
* char *dev: the name of an interface (or '\0'). MUST have enough
* space to hold the interface name if '\0' is passed
* int flags: interface flags (eg, IFF_TUN etc.)
*/
/* open the clone device */
if( (fd = open(clonedev, O_RDWR)) < 0 ) {
return fd;
}
/* preparation of the struct ifr, of type "struct ifreq" */
memset(&ifr, 0, sizeof(ifr));
ifr.ifr_flags = flags; /* IFF_TUN or IFF_TAP, plus maybe IFF_NO_PI */
if (*dev) {
/* if a device name was specified, put it in the structure; otherwise,
* the kernel will try to allocate the "next" device of the
* specified type */
strncpy(ifr.ifr_name, dev, IFNAMSIZ);
}
/* try to create the device */
if( (err = ioctl(fd, TUNSETIFF, (void *) &ifr)) < 0 ) {
close(fd);
return err;
}
/* if the operation was successful, write back the name of the
* interface to the variable "dev", so the caller can know
* it. Note that the caller MUST reserve space in *dev (see calling
* code below) */
strcpy(dev, ifr.ifr_name);
/* this is the special file descriptor that the caller will use to talk
* with the virtual interface */
return fd;
}
int main(void){
unsigned int seconds = 1;
char tap_name[IFNAMSIZ];
strcpy(tap_name, "tun0");
printf("%s\n", tap_name);
int tap_fd = tun_alloc(tap_name, IFF_TUN);
void *buffer = (void *)(malloc(3000));
printf("%s\n", tap_name);
if(tap_fd < 0){
perror("Allocating interface");
exit(1);
}
int nread;
while(1){
nread = read(tap_fd, buffer, sizeof(buffer));
if (nread < 0){
perror("Nread: ");
close(tap_fd);
free(buffer);
exit(1);
}
printf("Read %d Bytes from devies %s \n", nread, tap_name);
sleep(seconds);
}
}
I executed this program on one terminal and pinged the interface from another terminal.
But when I ping the interface from the command line (ping 192.168.0.24, I have assigned that IP to the interface), on the terminal of the program there's always written "read 8 Bytes from interface", although the number of bytes should vary when I ping the interface. Does anyone see the mistake?

Serial I/O in C with termios: unreliable output capitalization

I have a very small C program which sends and receives newline-terminated ASCII strings to and from a serial device. It's plugged into my computer with a USB adapter, on /dev/ttyUSB0.
Most of the time it sends the commands just find, but occasionally it will capitalize all the lower-case letters to upper-case. It leaves all special characters alone.
The string I am sending is /home\n. About 1 out of every five times I run the program (by simply running ./a.out without recompiling), the sent message understood by the device is /HOME\n.
Here is my source code:
#include <stdio.h>
#include <stdlib.h>
#include "zserial.h"
int main() {
char buf[256];
int fd = connect("/dev/ttyUSB0");
char *cmd = "/home\n";
send(fd, cmd);
receive(fd, buf, 256);
puts(buf);
exit(0);
}
And zserial.c:
#include <fcntl.h>
#include <termios.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include "zserial.h"
int send(int fd, char *buf) {
int len = strlen(buf);
int nb = write(fd, buf, len);
if (len != nb || nb < 1)
perror("Error: wrote no bytes!");
tcdrain(fd);
return nb;
}
int receive(int fd, char *dst, int nbytes) {
int i;
char c;
for(i = 0; i < nbytes;) {
int r = read(fd, &c, 1);
/* printf("Read %d bytes\n", r); */
if (r > 0) {
dst[i++] = c;
if (c == '\n') break;
}
}
dst[i] = 0; /* null-terminate the string */
return i;
}
int connect(char *portname) {
int fd;
struct termios tio;
fd = open(portname, O_RDWR | O_NOCTTY | O_NONBLOCK);
tio.c_cflag = CS8|CREAD|CLOCAL;
if ((cfsetospeed(&tio, B115200) & cfsetispeed(&tio, B115200)) < 0) {
perror("invalid baud rate");
exit(-1);
}
tcsetattr(fd, TCSANOW, &tio);
return fd;
}
What am I doing wrong? Is there some termios flag which modifies the output on a serial port?
c_oflag & OLCUC turns on the mapping of lowercase to uppercase on output. Since you never initialized tio, it's not surprising you got some random flags set.
You have two choices:
tcgetattr the current settings into a termios struct to initialize it, then modify the ones you're interested in, then write them back with tcsetattr
initialize all the termios fields to known values, not just c_cflag and the speed fields.

Redundancy when reading USB serial port (C;Mac OSX;Arduino)

I'm writing a simple C program that can read data from a USB port that is connected to my Arduino device. The Arduino outputs data at a baud rate of 9600 in chunks of 4 bytes.
I want the input from the Arduino to my computer to look something like this:
136.134.132.130.129.127.126.124.121.119.117.115.113.111.
However, I'm getting something like this:
271.274.281..2.4062.4022.40225.4021
Question: How do I get the input in my C program to neatly synchronize with out loosing data/ rereading data? Are there some kind of flags that could tell my program when the port has new data?
Code:
#include <stdio.h> /* Standard input/output definitions */
#include <string.h> /* String function definitions */
#include <unistd.h> /* UNIX standard function definitions */
#include <fcntl.h> /* File control definitions */
#include <errno.h> /* Error number definitions */
#include <termios.h> /* POSIX terminal control definitions */
#include <sys/types.h>
int open_port(void)
{
int fd; /* File descriptor for the port */
fd = open("/dev/tty.usbmodemfd121", O_RDWR | O_NOCTTY | O_NDELAY);
if (fd == -1)
{
perror("open_port: Unable to open /dev/tty");
}
else
fcntl(fd, F_SETFL, 0);
struct termios options;
tcgetattr(fd,&options);
cfsetospeed(&options,B9600);
options.c_cflag |=(CLOCAL | CREAD);
tcsetattr(fd, TCSANOW, &options);
return (fd);
}
int main (){
int i;
for(i=0; i<50; i++){
fcntl(open_port(), F_SETFL, FNDELAY);
char buf[5];
size_t nbytes;
ssize_t bytes_read;
nbytes = sizeof(buf);
bytes_read = read(open_port(), buf, nbytes);
printf("%s ", buf);
buf[0]=0;
}
return 0;
}
Your program does not properly open() the serial port for reading it.
In fact it repeatedly opens it two times every iteration of the for loop.
The device should be opened only once by your program.
Instead of
for (i=0; i<50; i++) {
fcntl(open_port(), F_SETFL, FNDELAY);
bytes_read = read(open_port(), buf, nbytes);
}
the main program should be structured like
fd = open_port();
if (fd < 0) {
/* handle error condition */
}
rc = fcntl(fd, F_SETFL, FNDELAY);
if (rc < 0) {
/* handle error condition */
}
for (i=0; i<50; i++) {
bytes_read = read(fd, buf, nbytes);
if (bytes_read < 0) {
/* handle error condition */
}
}
close(fd);
Your program is too "simple". It sets only a few attributes, and doesn't bother to check the return codes of system calls.
Is this supposed to be canonical or non-canonical (aka raw) mode (i.e. is the data ASCII text or binary)?
Refer to this Serial Programming Guide for proper setup of the serial port.
read data from a USB port
USB is a bus.
The device your program reads from is a serial port attached to that USBus.
Second coding issue
Your original code may print garbage data.
nbytes = sizeof(buf);
bytes_read = read(open_port(), buf, nbytes);
printf("%s ", buf);
buf[0]=0;
The bytes returned by the read() operation are not likely to be terminated by a NULL byte, so a string operation on that read buffer could exceed the bounds of the allocated array.
Code that would not misbehave would be something like:
nbytes = sizeof(buf) - 1;
bytes_read = read(fd, buf, nbytes);
if (bytes_read < 0) {
/* handle error condition */
} else {
buf[bytes_read] = 0; /* append terminator */
printf("%s ", buf);
}
Note that nbytes is one less than the allocated size of the buffer.
This is to ensure that there is an available byte to store the string terminator byte when the read() operation returns a "full" buffer of nbytes.
For efficiency the assignment of nbytes should be performed before entering the for loop, rather than within the loop.

Reading from a pty

I'd like to receive (and later process) write(1) and wall(1) messages using a (Unix 98-style) pseudo tty on Linux.
I already have the following minimal implementation:
#include <stdlib.h>
#include <string.h>
#include <stdio.h>
#include <unistd.h>
#include <errno.h>
#include <fcntl.h>
#include <signal.h>
#include <utempter.h>
#define BUF_LENGTH 1024
int
main (void)
{
FILE *lf;
int masterfd, slavefd;
char *slave_name = NULL;
char buf[BUF_LENGTH];
size_t nbytes = sizeof(buf);
ssize_t bytes_read;
int exit_code = EXIT_SUCESS;
if ((masterfd = posix_openpt (O_RDWR | O_NOCTTY)) == -1
|| grantpt (masterfd) == -1
|| unlockpt (masterfd) == -1
|| (slave_name = ptsname (masterfd)) == NULL)
exit (EXIT_FAILURE);
if (!(lf = fopen("term.log","w")))
exit (EXIT_FAILURE);
addToUtmp (slave_name, NULL, masterfd);
for (;;)
{
bytes_read = read(masterfd, buf, nbytes);
if (bytes_read <= 0)
break
fwrite (buf, 1, bytes_read, lf);
}
if (bytes_read < 0)
{
fprintf (stderr, "error reading from master pty: %s\n", strerror (errno));
exit_code = EXIT_FAILURE;
}
fclose (lf);
if (slavefd >= 0)
close (slavefd);
if (masterfd >= 0)
{
removeLineFromUtmp (slave_name, masterfd);
close (masterfd);
}
exit (exit_code);
}
The problem is now that it only works for reading the first message, then read gives me a EIO error. Why is that?
It looks like this happens simply when the last slave file descriptor is closed. Considering write(1) and wall(1) will have the only file descriptor to the slave, you get EIO as soon as those finish writing.
The easiest way to keep this from happening is by keeping a file descriptor around. Right after your ptsname call, do an open(slave_name, O_RDRW).
(Curiously, you already have a slavefd variable, and the code to clean it up. Are you testing us? :p)

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