多进程的共享空间疑问

[影子传说_Hades]

apue.h

/* Our own header, to be included before all standard system headers */

#ifndef	_APUE_H
#define	_APUE_H

#if defined(SOLARIS)
#define _XOPEN_SOURCE	500	/* Single UNIX Specification, Version 2  for Solaris 9 */
#define CMSG_LEN(x)	_CMSG_DATA_ALIGN(sizeof(struct cmsghdr)+(x))
#elif !defined(BSD)
#define _XOPEN_SOURCE	600	/* Single UNIX Specification, Version 3 */
#endif

#include <sys/types.h>		/* some systems still require this */
#include <sys/stat.h>
#include <sys/termios.h>	/* for winsize */
#ifndef TIOCGWINSZ
#include <sys/ioctl.h>
#endif
#include <stdio.h>		/* for convenience */
#include <stdlib.h>		/* for convenience */
#include <stddef.h>		/* for offsetof */
#include <string.h>		/* for convenience */
#include <unistd.h>		/* for convenience */
#include <signal.h>		/* for SIG_ERR */

#define	MAXLINE	4096			/* max line length */

/*
 * Default file access permissions for new files.
 */
#define	FILE_MODE	(S_IRUSR | S_IWUSR | S_IRGRP | S_IROTH)

/*
 * Default permissions for new directories.
 */
#define	DIR_MODE	(FILE_MODE | S_IXUSR | S_IXGRP | S_IXOTH)

typedef	void	Sigfunc(int);	/* for signal handlers */

#if	defined(SIG_IGN) && !defined(SIG_ERR)
#define	SIG_ERR	((Sigfunc *)-1)
#endif

#define	min(a,b)	((a) < (b) ? (a) : (b))
#define	max(a,b)	((a) > (b) ? (a) : (b))

/*
 * Prototypes for our own functions.
 */
char	*path_alloc(int *);				/* {Prog pathalloc} */
long	 open_max(void);				/* {Prog openmax} */
void	 clr_fl(int, int);				/* {Prog setfl} */
void	 set_fl(int, int);				/* {Prog setfl} */
void	 pr_exit(int);					/* {Prog prexit} */
void	 pr_mask(const char *);			/* {Prog prmask} */
Sigfunc	*signal_intr(int, Sigfunc *);	/* {Prog signal_intr_function} */

int		 tty_cbreak(int);				/* {Prog raw} */
int		 tty_raw(int);					/* {Prog raw} */
int		 tty_reset(int);				/* {Prog raw} */
void	 tty_atexit(void);				/* {Prog raw} */
#ifdef	ECHO	/* only if <termios.h> has been included */
struct termios	*tty_termios(void);		/* {Prog raw} */
#endif

void	 sleep_us(unsigned int);			/* {Ex sleepus} */
ssize_t	 readn(int, void *, size_t);		/* {Prog readn_writen} */
ssize_t	 writen(int, const void *, size_t);	/* {Prog readn_writen} */
void	 daemonize(const char *);			/* {Prog daemoninit} */

int		 s_pipe(int *);					/* {Progs streams_spipe sock_spipe} */
int		 recv_fd(int, ssize_t (*func)(int,
		         const void *, size_t));/* {Progs recvfd_streams recvfd_sockets} */
int		 send_fd(int, int);				/* {Progs sendfd_streams sendfd_sockets} */
int		 send_err(int, int,
		          const char *);		/* {Prog senderr} */
int		 serv_listen(const char *);		/* {Progs servlisten_streams servlisten_sockets} */
int		 serv_accept(int, uid_t *);		/* {Progs servaccept_streams servaccept_sockets} */
int		 cli_conn(const char *);		/* {Progs cliconn_streams cliconn_sockets} */
int		 buf_args(char *, int (*func)(int,
		          char **));			/* {Prog bufargs} */

int		 ptym_open(char *, int);	/* {Progs3 ptyopen_streams ptyopen_bsd ptyopen_linux} */
int		 ptys_open(char *);			/* {Progs3 ptyopen_streams ptyopen_bsd ptyopen_linux} */
#ifdef	TIOCGWINSZ
pid_t	 pty_fork(int *, char *, int, const struct termios *,
		          const struct winsize *);		/* {Prog ptyfork} */
#endif

int		lock_reg(int, int, int, off_t, int, off_t); /* {Prog lockreg} */
#define	read_lock(fd, offset, whence, len) \
			lock_reg((fd), F_SETLK, F_RDLCK, (offset), (whence), (len))
#define	readw_lock(fd, offset, whence, len) \
			lock_reg((fd), F_SETLKW, F_RDLCK, (offset), (whence), (len))
#define	write_lock(fd, offset, whence, len) \
			lock_reg((fd), F_SETLK, F_WRLCK, (offset), (whence), (len))
#define	writew_lock(fd, offset, whence, len) \
			lock_reg((fd), F_SETLKW, F_WRLCK, (offset), (whence), (len))
#define	un_lock(fd, offset, whence, len) \
			lock_reg((fd), F_SETLK, F_UNLCK, (offset), (whence), (len))

pid_t	lock_test(int, int, off_t, int, off_t);		/* {Prog locktest} */

#define	is_read_lockable(fd, offset, whence, len) \
			(lock_test((fd), F_RDLCK, (offset), (whence), (len)) == 0)
#define	is_write_lockable(fd, offset, whence, len) \
			(lock_test((fd), F_WRLCK, (offset), (whence), (len)) == 0)

void	err_dump(const char *, ...);		/* {App misc_source} */
void	err_msg(const char *, ...);
void	err_quit(const char *, ...);
void	err_exit(int, const char *, ...);
void	err_ret(const char *, ...);
void	err_sys(const char *, ...);

void	log_msg(const char *, ...);			/* {App misc_source} */
void	log_open(const char *, int, int);
void	log_quit(const char *, ...);
void	log_ret(const char *, ...);
void	log_sys(const char *, ...);

void	TELL_WAIT(void);		/* parent/child from {Sec race_conditions} */
void	TELL_PARENT(pid_t);
void	TELL_CHILD(pid_t);
void	WAIT_PARENT(void);
void	WAIT_CHILD(void);

#endif	/* _APUE_H */

apueerror.h

#include <errno.h>      /* for definition of errno */
#include <stdarg.h>     /* ISO C variable aruments */

#define MAXLINE 4096
static void err_doit(int, int, const char *, va_list);

/*
 * Nonfatal error related to a system call.
 * Print a message and return.
 */
void err_ret(const char *fmt, ...) {
	va_list ap;

	va_start(ap, fmt);
	err_doit(1, errno, fmt, ap);
	va_end(ap);
}

/*
 * Fatal error related to a system call.
 * Print a message and terminate.
 */
void err_sys(const char *fmt, ...) {
	va_list ap;

	va_start(ap, fmt);
	err_doit(1, errno, fmt, ap);
	va_end(ap);
	exit(1);
}

/*
 * Fatal error unrelated to a system call.
 * Error code passed as explict parameter.
 * Print a message and terminate.
 */
void err_exit(int error, const char *fmt, ...) {
	va_list ap;

	va_start(ap, fmt);
	err_doit(1, error, fmt, ap);
	va_end(ap);
	exit(1);
}

/*
 * Fatal error related to a system call.
 * Print a message, dump core, and terminate.
 */
void err_dump(const char *fmt, ...) {
	va_list ap;

	va_start(ap, fmt);
	err_doit(1, errno, fmt, ap);
	va_end(ap);
	abort(); /* dump core and terminate */
	exit(1); /* shouldn't get here */
}

/*
 * Nonfatal error unrelated to a system call.
 * Print a message and return.
 */
void err_msg(const char *fmt, ...) {
	va_list ap;

	va_start(ap, fmt);
	err_doit(0, 0, fmt, ap);
	va_end(ap);
}

/*
 * Fatal error unrelated to a system call.
 * Print a message and terminate.
 */
void err_quit(const char *fmt, ...) {
	va_list ap;

	va_start(ap, fmt);
	err_doit(0, 0, fmt, ap);
	va_end(ap);
	exit(1);
}

/*
 * Print a message and return to caller.
 * Caller specifies "errnoflag".
 */
static void err_doit(int errnoflag, int error, const char *fmt, va_list ap) {
	char buf[MAXLINE];
	vsnprintf(buf, MAXLINE, fmt, ap);
	if (errnoflag)
		snprintf(buf + strlen(buf), MAXLINE - strlen(buf), ": %s", strerror(
				error));
	strcat(buf, "\n");
	fflush(stdout); /* in case stdout and stderr are the same */
	fputs(buf, stderr);
	fflush(NULL); /* flushes all stdio output streams */
}

#include "apue.h"
#include "apueerror.h"

//./a.out > temp.out 会产生两次before fork
int glob = 6; /* external variable in initialized data */
char buf[] = "a write to stdout\n";
int main(void) {
	int var; /* automatic variable on the stack */
	pid_t pid;
	var = 88;
	if (write(STDOUT_FILENO, buf, sizeof(buf) - 1) != sizeof(buf) - 1)
		err_sys("write error");
	printf("before fork\n"); /* we don't flush stdout */
	if ((pid = fork()) < 0) {
		err_sys("fork error");
	} else if (pid == 0) { /* child */
		glob++; /* modify variables */
		var++;
	} else {
		sleep(2); /* parent */
	}
	printf("pid = %d, glob = %d, var = %d\n", getpid(), glob, var);
	exit(0);
}

为什么
./a.out 只会产生一次 before fork
./a.out > temp.out 会产生两次before fork

为什么重定向才会有两次呢？

[赵4老师]

多进程用共享临时文本文件

[影子传说_Hades]

还需要您再细细讲一下

为啥./a.out 只会产生一次 before fork

多进程共享STD out 为啥只产生一次 before fork

[qq120848369]

因为终端是行缓冲，文件是全缓冲，你的输出printf没有\n，所以fork的子进程的stdin._iobuf里也有那句话，两个进程都结束的时候冲刷缓冲区都写到这个文件里了。

[影子传说_Hades]

[Quote=引用 3 楼 qq120848369 的回复:]
因为终端是行缓冲，文件是全缓冲，你的输出printf没有\n，所以fork的子进程的stdin._iobuf里也有那句话，两个进程都结束的时候冲刷缓冲区都写到这个文件里了。
[/Quote]

非常感谢，想了半天也没想通，您一说就懂了。