在处理文件系统路径的时候,我们一般会先开辟一块内存区,用来接收路径、或者拼接好路径传递给系统调用。这是因为路径在各个系统上都有最大长度限制,在 Windows 上这个值是 MAX_PATH,一般不能超过 260;在 Linux 上这个值是 PATH_MAX,一般不能超过 4096 (或者通过 pathconf (_PC_PATH_MAX, ...) 来获取,但是一般也是 4096),就像下面这段典型的代码:
1 int main () 2 { 3 #ifdef WIN32 4 char buf[MAX_PATH + 1] = { 0 }; 5 if (GetModuleFileNameA(NULL, buf, MAX_PATH) == 0) 6 { 7 printf("get current module path failed, errno %d", GetLastError()); 8 return -1; 9 } 10 #else 11 char buf[PATH_MAX + 1] = { 0 }; 12 if (readlink("/proc/self/exe", buf, PATH_MAX) < 0) 13 { 14 printf("read exe path failed, errno %d", errno); 15 return -1; 16 } 17 #endif 18 19 printf("current executable file path: %s\n", buf); 20 return 0; 21 }
它用来获取当前可执行文件的完整路径。在 Windows 与 Linux 上分别调用了 GetModuleFileName 与 readlink 系统调用,且事先在栈上分配了接收完整路径的内存区 buf。最后将成功获取的路径打印到控制台。在两种系统上,输出分别如下:
Windows
current executable file path: E:\code\apue\02.chapter\Release\path_max.exe
Linux
current executable file path: /home/yunhai/code/apue/02.chapter/path_max
其它与路径相关的调用也是类似的。那么问题来了,这个最大路径长度是为了方便程序编写 (不然需要动态分配内存,且需要两次调用,其中一次用于获取最终的路径长度),还是说底层的文件系统就只能支持这么长的路径呢?
为了弄清楚这个问题,我专门写了一个测试程序:
1 #ifdef WIN32 2 # ifndef _WIN32_WINNT // 指定要求的最低平台是 Windows Vista。 3 # define _WIN32_WINNT 0x0600 // 将此值更改为相应的值,以适用于 Windows 的其他版本。 4 # endif 5 #include <windows.h> 6 #include <direct.h> 7 #else 8 #include <limits.h> 9 #include <unistd.h> 10 #include <fcntl.h> 11 #include <errno.h> 12 #include <sys/stat.h> 13 #endif 14 #include <string.h> 15 #include <stdio.h> 16 #include <stdlib.h> 17 #include <time.h> 18 19 void get_random_name (char *name, int len, int level) 20 { 21 int i, n; 22 sprintf (name, "%d", level); 23 n = strlen (name); 24 for (i=n; i<len; ++ i) 25 name[i] = rand () % 26 + 'a'; 26 27 name [len] = 0; 28 } 29 30 int main (int argc, char *argv[]) 31 { 32 int dir_len = 0, file_len = 0; 33 char *name = 0; 34 int ret = 0, level = 0; 35 #ifdef WIN32 36 int name_max = MAX_PATH; 37 int path_max = MAX_PATH; 38 HANDLE fd = 0; 39 #else 40 char const* path = "/"; 41 int name_max = pathconf (path, _PC_NAME_MAX); 42 int path_max = pathconf (path, _PC_PATH_MAX); 43 #endif 44 45 printf ("NAME_MAX = %d, PATH_MAX = %d\n", 46 name_max, path_max); 47 48 // add 1 (/) to 10 to be conveniently to compute how low the path is with level. 49 dir_len = 9; 50 file_len = 100; 51 52 srand (time(0)); 53 name = (char *) calloc (1, (dir_len > file_len ? dir_len : file_len) + 1); 54 if (name == 0) 55 return -1; 56 57 58 do 59 { 60 get_random_name (name, dir_len, ++level); 61 #ifdef WIN32 62 //ret = _mkdir (name); 63 //if (ret == -1) 64 ret = CreateDirectory(name, NULL); 65 if (!ret) 66 { 67 printf ("CreateDirectory %s failed, errno = %d\n", name, GetLastError ()); 68 break; 69 } 70 #else 71 ret = mkdir (name, 0777); 72 if (ret == -1) 73 { 74 printf ("mkdir %s failed, errno = %d\n", name, errno); 75 break; 76 } 77 #endif 78 79 printf ("mkdir %s\n", name); 80 #ifdef WIN32 81 //ret = _chdir (name); 82 //if (ret == -1) 83 ret = SetCurrentDirectory(name); 84 if (!ret) 85 { 86 printf ("SetCurrentDirectory %s failed, errno = %d\n", name, GetLastError ()); 87 break; 88 } 89 #else 90 ret = chdir (name); 91 if (ret == -1) 92 { 93 printf ("chdir %s failed, errno = %d\n", name, errno); 94 break; 95 } 96 #endif 97 98 //printf ("change to that dir\n"); 99 get_random_name (name, file_len, level); 100 #ifdef WIN32 101 fd = CreateFile (name, GENERIC_READ | GENERIC_WRITE, FILE_SHARE_READ, NULL, OPEN_ALWAYS, 0, 0); 102 if (fd == INVALID_HANDLE_VALUE) 103 printf ("open %s failed, errno = %d\n", name, GetLastError ()); 104 else 105 { 106 printf ("open %s OK.\n", name); 107 CloseHandle (fd); 108 } 109 #else 110 int fd = open (name, O_RDWR | O_CREAT, 0644); 111 if (fd == -1) 112 printf ("open %s failed, errno = %d\n", name, errno); 113 else 114 { 115 printf ("open %s OK.\n", name); 116 close (fd); 117 } 118 #endif 119 } while (1); 120 121 free (name); 122 return 0; 123 }
这个小程序很简单,每次创建一个目录,切换进去,创建一个文件。这样周而复始,直到出错退出。目录名长度固定为 9,这样加上目录分隔符(/ 或 \)就正好凑成 10,方便之后根据目录深度计算路径总长度;文件名长度固定为 100,也是为了方便计算路径总长度。每个名字都是由 26 个字母随机组成的,同时将目录深度 (level)记录在名字开始。例如第 10 级目录可能长这样:“10jmvrfqv”,前面的数字 10 明确的标识了目录层级,同理,文件名前面也是层级数字,也是为了方便计算。好了,我们分别在两个系统上跑一下这个测试程序。
NAME_MAX = 260, PATH_MAX = 260 mkdir 1tdobxciu open 1gybhgvdtngozbkzvcuvpnitcpntxgtjhasljfycfowuuqmcvvitsthzdxpckttobaqtccxeabfmnvhoeknyjifabnynkrjwzhiq OK. mkdir 2wafsehgp open 2ssywxcafcvpuppgxjcpctjkleftvyhwpuzydlcaaxkbsgumljccmokznqpkvliklndweyegqxvmrmtcrfwzqyllezvneztvqwma OK. mkdir 3vmkjwgrp open 3bclyodmcwjgzmokchhkrmharodswgmpbusxozgqogaguvppohpthetfgqxrmihdjjmqsxvdzgondfsirxfmqbmgmexdnbdjyrqr OK. mkdir 4xmvhqrfh open 4hclzbqzoniuuolhumysibxonqutbpqgvroamdwdorhypsfkrkyskqykdukfrngtfipgjidvazgsvdfejofbjqqwpthkfxzgwubw OK. mkdir 5rwoxiiwh open 5sdeemddmvkhszsubkojdemrconicercdpdcmsmitbbuxowvsqbfghagpwkgmdrytzfnafoqvwsktiwdkjskfukxtxjeknctdjcm OK. mkdir 6fnmlhdor open 6vjrqxecbutxcospyzihwjabulwzbvbwimubvvchdxgyhqfjebjnpbbhvtwrkjlowmzbpoqkshwjbzoqysxfthztefvzwgarrdon OK. mkdir 7sbvhcsxw open 7ddmdylgkhghdeefydnjfsmiyxwutewbmujsppdezpoamtwodvqvkyaeswywbfnvjctofiaftmkiwapbjawuhjvuqsmefjpfgbew OK. mkdir 8dwhpsgaj open 8xepfibfufsaefmghnjukuddbqhyijunkazscfjxkznnylylpcaiauwyrzcfixbjfguvimzwrabcnvxomkaelrwtgnhsjxpiwmii OK. mkdir 9wmpqcalh open 9ldugmypucabxtjnpxqdvinxtevztnspuakrwwhebpshzsgbkedvmxbibkwyaoxctfelwjsglvdwjfjffuleijofuoinnrdcdymf OK. mkdir 10vlwydie open 10jihhaxrcpsviddxserikiiyrtbiwdumzdsyhkkodreimjxivkpnnxxtgpuathmqatlajmfueupwsdwjhyferopxiqggkywccvz OK. mkdir 11awvhauw open 11ypsctazqpuebetmzzgrzasmjmzppymtdabgcajahhwuwilzxlgvpupuksrjtmigjogjyqxhczuunlknxupfixnyxlyjsdhgwuf OK. mkdir 12pdznsnw open 12easlcjvcdudtktfkseedbtlbvnjhjtlchtjbbshmjbdsnpifidqjwsttalzwlyjezixtghwortsusrifigbxhpznmiigtvviym OK. mkdir 13pvvjtcb open 13lbpsmpbwyzzcnjlixarmvbxpqeketgycbduhzlhhakxykncmxwhmgqrwigcywiehemmhlinrhxnxbktentpvnlazrzgniwgcnf OK. mkdir 14jrhanvs open 14nlvfzgxyfzekthrzidzoeugldefwnxlelbntfetdnxvxkcupkpnzxhytidycrstkchdojzrsdjxclkvfgvmmsenysyyinkylsd OK. mkdir 15yilwbim open 15dhjbeoeybggfrjpstbyobosrmigtfhmpbajgwhjgczgklphxweefjoliwbuabyxyfsnzjzpiaxdagfbkujrqistslqkffqmilx OK. mkdir 16owgedmp open 16cqauvpdzsghgmzdbdxaqdclrvuyophifemvsoygwuhbtlkkmwmwxekwcxgmauagexxtyirthfgjbztmutljstjiwcbwmntkfri failed, errno = 3 mkdir 17xljxtht open 17umkwtdbxiayekrljssfnxgvcmytthwevlmrxptivqmvwrkpqkvxoircuwadhkyidoaydlvnnafejpssqdewcymluzmrwrrkwmn failed, errno = 3 mkdir 18aywtihk open 18ynydjmncmakzoezcupoeqfiahsunxhqvczbuapntehglrrhubpbydnnpnbpjdschmrkzkkpvgfajemsfsvofsjoclijforjkex failed, errno = 3 mkdir 19qsvtxwm open 19qpfdrjgnxthljturhvuymuoqctsmixxrircoievqpgrnxpzufozwvpsjocdxfhjodpfzljbgegutfsquaehjoaswqhwtsqbgmh failed, errno = 3 mkdir 20vzvuovy open 20zqqxsfnxrccsubxwevfffunrxfugkovltzidalrmppjjyrbnohxgmwnmwcseijuhzabngyfodgodnblmvhmvnmkvqfijucfqfz failed, errno = 3 mkdir 21idgwupz open 21vlkqahypwulaaykizzvqviqztswdwikvloswluydfyqmgjcaixqqrilsrjireprfrvbgfvspulfmnuksbvsnfkdncybuhxddtw failed, errno = 3 mkdir 22plbyhbe open 22rbekcixmxcohnvqlyhlppmgqtpsgsjwxhuwlmhnzspkbvdrnzechjbjsglzajiadiqguutgfjfkaqdtfqeevpcrsejiiotytse failed, errno = 3 mkdir 23vggwvra open 23tlbobatwbdpaxouojluctxqyddllqedqbeotpkkxgddristpprablooxlomgvneixubhxhemtlvipqwzzfgqcxjtdltytpialw failed, errno = 3 mkdir 24dkpkfup open 24fujnicyzpudgnsuzcqkkgihsousejefcuepnkkxdpdxbxxfryeglrmikgxxckazfokmdrgttnqxavphpoekagfbenjyuxpfntu failed, errno = 3 CreateDirectory 25aajjgcq failed, errno = 206
我是在 G:\ 根目录运行这个程序的,跑到第 16 级目录时,CreateFile 失败返回了,错误码为 3 (ERROR_PATH_NOT_FOUND:系统找不到指定的路径),对于文件创建失败,这里忽略,所以程序继续运行;跑到第 25 级目录时,CreateDirectory 也失败返回了,错误码为 206 (ERROR_FILENAME_EXCED_RANGE:文件名或扩展名太长),因为不能再继续了,所以整个程序终止。看了一下,到 16 级目录时整个路径是这样:
总长度达到 162,此时如果想再创建文件的话,总长度会升至 162 + 1 + 100 = 263 > MAX_PATH,所以接下来的文件创建失败了。然后再看下到 24 级目录时路径的样子:
总长度达到 243,如果再加上一个目录的长度的话,应该是253 < MAX_PATH,按理说应该还可以容纳一个目录的创建,但是我实地在这个目录下面尝试创建目录,得到了这样的错误:
如果是创建文件的话,会发现输入一定长度的文件名之后,就输入不了了:
这个长度目前是 16 (算上后缀 .txt 4个字符),加上之前目录的长度 243,总长度为 243 + 1 + 16 = 260,正好等于 MAX_PATH。可见,对于 Windows 而言,这个 MAX_PATH 就是底层文件系统的限制,再怎么玩也玩不出这个杠杠。尝试切换 CreateDirectory / SetCurrentDirectory 实现为 _mkdir / _chdir,看看有什么不同 :
line 62-65
ret = _chdir (name); if (ret == -1) //ret = SetCurrentDirectory(name); //if (!ret)
line 81-84
ret = _mkdir (name); if (ret == -1) //ret = CreateDirectory(name, NULL); //if (!ret)
重跑上面的例子,会得到完全一样的结果。
一开始我是在 CentOS i686 环境上跑的,没想到一阵跑之后再回来看,虚拟机桌面居然重置了,所有 shell 都退出了,没有截到任何输出,无奈只好采用另一个平台了。在这个中标麒麟的环境跑完了全程:
$ ./path_max NAME_MAX = 255, PATH_MAX = 4096 mkdir 1rkxtumwh open 1tovmschzmqujtonfqbqyfomyykrtwqcrezfxbmypesyygmfxpvxvjmvkwnfudhnhgsghgfwkzxkgjpfamcxwotimgniluvucpbl OK. mkdir 2vijihgsn open 2rkuswxpuniccqrkbliwpzzaxhkfqqzgklackzrgpclrscduqnsfnrikasrqkswuexwqwqwlshekkigaxyhkrpwrjniuieqodnea OK. mkdir 3fanxiuju open 3cqwzqdmivizeyjygopureytlajkkdtehlciegumddnhdxgjnxfedfaofjysmtytfbbjjyxmdmtgjbrwywdddfrkopcclbxqcabn OK. mkdir 4yazbmsjy open 4wawuybydgrowjqyutxmxynkypjcbelzaowwoxwsgqiczabtuaftyuezjpbmvomxekutjsnpixuhxxctxhnwdtvmiybemnbrzxmk OK. mkdir 5sbcaywkx open 5torqwgobzlqjlrnygrrhqdrigvletvbmjufhcvldjbnuscuavlhmqawwwhcpegeqalzfgkirnvmfaghvuojkpghlnlcsrgkurjz OK. mkdir 6aujthffp open 6howeinoseubrjntbgclavxzxrjraqwrzmqdudrphnsaxhtaovootlqqebheteysqoxntpccewcbevcstqimbacfbklwqjqixovs OK. mkdir 7fzujwwnc open 7upunhebiffpircfhmxsllpespacnxpptglgnrjxyoohgqopclhqzxuroutdtkumshufydexusgcjwtnhbdiaxaquvvnircaywgy OK. mkdir 8cmxwffao open 8btdlwhtygtqaplqxcszcofbsuyqzeqphmssizmjgfziwlyvpsvsjcvbwtsxzkmhwhzhinqovrztezpusmodqjepeyoejdnhkoos OK. mkdir 9cgjxyiqe open 9jhydvmgovlfaltguwvegjubndkmbufhdoghmupasahuoackzxohijiwmukpopwueedqbsstvzpjbrtcokkyvuujrfahwwbcdeue OK. mkdir 10xmzunod open 10rfytviftdcqmtxmatlewqkqujfvdsjgjqffnpkgtoyhhvwkrhqnxaftkmqphbysrdxeukmpylxigtuxckmbmuwyimopnmkgrjk OK. …… mkdir 1341932sc open 1341932rlcbgduqybifdacrgopzdnulnwxjtrlldpmltibrjlzolbhuqytvnpgbmgmhzzscpfniprbbearsezmuxhrnwaqkgcsfb OK. mkdir 1341933kk open 1341933qrzyhsckxcbpgcdcckupgufrchjjivvbnvbwndgmgjdomgroqndzjiqnrbwbwuclpejejrtrbwgndzdvoguxqmkhpikoc OK. mkdir 1341934nz open 1341934ttlyecrwdpcssdvqrdmovzaeqipeneggxrgbvxzzpbthgrzxwmnsnnwdylilpotohasczrdovzybqxzmjmewcdaaokngy OK. mkdir 1341935gu open 1341935hgmjgenwzmuaesbsbozaqccsqmfwmnttufcclrykfvnjnocqebswduovgvusjnlduqhghhspcfypuciafaxivlgdhcwqr OK. mkdir 1341936jv open 1341936lzerjlkaqryhndpoisltnwzqfdoxuysfzyzkklkccljqozgyrrrgpsyvvnupnpxonwbzjlblyldomjndcglszlpwyjony OK. mkdir 1341937le open 1341937ojfnsspftcjhouwtxderepgcqrqfsdjgmqvejmkcovlcpivonbfttnvjhmqzpahcqeibruffrrijbfxojfhcuelbqdcgd OK. mkdir 1341938lk open 1341938wquzhrfoiyytzerpnyyrscewvhadnmplejkncrblpaeqgxhtxhkqkoohxokkczvikgvnzxarxghderybyktkajrxybkca OK. mkdir 1341939fl open 1341939lnganfaecgnhmehqftjpvthvtkfvlkiyypynuarzggitnsjsluhhpqckbjgotootfogcoxduglptfylrvuaklevondeit OK. mkdir 1341940sc open 1341940bgiduigqotghzgssbmuozylpnqtvjlzktipnqxggrmnsshkwvekvevmtngqwtqjnaacsyiyruljpstlnzxkfuxyhfodyg OK. mkdir 1341941ol open 1341941gpqypywgvhpkbkxrlubqoaovfeaelprrghqvimbdwspzemqpgshwsvrabrgmixgohyjpmmuigjikxybfsibnfunjltvws OK. mkdir 1341942dk failed, errno = 28
中间输出比较多,就省略了,只显示前后十几行内容。我是在 /home/vmware/code/apue/02.chapter 目录下运行这个程序的,跑到 1341942 级目录时,mkdir 失败返回了,错误码为 28 (ENOSPC:No space left on device)。总的目录路径长度达到 34 + 1341941 * 10 = 13419444,最深层的一个文件总路径长度达到 34 + 1341941 * 10 + 100 = 13419544,都远远大于 PATH_MAX (4096)了。此时我在运行程序的目录新建文件或目录,都会失败。
$ mkdir abc mkdir: 无法创建目录"abc": 设备上没有空间 $ touch abc touch: 无法创建"abc": 设备上没有空间
所以可以肯定的一点是,Linux 上的 PATH_MAX 并不是底层文件系统对路径最大长度的限制,只是一种便于程序编写的常量。那路径的最大长度到底由什么决定呢?看了上面 ENOSPC 的错误码,我第一反应是硬盘空间满了,然而在查看了磁盘剩余空间后,我否决了这个可能性。下面是我在执行测试程序前后分别记录的 df 输出。
可以看到所在的 /home 挂载点使用比例激增 (8%~22%),但是也没有达到 100%,所以磁盘空间还是比较充足的,很可能只是 inode 用光了(?)。不管怎样,本质上都是一种资源限制,这使得程序员可以在 Linux 上创建比较长的文件路径,比 Windows 提供了更大的灵活性。但是这样长的路径并不被所有程序所识别,例如,我在刚才创建测试目录的地方,递归列出所有文件,输出如下:
$ ls -R .: 1rkxtumwh Makefile name_max.c path_max path_max.o path_max.vcxproj apue.o name_max name_max.o path_max.c path_max.sln ./1rkxtumwh: 1tovmschzmqujtonfqbqyfomyykrtwqcrezfxbmypesyygmfxpvxvjmvkwnfudhnhgsghgfwkzxkgjpfamcxwotimgniluvucpbl 2vijihgsn ./1rkxtumwh/2vijihgsn: 2rkuswxpuniccqrkbliwpzzaxhkfqqzgklackzrgpclrscduqnsfnrikasrqkswuexwqwqwlshekkigaxyhkrpwrjniuieqodnea 3fanxiuju ./1rkxtumwh/2vijihgsn/3fanxiuju: 3cqwzqdmivizeyjygopureytlajkkdtehlciegumddnhdxgjnxfedfaofjysmtytfbbjjyxmdmtgjbrwywdddfrkopcclbxqcabn 4yazbmsjy ./1rkxtumwh/2vijihgsn/3fanxiuju/4yazbmsjy: 4wawuybydgrowjqyutxmxynkypjcbelzaowwoxwsgqiczabtuaftyuezjpbmvomxekutjsnpixuhxxctxhnwdtvmiybemnbrzxmk 5sbcaywkx ./1rkxtumwh/2vijihgsn/3fanxiuju/4yazbmsjy/5sbcaywkx: 5torqwgobzlqjlrnygrrhqdrigvletvbmjufhcvldjbnuscuavlhmqawwwhcpegeqalzfgkirnvmfaghvuojkpghlnlcsrgkurjz 6aujthffp ./1rkxtumwh/2vijihgsn/3fanxiuju/4yazbmsjy/5sbcaywkx/6aujthffp: 6howeinoseubrjntbgclavxzxrjraqwrzmqdudrphnsaxhtaovootlqqebheteysqoxntpccewcbevcstqimbacfbklwqjqixovs 7fzujwwnc ./1rkxtumwh/2vijihgsn/3fanxiuju/4yazbmsjy/5sbcaywkx/6aujthffp/7fzujwwnc: 7upunhebiffpircfhmxsllpespacnxpptglgnrjxyoohgqopclhqzxuroutdtkumshufydexusgcjwtnhbdiaxaquvvnircaywgy 8cmxwffao ./1rkxtumwh/2vijihgsn/3fanxiuju/4yazbmsjy/5sbcaywkx/6aujthffp/7fzujwwnc/8cmxwffao: 8btdlwhtygtqaplqxcszcofbsuyqzeqphmssizmjgfziwlyvpsvsjcvbwtsxzkmhwhzhinqovrztezpusmodqjepeyoejdnhkoos 9cgjxyiqe ./1rkxtumwh/2vijihgsn/3fanxiuju/4yazbmsjy/5sbcaywkx/6aujthffp/7fzujwwnc/8cmxwffao/9cgjxyiqe: 10xmzunod 9jhydvmgovlfaltguwvegjubndkmbufhdoghmupasahuoackzxohijiwmukpopwueedqbsstvzpjbrtcokkyvuujrfahwwbcdeue ./1rkxtumwh/2vijihgsn/3fanxiuju/4yazbmsjy/5sbcaywkx/6aujthffp/7fzujwwnc/8cmxwffao/9cgjxyiqe/10xmzunod: 10rfytviftdcqmtxmatlewqkqujfvdsjgjqffnpkgtoyhhvwkrhqnxaftkmqphbysrdxeukmpylxigtuxckmbmuwyimopnmkgrjk …… 410dntuzt ls: 无法打开目录./1rkxtumwh/2vijihgsn/3fanxiuju/4yazbmsjy/5sbcaywkx/6aujthffp/7fzujwwnc/8cmxwffao/9cgjx yiqe/10xmzunod/11ltybuja/12pddukks/13pgjmtme/14yxyuhvt/15qgddfps/16ipghtsx/17hrlvjqw/18mlwamyj/19ycqq oum/20gdekplr/21mgkqtef/22uxjelyo/23yuogjxv/24gdyuetr/25ekyseyq/26mohxeho/27yhfufeo/28emftmwp/29detfm nz/30ulhqorq/31lzvmcau/32vrkdiho/33pwrixvt/34ysjlzxm/35anbimen/36aazpcto/37dpzcvhb/38vbxsrwc/39urooly a/40gabvtkg/41fswpnkc/42eyozjbb/43mbsphto/44szukcad/45lobskag/46njddwrk/47qmyaqip/48qywwpge/49jymbads /50cevctwp/51dbrvvrg/52alaqbak/53wdxytcx/54iajcocc/55yulgnls/56xdvlohr/57rsyzzme/58avlsjct/59kzwpnmn/ 60zuuyott/61cmyywqh/62fyuzvce/63djljats/64aopcwyi/65rxefdvy/66egqsclw/67jxyausw/68atlilhv/69xrxqsel/7 0isglkct/71fwkjdms/72mhygulk/73nlnwbrx/74sroynrd/75dimglds/76ulettre/77cvdbchx/78imrnssw/79uqfezsr/80 lkhipxs/81rbdpqqz/82lvyzxqt/83sgxgrxx/84tsgcvwa/85jaamdba/86zoneybo/87glbanpe/88tkzaefg/89rrwsack/90u owxuoq/91rabkpyi/92dcnxiyz/93lacaahs/94frjwezd/95yhtpnte/96fblotmh/97mibhekj/98rzonuec/99topwrdn/100d blgkn/101drisrx/102aqjjso/103uefcom/104yrflxq/105hyfxyh/106tlpbnl/107ndiswq/108uvgksb/109ozcwup/110qx pxlo/111gttfdp/112nepxhw/113xwflah/114yibgjy/115dffvxj/116ukiosf/117fpvyvt/118kdreee/119ejgwiz/120lpo qvc/121gihnlr/122umltry/123ajaaeq/124unorwj/125iuipco/126uwghec/127lzrzvo/128zytdwi/129xvkgeh/130mgfx pr/131kekuik/132fjlbpy/133wogrgw/134eotsoo/135fquige/136smcyju/137obzwup/138fewkay/139dgizof/140gqrvp w/141lwxiji/142gprlwz/143geyxsn/144otqwya/145ooyfvk/146ucigye/147xstuji/148ziprio/149ufrzci/150lmgpli /151kczfla/152svbdit/153fzlyns/154yirhkv/155xbdixx/156mogbae/157goxflw/158fixfsg/159ncihug/160bnrvsj/ 161ynpfeh/162cmanrm/163utjekr/164qcvzim/165ypxvkh/166ggnvdw/167nszvsr/168uyuayf/169fatffy/170znjsdj/1 71jqobhe/172syewyj/173fbdzgu/174bufnki/175qasxqn/176rigmyh/177zwoexf/178bdrxbg/179ggdmzx/180ljyrtm/18 1qbywid/182wsjhtx/183moknmr/184haaeaa/185jdpxza/186eerxfa/187kpfjiq/188wrdbbh/189uuhvct/190ofpwct/191 qnmtmy/192gqyowy/193eckugg/194oonhah/195dbjuxd/196qzbvuh/197kcqziu/198akbhoa/199hkszfc/200aqieum/201a hkuqb/202hkikbi/203gywiex/204ibfmca/205vgczdp/206pchnch/207tfkxlo/208orztrq/209foperp/210umtatf/211rf qqzo/212xuspzm/213bsqdet/214kbvvmg/215qsbepc/216qqqvea/217mtyasr/218eunbgv/219stppxz/220ncqzlb/221qgl ips/222naqylv/223ymrfpk/224vavwhr/225dznual/226iuraac/227ztobic/228jynnfd/229iatquu/230vuokzi/231ykpc bl/232wqldxe/233qeebyv/234nvspvv/235iqxfye/236nwnztb/237qjtgnn/238qyzyas/239lhcqon/240piorsv/241upvvm j/242sjgemx/243ndojai/244whhuvw/245hmojut/246kwaiwh/247ohckmb/248ucmdrh/249eqzimi/250uymmqd/251cnglst /252iysywl/253jridil/254nmoqzh/255ygguhk/256nwuiap/257zvtolc/258urycli/259kpiuab/260jempzc/261jyqcbe/ 262sulagk/263mvljld/264ciiawa/265vjxora/266rxfgkg/267jwvpoy/268mpmgja/269xpxycf/270fjxobt/271zahncm/2 72soljdu/273jcdwrq/274wvrusy/275noyexu/276oksjfe/277wzfwvr/278tcsfue/279uhzgjs/280lfpypw/281cyjibv/28 2jwnlgu/283nnktxs/284udmuma/285xzbwnj/286jemqma/287xnedax/288dcvfle/289ddnbqe/290jheimg/291rscgbr/292 zlyhgw/293jbmjzx/294qocgsi/295mbyvyj/296ntwpkk/297jhofrm/298ibirpc/299kquens/300mbjnmo/301najyhm/302g yerut/303blongi/304tafajx/305suvetf/306faeoiu/307dunnih/308ifvtsq/309vulqxe/310ilvtjv/311gzknfk/312ju xeni/313bpaezt/314dcqgbb/315rqnuic/316ejhbrb/317wfzvzo/318hfndtr/319zgsznw/320dqahuh/321mdnyne/322xiu lms/323lplrvo/324qtvodk/325tiigxk/326kljzob/327lxxopm/328qwjlms/329jwcbic/330nowwgr/331jqqbce/332qael pn/333vyyvlr/334wvchpm/335afdbpb/336vsrvym/337ktyfii/338gasxaz/339gjjnco/340yqdxcc/341eysxtl/342ipsoc o/343xrjucz/344aoetoq/345rtnyzv/346ptrthp/347gnxqkl/348vfpcrp/349jmoobg/350glqvzi/351vkznzy/352cmjseg /353zgxeyh/354vfkgth/355fnhgjo/356cyouso/357hruehw/358jadbhv/359hklsla/360hwsife/361ragklc/362xydtpr/ 363tjrhwj/364gpsojy/365sphazl/366fgwang/367tovqhr/368ybyzbe/369wasspn/370jjkehg/371fvmqie/372bdigtd/3 73afocqc/374agluqq/375jlrfsu/376gtbemx/377lqbrlg/378xmxxvu/379pwznpd/380gdryol/381cgzdfe/382egzxfd/38 3ciccll/384mpoifm/385ygdqiy/386ahilzb/387bhmyia/388cmfhah/389kuxsqg/390winlgk/391jnjlim/392vurqut/393 yehkub/394vkgknd/395pxnmgr/396bsnizx/397bzbbbs/398kdfjjs/399ugopik/400klxzic/401kaqhcd/402iyrruo/403d tncvg/404feosyw/405cbohnn/406hfirse/407tqbohz/408udrjbo/409yhmpsz/410dntuzt: 文件名过长
中间输出比较多,就省略了。跑到 410 级目录时,ls 报错了,上一级目录路径的总长度达到 34 + 409 * 10 = 4124,其实已经比 PATH_MAX 大了,不过还是因为缓冲区不足而中断退出了。再尝试查找文件,输出如下:
$ find . . ./name_max.o ./path_max.c ./Makefile ./name_max ./path_max ./path_max.o ./name_max.c ./1rkxtumwh ./1rkxtumwh/1tovmschzmqujtonfqbqyfomyykrtwqcrezfxbmypesyygmfxpvxvjmvkwnfudhnhgsghgfwkzxkgjpfamcxwotimgniluvucpbl ./1rkxtumwh/2vijihgsn ./1rkxtumwh/2vijihgsn/3fanxiuju ./1rkxtumwh/2vijihgsn/3fanxiuju/3cqwzqdmivizeyjygopureytlajkkdtehlciegumddnhdxgjnxfedfaofjysmtytfbbjjyxmdmtgjbrwywdddfrkopcclbxqcabn ./1rkxtumwh/2vijihgsn/3fanxiuju/4yazbmsjy ./1rkxtumwh/2vijihgsn/3fanxiuju/4yazbmsjy/5sbcaywkx ./1rkxtumwh/2vijihgsn/3fanxiuju/4yazbmsjy/5sbcaywkx/6aujthffp ./1rkxtumwh/2vijihgsn/3fanxiuju/4yazbmsjy/5sbcaywkx/6aujthffp/6howeinoseubrjntbgclavxzxrjraqwrzmqdudrphnsaxhtaovootlqqebheteysqoxntpccewcbevcstqimbacfbklwqjqixovs ./1rkxtumwh/2vijihgsn/3fanxiuju/4yazbmsjy/5sbcaywkx/6aujthffp/7fzujwwnc ./1rkxtumwh/2vijihgsn/3fanxiuju/4yazbmsjy/5sbcaywkx/6aujthffp/7fzujwwnc/7upunhebiffpircfhmxsllpespacnxpptglgnrjxyoohgqopclhqzxuroutdtkumshufydexusgcjwtnhbdiaxaquvvnircaywgy ./1rkxtumwh/2vijihgsn/3fanxiuju/4yazbmsjy/5sbcaywkx/6aujthffp/7fzujwwnc/8cmxwffao ./1rkxtumwh/2vijihgsn/3fanxiuju/4yazbmsjy/5sbcaywkx/6aujthffp/7fzujwwnc/8cmxwffao/8btdlwhtygtqaplqxcszcofbsuyqzeqphmssizmjgfziwlyvpsvsjcvbwtsxzkmhwhzhinqovrztezpusmodqjepeyoejdnhkoos ./1rkxtumwh/2vijihgsn/3fanxiuju/4yazbmsjy/5sbcaywkx/6aujthffp/7fzujwwnc/8cmxwffao/9cgjxyiqe ./1rkxtumwh/2vijihgsn/3fanxiuju/4yazbmsjy/5sbcaywkx/6aujthffp/7fzujwwnc/8cmxwffao/9cgjxyiqe/10xmzunod ./1rkxtumwh/2vijihgsn/3fanxiuju/4yazbmsjy/5sbcaywkx/6aujthffp/7fzujwwnc/8cmxwffao/9cgjxyiqe/10xmzunod/10rfytviftdcqmtxmatlewqkqujfvdsjgjqffnpkgtoyhhvwkrhqnxaftkmqphbysrdxeukmpylxigtuxckmbmuwyimopnmkgrjk ……
最后有没有完成我没有得到结论,因为整个虚拟机历时 N 天这个命令还没跑完 (N > 10),运行中的 find 截图为证:
我是按内存占用从高到低排序的,可以看到经过 N 天的运行 find 命令的内存占用已经超过了整个图形界面(Xorg),另外与 find 命令关联的终端 (mate-terminal) 内存、CPU也在高位运行。不过至少可以说明 find 并没有使用 PATH_MAX 来简单限制路径长度,可能是通过动态分配内存来实现对长路径支持的。另外个人比较好奇这个命令的句柄开支,于是看了下 lsof 的输出:
$ lsof -p `pidof find` COMMAND PID USER FD TYPE DEVICE SIZE/OFF NODE NAME find 113339 vmware cwd DIR 8,5 4096 2095005 /home/vmware/code/apue/02.chapter find 113339 vmware rtd DIR 8,3 4096 2 / find 113339 vmware txt REG 8,3 203296 1180356 /usr/bin/find find 113339 vmware mem REG 8,3 106374736 1187939 /usr/lib/locale/locale-archive find 113339 vmware mem REG 8,3 141880 1191334 /usr/lib64/libpthread-2.20.so find 113339 vmware mem REG 8,3 19512 1190784 /usr/lib64/libdl-2.20.so find 113339 vmware mem REG 8,3 447240 1191294 /usr/lib64/libpcre.so.1.2.3 find 113339 vmware mem REG 8,3 2082456 1190695 /usr/lib64/libc-2.20.so find 113339 vmware mem REG 8,3 1167000 1191118 /usr/lib64/libm-2.20.so find 113339 vmware mem REG 8,3 154784 1191415 /usr/lib64/libselinux.so.1 find 113339 vmware mem REG 8,3 163184 1190512 /usr/lib64/ld-2.20.so find 113339 vmware mem REG 8,3 26254 1448972 /usr/lib64/gconv/gconv-modules.cache find 113339 vmware mem REG 8,3 30239 3015834 /usr/share/locale/zh_CN/LC_MESSAGES/findutils.mo find 113339 vmware 0u CHR 136,0 0t0 3 /dev/pts/0 find 113339 vmware 1u CHR 136,0 0t0 3 /dev/pts/0 find 113339 vmware 2u CHR 136,0 0t0 3 /dev/pts/0 find 113339 vmware 3r DIR 8,5 4096 2095005 /home/vmware/code/apue/02.chapter find 113339 vmware 4r unknown /proc/113339/fd/4 (readlink: File name too long) find 113339 vmware 6r unknown /proc/113339/fd/6 (readlink: File name too long) find 113339 vmware 7r unknown /proc/113339/fd/7 (readlink: File name too long) find 113339 vmware 9r unknown /proc/113339/fd/9 (readlink: File name too long) find 113339 vmware 10r unknown /proc/113339/fd/10 (readlink: File name too long) find 113339 vmware 11r unknown /proc/113339/fd/11 (readlink: File name too long)
出人意料的是并没有很多打开中的文件句柄。按我的理解,每遍历一层目录应该有一个目录文件的句柄被打开,但是现在看没有,是因为我造的例子比较特殊,每个目录下只有一个子目录?还是 find 另辟蹊径不用占用许多句柄?暂时不得而知。总的来讲,find 的表现要好于 ls,顿时对这个神奇的命令产生了敬仰。最后插一句题外话,从上面的输出我们可以看到另一个现象,就是因为路径太长了,readlink 返回了错误,可见对长路径,很多系统 api 也是不支持的。最后你可以使用 rm -rf xxxx 来尝试清理这些目录与文件:
不出所料 rm 占用了很多的内存和 CPU,不过好歹可以工作 (而且耗时没有 find 那么夸张,也就十几分钟),所以我的虚拟机可以恢复到测试前的工况。下面是 lsof 针对 rm 的一些输出
$ lsof -p `pidof rm`
COMMAND PID USER FD TYPE DEVICE SIZE/OFF NODE NAME
rm 70867 vmware cwd DIR 8,5 4096 2095005 /home/vmware/code/apue/02.chapter
rm 70867 vmware rtd DIR 8,3 4096 2 /
rm 70867 vmware txt REG 8,3 62872 1181241 /usr/bin/rm
rm 70867 vmware mem REG 8,3 106374736 1187939 /usr/lib/locale/locale-archive
rm 70867 vmware mem REG 8,3 2082456 1190695 /usr/lib64/libc-2.20.so
rm 70867 vmware mem REG 8,3 163184 1190512 /usr/lib64/ld-2.20.so
rm 70867 vmware 0u CHR 136,0 0t0 3 /dev/pts/0
rm 70867 vmware 1u CHR 136,0 0t0 3 /dev/pts/0
rm 70867 vmware 2u CHR 136,0 0t0 3 /dev/pts/0
rm 70867 vmware 3u unknown /proc/70867/fd/3 (readlink: File name too long)
rm 70867 vmware 4u unknown /proc/70867/fd/4 (readlink: No such file or directory)
rm 70867 vmware 5u unknown /proc/70867/fd/5 (readlink: File name too long)
rm 70867 vmware 6u unknown /proc/70867/fd/6 (readlink: File name too long)
rm 70867 vmware 7u unknown /proc/70867/fd/7 (readlink: File name too long)
rm 70867 vmware 8u unknown /proc/70867/fd/8 (readlink: File name too long)
和 find 一样,它没有多少打开中的文件句柄(否则打开文件句柄数早超限了)。另外从 lsof 的输出看,有一些节点已经被 rm 删除了,所以 readlink 报错。
对超长的文件路径来说,不同的 Linux 命令的支持程度也是不同的,有的支持,有的不支持。而且考虑到传递给进程的命令行总长度也是有限制的,除了一些支持递归目录处理的命令外,普通的命令是不会支持这么长的路径的。另一方面,从程序员的角度考虑,对于大路径的支持也是比较麻烦的,首先是系统支不支持输入、输出超长路径;如果支持,输入的场景还比较好办,输出的场景就比较麻烦了。最简单的办法是自己定义一个大于 PATH_MAX 值的常量并使用它分配内存,但是这样也存在问题,一方面日常处理比较浪费内存;另一方面如果路径超过你自己定义的这个值,还是会出现接收截断的问题。而且程序员没有办法去预知要得到的路径长度,从而提前动态分配内存。
总结一下,这个最大路径限制并不是系统不能支持超长的路径,而且作为一种系统与应用之间的一个约定,有了这个约定之后,程序对路径的处理将得到简化。况且我们没有对超长路径的硬需求,绝大部分 Windows 应用在 260 个字符路径的限制下就跑得很好,Linux 4096 的限制就更不用说了。
这个测试程序的一个额外用处就是耗尽所在分区的 inode(?),并观察系统在这种极限情况下的一些表现,例如,当我试图截取屏幕时不能成功,得到下面的提示框:
当你需要验证程序在这种极限状态下的行为时,这个程序不失为一种有用工具。
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