lstat:无法访问另一个目录中的文件 [英] lstat: can't access files in another directory

问题描述

我正在尝试编写类似ls的程序，该程序产生具有许可权，所有者，时间和文件名的类似 ls -l <​​/code>的输出.如果我通过.(或不执行任何操作)，则效果很好，因此它适用于当前目录.但是，如果我在当前目录中传入或传出其他任何目录，则 perror 会说它无法访问" 文件.

I'm trying to write ls-alike program that produces output like ls -l with permissions, owners, time and name of the file. It works good if I pass . (or nothing), so it works with the current directory. But if I pass any other directory in or out of the current one, perror says it "can't access" the files.

请帮助我找出阻止 lstat 访问其他目录中文件的原因.

Please, help me figure out what prevents lstat from accessing files in another dirs.

我使用gcc和一个没有IDE的文本编辑器，开始学习使用gdb(试图进行调试，但是没有找到可以使我指出要寻找哪种解决方案的东西).这就是为什么我决定将所有代码都放在这里，以便任何人都可以运行它的原因.也许我传递了错误的参数，也许我不知道是 lstat 的某种错误行为.我一直在尝试在网上找到有关此内容的信息，但没有结果.

I use gcc and a text editor, no IDE, started learning to use gdb (tried to debug but didn't find something that would point me to what kind of solution shall I look for). That's why I decided to put here all the code, so anybody can run it. Maybe I pass wrong arguments, maybe it is some kind of lstat's wrong behavior, I don't know. I've been trying to find something about it on the web, but with no result.

这是我到目前为止所做的:

Here is what I've done up to this moment:

#include <stdio.h>
#include <stdlib.h>
#include <dirent.h>
#include <sys/stat.h>
#include <errno.h>
#include <fcntl.h>     // open()
#include <unistd.h>    // close()
#include <sys/param.h>
#include <string.h>
#include <limits.h>    // PATH_MAX
#include <pwd.h>       // structure for getpwuid()
#include <grp.h>       // structure for getgrgid()
#include <time.h>      // ctime()

static void ls(const char *dir);
void get_info(const char *name, int offset);
int get_maxsize(const char *name);


int main(int argc, char ** argv)
{
    if (argc == 1)
        ls(".");
    else
        while (--argc > 0)
            ls(*++argv);
    return 0;
}


static void ls(const char *dir)
{
    struct dirent * entry;
    DIR *d = opendir(dir);
    char pathbuf[PATH_MAX + 1];
    int offset = 0;

    if (d == 0) {
        perror("ls");
        return;
    }

    /* find max file size for better ls-alike output */
    while ((entry = readdir(d)) != NULL) {
        realpath(entry->d_name, pathbuf);
        /* pathbuf OR entry->d_name here: */
        if (get_maxsize(entry->d_name) > offset)
            offset = get_maxsize(pathbuf);
    }
    closedir(d);

    d = opendir(dir);
    while ((entry = readdir(d)) != NULL) {
        /* pathbuf OR entry->d_name here: */
        realpath(entry->d_name, pathbuf);
        get_info(entry->d_name, offset);
    }
    closedir(d);
}


void get_info(const char *name, int offset)
{
    struct stat statbuf;
    struct passwd *pwdPtr;
    struct group *grpPtr;
    int length = 0;
    char *time = NULL;

    /* skip . and .. dirs */
    if (strcmp(name, ".") == 0 || strcmp(name, "..") == 0)
        return;

    if (lstat(name, &statbuf) == -1) {
        fprintf(stderr, "can't access %s\n", name);
        perror("get_info");
        return;
    } 
    else
        switch (statbuf.st_mode & S_IFMT) {
            case S_IFREG: printf("-"); break;
            case S_IFDIR: printf("d"); break;
            case S_IFCHR: printf("c"); break;
            case S_IFBLK: printf("b"); break;
            case S_IFLNK: printf("l"); break;
            case S_IFSOCK: printf("s"); break;
            case S_IFIFO: printf("p"); break;
        }

    /* owner */
    if (statbuf.st_mode & S_IREAD) printf("r");
    else printf("-");
    if (statbuf.st_mode & S_IWRITE) printf("w");
    else printf("-");
    if (statbuf.st_mode & S_IEXEC) printf("x");
    else printf("-");

    /* group */
    if (statbuf.st_mode & S_IRGRP) printf("r");
    else printf("-");
    if (statbuf.st_mode & S_IWGRP) printf("w");
    else printf("-");
    if (statbuf.st_mode & S_IXGRP) printf("x");
    else printf("-");

    /* other users */
    if (statbuf.st_mode & S_IROTH) printf("r");
    else printf("-");
    if (statbuf.st_mode & S_IWOTH) printf("w");
    else printf("-");
    if (statbuf.st_mode & S_IXOTH) printf("x");
    else printf("-");

    /* hard links */
    printf(" %2zu", statbuf.st_nlink);

    /* owner name */
    if ((pwdPtr = getpwuid(statbuf.st_uid)) == NULL) {
        perror("getpwuid");
        exit(EXIT_FAILURE);
    } 
    else {
        printf(" %s", pwdPtr->pw_name);
    }

    /* gruop name */
    if ((grpPtr = getgrgid(statbuf.st_gid)) == NULL) {
        perror("getgrgid");
        exit(EXIT_FAILURE);
    } 
    else {
        printf(" %s", grpPtr->gr_name);
    }

    /* size in bytes */
    /* "C uses an asterisk in the position of the field
       width specifier to indicate to printf that it will
       find the variable that contains the value of the field
       width as an additional parameter." 
       http://www.eecs.wsu.edu/~cs150/reading/printf.htm */
    while(offset != 0)
    {
        offset /= 10;             /* n=n/10 */
        ++length;
    }
    printf(" %*d", length, (int)statbuf.st_size);

    /* last modifying time */
    time = ctime(&statbuf.st_mtime);
    time[strlen(time) - 1] = 0;
    printf(" %s", time);

    /* index */
    // ToDo

    /* filename */
    printf(" %s", name);

    printf("\n");
    // -,d,c,b,l,s,p
    //if ((statbuf.st_mode & S_IFMT) == S_IFREG)
    //    printf("- %8ld %s\n", statbuf.st_size, name);
}


int get_maxsize(const char *name)
{
    struct stat statbuf;

    if (lstat(name, &statbuf) == -1) {
        fprintf(stderr, "can't access %s\n", name);
        perror("get_maxsize");
        return -1;
    }
    return statbuf.st_size;
}

正常工作时输出(仅适用于当前目录):

Output when it works fine (only with current directory):

yulian@deb:~/programming/os$ ./readDir .
-rw-rw-rw-  1 yulian yulian  4387 Mon Nov 30 06:31:51 2015 readDir.c
-rw-rw-rw-  1 yulian yulian   282 Sun Nov 29 04:43:03 2015 sometext.txt
-rwxr-xr-x  1 yulian yulian 13792 Sat Nov 28 11:54:09 2015 readDir
drwxr-xr-x  2 yulian yulian  4096 Fri Nov 27 05:26:42 2015 testDir 
// there is test dir called `testDir` where it fails

失败时输出:

yulian@deb:~/programming/os$ ./readDir testDir/
can't access 2.jpg
get_maxsize: No such file or directory
can't access ETicket_edc7cb12cdc23e6c04a308f34fd31c28.pdf
get_maxsize: No such file or directory

更新:将建议的解决方案添加到 get_info :

update: with the suggested solution I added to get_info:

....
char *filemane = NULL;

filemane = strrchr(name, '/') + 1;
/* to prevent . and .. from output */
if (strcmp(filemane, ".") == 0 || strcmp(filemane, "..") == 0)
    return;
...
/* filename */
printf(" %s", filemane);          // and changed the argument here

因此，输出现在生成的文件名与 ls -l <​​/code>完全相同，而不是其完整路径.

so the output now produces file names exactly as ls -l, not their full paths.

推荐答案

您缺少的是将要探索的 dir 添加到pathbuf变量中.只需考虑使用以下ls

What you're missing is to add the dir you want to explore into the pathbuf variable. Simply consider using the following implementation of ls

static void ls(const char *dir)
{
    struct dirent * entry;
    DIR *d = opendir(dir);
    char pathbuf[PATH_MAX + 1];
    char tmp[PATH_MAX+1];
    int offset = 0;

    if (d == 0) {
        perror("ls");
        return;
    }

    /* find max file size for better ls-alike output */
    while ((entry = readdir(d)) != NULL) {
        /* pathbuf OR entry->d_name here: */
        // realpath(entry->d_name, pathbuf);
        snprintf (tmp, PATH_MAX, "%s/%s", dir, entry->d_name);
        if (get_maxsize(tmp) > offset)
            offset = get_maxsize(tmp);
    }
    closedir(d);

    d = opendir(dir);
    while ((entry = readdir(d)) != NULL) {
        /* pathbuf OR entry->d_name here: */
        // realpath(entry->d_name, pathbuf);
        snprintf (tmp, PATH_MAX, "%s/%s", dir, entry->d_name);
        get_info(tmp, offset);
    }
    closedir(d);
}

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