I've been experimenting with pseudo-terminals and found something I don't quite understand. Writing an EOF character to the master end doesn't work quite as I expect. Once I've written any other data, the master pty seems to treat a single ^D as a seperator, i.e. writing "abcabc" would let cat do one read of six bytes, but "abc\x04abc" would make cat two reads of three bytes. Is this behavior documented anywhere?
Here's the code I test with. I asserted everything in case there were errors I was missing but didn't find any...
#include <unistd.h>
#include <fcntl.h>
#include <pty.h>
#include <errno.h>
#include <ctype.h>
#include <stdio.h>
#include <string.h>
#include <stdarg.h>
#include <assert.h>
// My system doesn't declare ptsname for some reason
// but it clearly exists.
#define _XOPEN_SOURCE
#include <stdlib.h>
extern char *ptsname(int fd);
#include <sys/types.h>
#include <sys/wait.h>
/**
* popen() and system() create a whole new shell, which
* clutters strace output with tons of garbage.
* this just exec's raw and does no such thing.
*/
int system_noshell(const char *cmd, ...);
int main(int argc, char *argv[])
{ // Open a pseudo-terminal, but don't make it our stdin/out/err
int fd_master=posix_openpt(O_RDWR|O_NOCTTY);
int ppid=getpid(); // Process ID of parent
int cpid=-1; // Get child PID later
const char *pts; // slave pseudoterm
int s;
char c;
assert( fd_master >= 0 ); // Did master terminal open?
fprintf(stderr, "[P] Opened fd_master = %d\n", fd_master);
assert( unlockpt(fd_master) >= 0 ); // Unlock PTY
fprintf(stderr, "[P] Unlocked fd_master\n");
assert( (pts=ptsname(fd_master)) != NULL ); // Get slave name
fprintf(stderr, "[P] Name of slave: %s\n", pts);
// Create a child process to use the slave pty
assert( (cpid=fork()) >= 0);
/**
* Following a specific series of events here:
* Parent Child
* ===================================
* Open Master
* fork
* Open Child
* Close Master
* read write '|'
* write 'abc'
* write EOF EOF
* read abc
* EOF
* exit
* wait()
* exit
*/
if(cpid == 0) // Child code
{
int fd_slave=-1; // Slave PTY
// Save real stderr so fprintf writes to it instead of pty
int STDERR=dup(STDERR_FILENO);
FILE *stderr=fdopen(STDERR, "w");
setvbuf(stderr, NULL, _IONBF, 0);
assert( close(fd_master) >= 0 ); // Ditch master PTY
assert( (fd_slave=open(pts,O_RDWR)) >= 0); // Open slave PTY
fprintf(stderr, "\t[C] Opened slave %s\n", pts);
// This will dup fd over stdin,out,err then close fd
// This function needs compilation with -lutil
assert( login_tty(fd_slave) >= 0 );
// We can't use assert after here, it'll print to the real stderr
if(system_noshell("/bin/stty", "-echo", NULL) != 0)
{
fprintf(stderr, "\t[C] Couldn't disable echo\n");
exit(1);
}
fprintf(stderr, "\t[C] Disabled echo\n");
// We let the parent know the child has control of
// the slave terminal by writing a char to it.
if(write(STDOUT_FILENO, "|", 1) != 1 )
{
fprintf(stderr, "\t[C] Couldn't send |\n");
exit(1);
}
fprintf(stderr, "\t[C] Wrote char\n");
// We exec cat, to read until EOF.
execl("/bin/cat", "/bin/cat", NULL);
fprintf(stderr, \t[C] Couldn't exec: %s\n",
strerror(errno));
exit(1);
}
fprintf(stderr, "[P] Created child pid=%d\n", cpid);
// Parent code
assert( read(fd_master, &c, 1) == 1 );
assert( c == '|' ); // Child should have written |
fprintf(stderr, "[P] Read first char from child\n");
// Write "abc" to master end of terminal
assert( write(fd_master, "abc", 3) == 3 );
fprintf(stderr, "[P] Wrote data to child\n");
// The child hangs if I don't write two EOF chars.
assert( write(fd_master, "\x04\x04", 2) == 2 );
fprintf(stderr, "[P] Wrote EOF\n");
while(read(fd_master, &c, 1) == 1) // Read bytes until EOF
{
if(c<0x20) fprintf(stderr, "[P] Read ^%c\n", c+'@');
else fprintf(stderr, "[P] Read '%c'\n", c);
}
assert( wait(&s) == cpid ); // Wait for child to exit
fprintf(stderr, "[P] Child has exited\n");
assert( close(fd_master) >= 0 );
fprintf(stderr, "[P] All finished.\n");
return(0);
}
#define MAX_ARGS 16
int system_noshell(const char *cmd, ...)
{
const char *args[MAX_ARGS]={cmd};
int n=0, status;
va_list ap;
// Assemble varargs into the args array
va_start(ap, cmd);
do
args[++n]=va_arg(ap, void *);
while((args[n]) && (n<(MAX_ARGS-2)));
va_end(ap);
args[++n]=NULL; // Terminate argument list
n=fork();
if(n < 0)
return(-1);
else if(n == 0)
{
execvp(cmd, args);
exit(255);
}
assert( waitpid(n, &status, 0) == n);
return(WEXITSTATUS(status));
}