重新打开与另一个访问的文件描述符？ [英] Reopen a file descriptor with another access?

问题描述

假设操作系统是Linux。假设我打开写的文件，并得到一个文件描述符 FDW 。是否有可能得到另一个文件描述符 FDR ，与该文件只读访问，而无需调用打开一遍吗？我不想打电话的原因打开是底层文件可能已被移动或甚至被其他进程的文件系统中取消链接，所以重复使用相同的文件名不反对这样的行动可靠。所以我的问题是：反正有权利，如果只给一个文件描述符打开不同的访问文件描述符？ DUP 或 dup2 不改变访问权，我想。

Assume the OS is linux. Suppose I opened a file for write and get a file descriptor fdw. Is it possible to get another file descriptor fdr, with read-only access to the file without calling open again? The reason I don't want to call open is the underlying file may have been moved or even unlinked in the file system by other processes, so re-use the same file name is not reliable against such actions. So my question is: is there anyway to open a file descriptor with different access right if given only a file descriptor? dup or dup2 doesn't change the access right, I think.

推荐答案

是的！诀窍是通过来访问已删除的文件/ proc /自/ FD / N 。这是一个只有Linux帽子戏法，据我所知。

Yes! The trick is to access the deleted file via /proc/self/fd/n. It’s a linux-only trick, as far as I know.

运行此程​​序：

#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>

int main() {
    FILE* out_file;
    FILE* in_file;
    char* dev_fd_path;
    char buffer[128];

    /* Write "hi!" to test.txt */
    out_file = fopen("test.txt", "w");
    fputs("hi!\n", out_file);
    fflush(out_file);

    /* Delete the file */
    unlink("test.txt");

    /* Verify that the file is gone */
    system("ls test.txt");

    /* Reopen the filehandle in read-mode from /proc */
    asprintf(&dev_fd_path, "/proc/self/fd/%d", fileno(out_file));
    in_file = fopen(dev_fd_path, "r");
    if (!in_file) {
        perror("in_file is NULL");
        exit(1);
    }
    printf("%s", fgets(buffer, sizeof(buffer), in_file));

    return 0;
}

它写入一些文本文件，将其删除，但保持文件描述符打开，然后通过不同的路线重新打开它。文件不会被真正删除，直到最后一个进程抱着最后的文件描述符关闭它，在那之前，你可以通过的/ proc 在该文件的内容得到。

感谢我的老东家阿纳托利教我这一招，当我删除了那个幸运的仍在另一个进程附加到一些重要的文件！

Thanks to my old boss Anatoly for teaching me this trick when I deleted some important files that were fortunately still being appended to by another process!

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