C 遍历目录并成对打印条目

Question

假设我有以下代码抽象:

  DIR *d;
  struct dirent *sd; 
  d = opendir("SOME DIRECTORY");
  // handle errors 
  sd = readdir(d);
  while(sd != NULL){
    printf("(%s, %s)\n", sd->d_name, sd->the_next_d_name);
    sd = readdir(d);
  }
  closedir(dir);

我对 c 有点陌生，正在详细了解 man 页面。我只是想了解如何在循环体中访问目录的两个条目。该代码使用一些假名来表达这个想法。另外，如果有奇数个条目，我希望最后一对有一个与 NULL

配对的条目

无论如何，谢谢。

Answer 1

主要问题是 readdir()函数在第二次调用时不会保留先前返回的值:

The application shall not modify the structure to which the return value of readdir() points, nor any storage areas pointed to by pointers within the structure. The returned pointer, and pointers within the structure, might be invalidated or the structure or the storage areas might be overwritten by a subsequent call to readdir() on the same directory stream. They shall not be affected by a call to readdir() on a different directory stream.

因此，您必须复制每对条目中第一个条目的名称。想必，您有 strdup()可用，所以这是最小的痛苦。 (如果您没有 strdup()，则很容易编写自己的版本 — 7 行，包括仅带有大括号的行。)

#include <dirent.h>
#include <stdio.h>
#include <string.h>
#include <stdlib.h>
#include "stderr.h"

int main(int argc, char **argv)
{
    err_setarg0(argv[0]);
    if (argc > 2)
        err_usage("[directory]");
    char *dir = ".";
    if (argc == 2)
        dir = argv[1];
    DIR *dp = opendir(dir);
    if (dp == NULL)
        err_syserr("failed to open directory '%s': ", dir);
    char *name_0 = 0;
    int counter = 0;
    struct dirent *name;
    while ((name = readdir(dp)) != NULL)
    {
        if (++counter % 2 == 1)
        {
            if ((name_0 = strdup(name->d_name)) == NULL)
                err_syserr("failed to duplicate name '%s': ", name->d_name);
        }
        else
        {
            printf("(%s, %s)\n", name_0, name->d_name);
            free(name_0);
            name_0 = NULL;
        }
    }
    if (counter % 2 == 1)
        printf("(%s, %s)\n", name_0, "NULL");
    free(name_0);
    closedir(dp);
    return 0;
}

(stderr.h 中声明的函数均以 err_ 开头，大大简化了错误报告。您可以在 GitHub 上找到它们的代码 https://github.com/jleffler/soq/tree/master/src/libsoq 。)