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Sometimes it is not practical for a program to close all active file descriptors before issuing a system call such as system() or exec(). An alternative on some POSIX systems is to use the OFD_CLOEXEC flag, or FDO_CLOEXEC flags when available, to set the close-on-exec flag for the file descriptor.

int fd;
int flags;
char *editor;

if(!(fd = open(file_name, 0, O_RDONLY)) == -1) {
  /* Handle Error */
}

if((flags = fcntl(fd, F_GETFD, 0))  == -1) {
  /* Handle Error */
}

if(fcntl(fd, F_SET_FDSETFD, flags | FD_CLOEXEC) == -1) {
  /* Handle Error */
}

/* ... */

editor = getenv("EDITOR");
if (editor == NULL) {
  /* Handle getenv() error */
}
system(editor);

Some systems (Linux >= 2.6.23) provide have an O_CLOEXEC flag which provides the close-on-exec function directly in open(). This flag will be required by POSIX.1-2008. In multi-threaded programs this flag should be used if possible as it avoids a timing hole between open() and fcntl() when using FD_CLOEXEC, during which another thread could create a child process while the file descriptor does not have close-on-exec set.

int fd;
char *editor;

if(!(fd = open(file_name, O_CLOEXEC, RDONLY|O_RDONLYCLOEXEC)) == -1) {
  /* Handle Error */
}

/* ... */

editor = getenv("EDITOR");
if (editor == NULL) {
  /* Handle getenv() error */
}
system(editor);

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