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Reply-To: <sbardwell AT lbmsys DOT com>
From: "Steven Bardwell" <SBardwell AT lbmsys DOT com>
To: <cygwin AT cygwin DOT com>
Subject: spawnv()  unlocks files in the calling program
Date: Sun, 9 Feb 2014 13:25:30 -0500
Message-ID: <040201cf25c4$54e9cd20$febd6760$@lbmsys.com>
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I have a simple programs that show the following issue:

1) program locks a file (in my test /tmp/yyy)
2) program then calls spawnv() (in my test    "/bin/sh  -c  /bin/touch
/tmp/xxx").
3) after the spawnv(), the file /tmp/yyy is no longer locked.

It seems to me that this is wrong. Perhaps I am misunderstanding how file 
locking works. On the other hand, the file lock is preserved if the child
task is started with posix_spawn() rather than spawnv().

#include <stdio.h>
#include <string.h>
#include <stdlib.h>
#include <errno.h>
#include <unistd.h>
#include <process.h>
#include <fcntl.h>
#include <time.h>
#include <spawn.h>

extern char **environ;

main()
{
  int status;
  pid_t pid, task_pid;
  struct flock lock;
  const char * argv[] = {"/bin/sh", "-c", "/usr/bin/touch /tmp/xxx", (char
*) 0};
  pid = open("/tmp/yyy", O_WRONLY|O_CREAT,0666);  // create file to be
locked
  if (pid==(pid_t)-1) {
    fprintf(stderr, "Error creating /tmp/yyy - %s\n", strerror(errno));
    exit(1);
  }
  lock.l_type = F_WRLCK;                    // lock /tmp/yyy
  lock.l_start = 0;
  lock.l_whence = 0;
  lock.l_len = 0;
  errno = 0;
  if (fcntl(pid, F_SETLKW, &lock) < 0) {
    fprintf(stderr, "Error locking /tmp/yyy - %s\n", strerror(errno));
    exit(1);
  }

  write_status("/tmp/yyy is locked ... sleeping for 10 seconds\n");

  sleep(10);

  status = spawnv(_P_NOWAIT, "/bin/sh", argv);  // spawn some random program
/*  status = posix_spawn(&task_pid, "/bin/sh", NULL, NULL, argv, environ);
*/
  if (errno != 0)
    fprintf(stderr, "Error starting %s (%s)\n", argv[2], strerror(errno));
  else {
    write_status("spawnv() succeeded\n");
  }
  if (access("/tmp/xxx", F_OK)) {               //  check to make sure our
random program worked
    write_status("spawned program did not complete\n");
  } else {
    write_status("spawned program completed\n");
  }

  while (1) {
    write_status("main program idle\n");
    sleep(2);
  }
  exit(0);
}
write_status(char *message)
{
  char ctbuf[90];
  time_t t,time(time_t *);
  t=time((time_t*)0);         /* make a time stamp */
  strcpy(ctbuf,(char *)ctime(&t));
  if (ctbuf[strlen(ctbuf)-1]=='\n') ctbuf[strlen(ctbuf)-1]=(char)0;
  strcat(ctbuf, ": ");
  strcat(ctbuf, message);
  fprintf(stderr, ctbuf);
}

Steve Bardwell





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