多进程COW问题求教

ppm10103

#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？

源代码在附件中

鸡丝拌面

The standard I/O library, however, is buffered. Recall from Section 5.12 that standard output is line buffered if it's connected to a terminal device; otherwise, it's fully buffered. When we run the program interactively, we get only a single copy of the printf line, because the standard output buffer is flushed by the newline. But when we redirect standard output to a file, we get two copies of the printf line. In this second case, the printf before the fork is called once, but the line remains in the buffer when fork is called. This buffer is then copied into the child when the parent's data space is copied to the child. Both the parent and the child now have a standard I/O buffer with this line in it. The second printf, right before the exit, just appends its data to the existing buffer. When each process terminates, its copy of the buffer is finally flushed.

鸡丝拌面

ps，lz给出的代码里不是腔调了“/* we don't flush stdout */”嘛。

ppm10103

多谢指点了，就是因为没想明白为什么文本是全缓冲。

现在知道了，块设备是全缓冲。

不同设备的缓冲策略不同