我制作了制作 fork 的程序，我认为 child 不会影响 parent 。

虽然我没有在父级中做任何更改，但是文件指针已更改。

#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <sys/types.h>
#include <sys/wait.h>

int main(void) {
    FILE *fp = fopen("sm.c", "r");
    char buf[1000];
    char *args[] = {"invailid_command", NULL};

    fgets(buf, sizeof(buf), fp);
    printf("I'm one %d %ld\n", getpid(), ftell(fp));
    if (fork() == 0) {
        execvp(args[0], args);
        exit(EXIT_FAILURE);
    }
    wait(NULL);
    printf("I'm two %d %ld\n", getpid(), ftell(fp));
} 

这输出

I'm one 21500 20
I'm two 21500 -1

而且我想让文件指针在两次 printf 调用之间不变。

为什么文件指针会改变，即使 execvp 失败，我能否使文件指针不变？

最佳答案

感谢 Jonathan Leffler 为我们指明了正确的方向。

尽管您的程序在 CentOS 7/GCC 4.8.5/GLIBC 2.17 上没有对我产生相同的意外行为，但您观察到不同的行为是合理的。根据 POSIX(fork 所依赖的)，您的程序的行为实际上是未定义。以下是 the relevant section 的一些摘录(强调):

An open file description may be accessed through a file descriptor, which is created using functions such as open() or pipe(), or through a stream, which is created using functions such as fopen() or popen(). Either a file descriptor or a stream is called a "handle" on the open file description to which it refers; an open file description may have several handles.

[...]

The result of function calls involving any one handle (the "active handle") is defined elsewhere in this volume of POSIX.1-2017, but if two or more handles are used, and any one of them is a stream, the application shall ensure that their actions are coordinated as described below. If this is not done, the result is undefined.

[...]

For a handle to become the active handle, the application shall ensure that the actions below are performed between the last use of the handle (the current active handle) and the first use of the second handle (the future active handle). The second handle then becomes the active handle. [...]

The handles need not be in the same process for these rules to apply.

Note that after a fork(), two handles exist where one existed before. The application shall ensure that, if both handles can ever be accessed, they are both in a state where the other could become the active handle first. [Where subject to the preceding qualification, the] application shall prepare for a fork() exactly as if it were a change of active handle. (If the only action performed by one of the processes is one of the exec functions or _exit() (not exit()), the handle is never accessed in that process.)

For the first handle, the first applicable condition below applies. [An impressively long list of alternatives that do not apply to the OP's situation ...]

• If the stream is open with a mode that allows reading and the underlying open file description refers to a device that is capable of seeking, the application shall either perform an fflush(), or the stream shall be closed.

For the second handle:

• If any previous active handle has been used by a function that explicitly changed the file offset, except as required above for the first handle, the application shall perform an lseek() or fseek() (as appropriate to the type of handle) to an appropriate location.

因此，为了让 OP 的程序在父级和子级中访问相同的流，POSIX 要求父级 fflush() stdin在 fork 之前，那个 child fseek()启动后。然后，在等待 child 终止后， parent 必须 fseek()流。鉴于我们知道 child 的 exec 会失败，但是，可以通过让 child 使用 _exit() 来避免对所有刷新和查找的要求。 (不访问流)而不是 exit() .

遵守 POSIX 的规定会产生以下结果:

When these rules are followed, regardless of the sequence of handles used, implementations shall ensure that an application, even one consisting of several processes, shall yield correct results: no data shall be lost or duplicated when writing, and all data shall be written in order, except as requested by seeks.

但值得注意的是

It is implementation-defined whether, and under what conditions, all input is seen exactly once.

我理解仅仅听到您对程序行为的期望不符合相关标准可能有点不满意，但事实仅此而已。父进程和子进程确实有一些相关的共享数据，其形式为公共(public)打开文件描述(它们有单独的关联句柄)，这似乎很可能是意外(和undefined) 行为，但没有任何基础可以预测您看到的特定行为，也没有预测我看到的同一程序的不同行为。

关于fork 和(失败的)exec 后 C 文件指针发生变化，我们在Stack Overflow上找到一个类似的问题： https://stackoverflow.com/questions/50914178/