PCloseとfcloseの違い

結論を先に書きます:pcloseはwaitpidがpopenの時forkのサブプロセスを呼び出して死体を収めますが、fcloseはしません.したがって、popenを呼び出してfcloseで閉じると、ゾンビプロセスが発生する可能性があります.
醜いテストコード:

#include
#include 
ma()
{
    FILE * fp;
    char buffer[80];
    fp=popen("cat /etc/cwmp.config","r");
    fgets(buffer,sizeof(buffer),fp);
    printf("%s",buffer);
    fclose(fp);     // fclose  
}
main()
{
    int pid = 0;

    ma();
    ma();
    ma();
    sleep(10);
}

プログラムをバックグラウンドで実行し、pstreeで表示すると、3つのゾンビプロセスが発生し、プログラムが終了すると、ゾンビプロセスは親プロセスによって死体を回収されて終了します.
次に、uclibcにおけるpopenとpcloseのソースコードを見てみましょう.

static struct popen_list_item *popen_list /* = NULL (bss initialized) */;

FILE *popen(const char *command, const char *modes)
{
	FILE *fp;
	struct popen_list_item *pi;
	struct popen_list_item *po;
	int pipe_fd[2];
	int parent_fd;
	int child_fd;
	int child_writing;			/* Doubles as the desired child fildes. */
	pid_t pid;

	child_writing = 0;			/* Assume child is writing. */
	if (modes[0] != 'w') {		/* Parent not writing... */
		++child_writing;		/* so child must be writing. */
		if (modes[0] != 'r') {	/* Oops!  Parent not reading either! */
			__set_errno(EINVAL);
			goto RET_NULL;
		}
	}

	if (!(pi = malloc(sizeof(struct popen_list_item)))) {
		goto RET_NULL;
	}

	if (pipe(pipe_fd)) {
		goto FREE_PI;
	}

	child_fd = pipe_fd[child_writing];
	parent_fd = pipe_fd[1-child_writing];

	if (!(fp = fdopen(parent_fd, modes))) {
		close(parent_fd);
		close(child_fd);
		goto FREE_PI;
	}

	VFORK_LOCK;
	if ((pid = vfork()) == 0) {	/* Child of vfork... */
		close(parent_fd);
		if (child_fd != child_writing) {
			dup2(child_fd, child_writing);
			close(child_fd);
		}

		/* SUSv3 requires that any previously popen()'d streams in the
		 * parent shall be closed in the child. */
		for (po = popen_list ; po ; po = po->next) {
			close(po->f->__filedes);
		}

		execl("/bin/sh", "sh", "-c", command, (char *)0);

		/* SUSv3 mandates an exit code of 127 for the child if the
		 * command interpreter can not be invoked. */
		_exit(127);
	}
	VFORK_UNLOCK;

	/* We need to close the child filedes whether vfork failed or
	 * it succeeded and we're in the parent. */
	close(child_fd);

	if (pid > 0) {				/* Parent of vfork... */
		pi->pid = pid;
		pi->f = fp;
		VFORK_LOCK;
		pi->next = popen_list;
		popen_list = pi;
		VFORK_UNLOCK;

		return fp;
	}

	/* If we get here, vfork failed. */
	fclose(fp);					/* Will close parent_fd. */

 FREE_PI:
	free(pi);

 RET_NULL:
	return NULL;
}

#warning is pclose correct wrt the new mutex semantics?

int pclose(FILE *stream)
{
	struct popen_list_item *p;
	int stat;
	pid_t pid;

	/* First, find the list entry corresponding to stream and remove it
	 * from the list.  Set p to the list item (NULL if not found). */
	VFORK_LOCK;
	if ((p = popen_list) != NULL) {
		if (p->f == stream) {
			popen_list = p->next;
		} else {
			struct popen_list_item *t;
			do {
				t = p;
				if (!(p = t->next)) {
					__set_errno(EINVAL); /* Not required by SUSv3. */
					break;
				}
				if (p->f == stream) {
					t->next = p->next;
					break;
				}
			} while (1);
		}
	}
	VFORK_UNLOCK;

	if (p) {
		pid = p->pid;			/* Save the pid we need */
		free(p);				/* and free the list item. */

		fclose(stream);	/* The SUSv3 example code ignores the return. */

		/* SUSv3 specificly requires that pclose not return before the child
		 * terminates, in order to disallow pclose from returning on EINTR. */
		do {
			if (waitpid(pid, &stat, 0) >= 0) {
				return stat;
			}
			if (errno != EINTR) {
				break;
			}
		} while (1);
	}

	return -1;
}