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/* functions to spawn foreground/background jobs */
#include <stdio.h>
#include <unistd.h>
#include <stdlib.h>
#include <fcntl.h>
#include <signal.h>
#include <sys/types.h>
#include <sys/wait.h>
#include <errno.h>
/* Some of the functions below fail if the control tty can't be
* located, or if the calling process isn't in the foreground. In the
* first case, we are assuming that a foreground process will have the
* ctty open on either stdin, stdout or stderr, and we return ENOTTY
* if it isn't. In the second case, we return EPERM if a
* non-foreground process attempts to put something into the
* foreground (probably overly paranoid) except for the special case
* of foreground_self().
*/
/* assign the terminal (open on ctty) to a specific pgrp. This wrapper
* around tcsetpgrp() is needed only because of extreme bogosity on the
* part of POSIX; conforming systems deliver STGTTOU if tcsetpgrp is
* called from a non-foreground process (which it almost invariably is).
* A triumph of spurious consistency over common sense.
*/
int assign_terminal(int ctty, pid_t pgrp)
{
sigset_t sigs;
sigset_t oldsigs;
int rc;
sigemptyset(&sigs);
sigaddset(&sigs,SIGTTOU);
sigprocmask(SIG_BLOCK, &sigs, &oldsigs);
rc = tcsetpgrp(ctty, pgrp);
sigprocmask(SIG_SETMASK, &oldsigs, NULL);
return rc;
}
/* Like fork(), but does job control. FG is true if the newly-created
* process is to be placed in the foreground. (This implicitly puts
* the calling process in the background, so watch out for tty I/O
* after doing this.) PGRP is -1 to create a new job, in which case
* the returned pid is also the pgrp of the new job, or specifies an
* existing job in the same session (normally used only for starting
* second or subsequent process in a pipeline). */
pid_t spawn_job(int fg, pid_t pgrp)
{
int ctty = -1;
pid_t pid;
/* If spawning a *new* foreground job, require that at least one
* of stdin, stdout or stderr refer to the control tty, and that
* the current process is in the foreground.
* Only check for controlling tty if starting a new foreground
* process in an existing job.
* A session without a control tty can only have background jobs
*/
if (fg)
{
pid_t curpgrp;
if ((curpgrp = tcgetpgrp(ctty = 2)) < 0
&& (curpgrp = tcgetpgrp(ctty = 0)) < 0
&& (curpgrp = tcgetpgrp(ctty = 1)) < 0)
return errno = ENOTTY, (pid_t)-1;
if (pgrp < 0 && curpgrp != getpgrp())
return errno = EPERM, (pid_t)-1;
}
switch (pid = fork())
{
case -1: /* fork failure */
return pid;
case 0: /* child */
/* establish new process group, and put ourselves in
* foreground if necessary
* unclear what to do if setpgid fails ("can't happen")
*/
if (pgrp < 0)
pgrp = getpid();
if (setpgid(0,pgrp) == 0 && fg)
assign_terminal(ctty, pgrp);
return 0;
default: /* parent */
/* establish child process group here too. */
if (pgrp < 0)
pgrp = pid;
setpgid(pid, pgrp);
return pid;
}
/*NOTREACHED*/
}
/* Kill job PGRP with signal SIGNO */
int kill_job(pid_t pgrp, int signo)
{
return kill(-pgrp, signo);
}
/* Suspend job PGRP */
int suspend_job(pid_t pgrp)
{
return kill_job(pgrp, SIGSTOP);
}
/* Resume job PGRP in background */
int resume_job_bg(pid_t pgrp)
{
return kill_job(pgrp, SIGCONT);
}
/* resume job PGRP in foreground */
int resume_job_fg(pid_t pgrp)
{
pid_t curpgrp;
int ctty;
if ((curpgrp = tcgetpgrp(ctty = 2)) < 0
&& (curpgrp = tcgetpgrp(ctty = 0)) < 0
&& (curpgrp = tcgetpgrp(ctty = 1)) < 0)
return errno = ENOTTY, (pid_t)-1;
if (curpgrp != getpgrp())
return errno = EPERM, (pid_t)-1;
if (assign_terminal(ctty, pgrp) < 0)
return -1;
return kill_job(pgrp, SIGCONT);
}
/* put ourselves in the foreground, e.g. after suspending a foreground
* job
*/
int foreground_self()
{
pid_t curpgrp;
int ctty;
if ((curpgrp = tcgetpgrp(ctty = 2)) < 0
&& (curpgrp = tcgetpgrp(ctty = 0)) < 0
&& (curpgrp = tcgetpgrp(ctty = 1)) < 0)
return errno = ENOTTY, (pid_t)-1;
return assign_terminal(ctty, getpgrp());
}
/* closeall() - close all FDs >= a specified value */
void closeall(int fd)
{
int fdlimit = sysconf(_SC_OPEN_MAX);
while (fd < fdlimit)
close(fd++);
}
/* like system(), but executes the specified command as a background
* job, returning the pid of the shell process (which is also the pgrp
* of the job, suitable for kill_job etc.)
* If INFD, OUTFD or ERRFD are non-NULL, then a pipe will be opened and
* a descriptor for the parent end of the relevent pipe stored there.
* If any of these are NULL, they will be redirected to /dev/null in the
* child.
* Also closes all FDs > 2 in the child process (an oft-overlooked task)
*/
pid_t spawn_background_command(const char *cmd,
int *infd, int *outfd, int *errfd)
{
int nullfd = -1;
int pipefds[3][2];
int error = 0;
if (!cmd)
return errno = EINVAL, -1;
pipefds[0][0] = pipefds[0][1] = -1;
pipefds[1][0] = pipefds[1][1] = -1;
pipefds[2][0] = pipefds[2][1] = -1;
if (infd && pipe(pipefds[0]) < 0)
error = errno;
else if (outfd && pipe(pipefds[1]) < 0)
error = errno;
else if (errfd && pipe(pipefds[2]) < 0)
error = errno;
if (!error && !(infd && outfd && errfd))
{
nullfd = open("/dev/null",O_RDWR);
if (nullfd < 0)
error = errno;
}
if (!error)
{
pid_t pid = spawn_job(0, -1);
switch (pid)
{
case -1: /* fork failure */
error = errno;
break;
case 0: /* child proc */
dup2(infd ? pipefds[0][0] : nullfd, 0);
dup2(outfd ? pipefds[1][1] : nullfd, 1);
dup2(errfd ? pipefds[2][1] : nullfd, 2);
closeall(3);
execl("/bin/sh","sh","-c",cmd,(char*)NULL);
_exit(127);
default: /* parent proc */
close(nullfd);
if (infd)
close(pipefds[0][0]), *infd = pipefds[0][1];
if (outfd)
close(pipefds[1][1]), *outfd = pipefds[1][0];
if (errfd)
close(pipefds[2][1]), *errfd = pipefds[2][0];
return pid;
}
}
/* only reached if error */
{
int i,j;
for (i = 0; i < 3; ++i)
for (j = 0; j < 2; ++j)
if (pipefds[i][j] >= 0)
close(pipefds[i][j]);
}
if (nullfd >= 0)
close(nullfd);
return errno = error, (pid_t) -1;
}
/*--------------------------------------------------------------------*/
/* This bit is a somewhat trivial example of using the above. */
pid_t bgjob = -1;
volatile int signo = 0;
#ifndef WCOREDUMP
/* If WCOREDUMP is missing, you might want to supply a correct
* definition for your platform (this is usually (status & 0x80) but
* not always) or punt (as in this example) by assuming no core dumps.
*/
# define WCOREDUMP(status) (0)
#endif
int check_children()
{
pid_t pid;
int status;
int count = 0;
while ((pid = waitpid(-1, &status, WNOHANG | WUNTRACED)) > 0)
{
if (pid == bgjob && !WIFSTOPPED(status))
bgjob = -1;
++count;
if (WIFEXITED(status))
fprintf(stderr,"Process %ld exited with return code %d\n",
(long)pid, WEXITSTATUS(status));
else if (WIFSIGNALED(status))
fprintf(stderr,"Process %ld killed by signal %d%s\n",
(long)pid, WTERMSIG(status),
WCOREDUMP(status) ? " (core dumped)" : "");
else if (WIFSTOPPED(status))
fprintf(stderr,"Process %ld stopped by signal %d\n",
(long)pid, WSTOPSIG(status));
else
fprintf(stderr,"Unexpected status - pid=%ld, status=0x%x\n",
(long)pid, status);
}
return count;
}
void sighandler(int sig)
{
if (sig != SIGCHLD)
signo = sig;
}
int main()
{
struct sigaction act;
int sigcount = 0;
act.sa_handler = sighandler;
act.sa_flags = 0;
sigemptyset(&act.sa_mask);
sigaction(SIGINT,&act,NULL);
sigaction(SIGQUIT,&act,NULL);
sigaction(SIGTERM,&act,NULL);
sigaction(SIGTSTP,&act,NULL);
sigaction(SIGCHLD,&act,NULL);
fprintf(stderr,"Starting background job 'sleep 60'\n");
bgjob = spawn_background_command("sleep 60", NULL, NULL, NULL);
if (bgjob < 0)
{
perror("spawn_background_command");
exit(1);
}
fprintf(stderr,"Background job started with id %ld\n", (long)bgjob);
while (bgjob >= 0)
{
if (signo)
{
fprintf(stderr,"Signal %d caught\n", signo);
if (sigcount++)
kill_job(bgjob, SIGKILL);
else
{
kill_job(bgjob, SIGTERM);
kill_job(bgjob, SIGCONT);
}
}
if (!check_children())
pause();
}
fprintf(stderr,"Done - exiting\n");
return 0;
}
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