why this linux pseudo terminal program doesn't work? - c

Question

Master reads input from stdin and writes to pty-master, slave reads input from pty-slave and writes to stdout.
But this code/program seems doesn't work.
The Master writes to pty-master is OK, but the slave hungs when reads from pty-slave.
anyone can help me? thx in advance.
#include <assert.h>
#include <fcntl.h>
#include <errno.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <sys/stat.h>
#include <unistd.h>
/*
* copy from apue
*/
int ptym_open(char *pts_name, int pts_namesz)
{
char *ptr;
int fdm, err;
if ((fdm = posix_openpt(O_RDWR)) < 0) {
assert(0);
return -1;
}
if (grantpt(fdm) < 0) {
assert(0);
return -1;
}
if (unlockpt(fdm) < 0) {
assert(0);
return -1;
}
if ((ptr = ptsname(fdm)) == NULL) {
assert(0);
return -1;
}
strncpy(pts_name, ptr, pts_namesz);
pts_name[pts_namesz - 1] = 0;
printf("pts_name:%s\n", pts_name);
return fdm;
}
int ptys_open(char *pts_name)
{
int fds;
if ((fds = open(pts_name, O_RDWR)) < 0) {
return -1;
}
return fds;
}
int pty_fork(int *ptrfdm, char *slave_name, int slave_namesz)
{
int fdm, fds;
pid_t pid;
char pts_name[1024];
if ((fdm = ptym_open(pts_name, sizeof(pts_name))) < 0) {
assert(0);
return -1;
}
if (slave_name != NULL) {
strncpy(slave_name, pts_name, slave_namesz);
slave_name[slave_namesz - 1] = 0;
}
if ((pid = fork()) < 0) {
assert(0);
return -1;
} else if (pid == 0) {
if (setsid() < 0) {
assert(0);
}
if ((fds = ptys_open(pts_name)) < 0) {
assert(0);
}
close(fdm);
if (dup2(fds, STDIN_FILENO) != STDIN_FILENO) {
assert(0);
}
//if (dup2(fds, STDOUT_FILENO) != STDOUT_FILENO) {
// assert(0);
//}
//if (dup2(fds, STDERR_FILENO) != STDERR_FILENO) {
// assert(0);
//}
if ((fds != STDIN_FILENO) && (fds != STDOUT_FILENO) && (fds != STDERR_FILENO)) {
close(fds);
}
return 0;
} else {
*ptrfdm = fdm;
return pid;
}
}
int loop(int ptym)
{
pid_t pid;
int nread;
#define BUFFSIZE 512
char buf[BUFFSIZE];
if ((pid = fork()) < 0) {
assert(0);
} else if (pid == 0) {
while (1) {
if ((nread = read(STDIN_FILENO, buf, BUFFSIZE)) < 0) {
int errr = errno;
printf("%s\n", strerror(errr));
assert(0);
} else if (nread = 0) {
break;
}
if (write(ptym, buf, nread) != nread) {
int errr = errno;
printf("%s\n", strerror(errr));
assert(0);
}
fsync(ptym);
}
exit(0); // child
}
while (1) {
if ((nread = read(ptym, buf, BUFFSIZE)) <= 0) {
printf("%d break read\n", getpid());
break;
}
if (write(STDOUT_FILENO, buf, nread) != nread) {
assert(0);
}
}
}
int main(void)
{
int fdm;
char slave_name[1024];
pid_t pid = pty_fork(&fdm, slave_name, sizeof(slave_name));
if (pid < 0) {
assert(0);
} else if (pid == 0) {
int nread;
char buf[1024];
while(1){
if ((nread = read(STDIN_FILENO, buf, 3)) < 0) {
break;
}
printf("buf:%s\n", buf);
}
} else {
printf("child:%d#%s\n", pid, slave_name);
loop(fdm);
}
return 0;
}

Answer 1

Besides opening the pseudo-terminal, you have to initialize it (something referred to as line discipline). I don't see any of that in your example. You could compare with luit, which does do this (look for instance at the openTty function in sys.c).
Further reading:
Notes 7: Terminal I/O
Writing a Kernel Line Discipline
What are the responsibilities of each Pseudo-Terminal (PTY) component (software, master side, slave side)?