這個最大路徑長度是為了方便程序編寫？還是說底層的文件系統就只能支持這么長的路徑呢？本文為你揭開謎底

問題的提出

在處理文件系統路徑的時候，我們一般會先開辟一塊內存區，用來接收路徑、或者拼接好路徑傳遞給系統調用。這是因為路徑在各個系統上都有最大長度限制，在 Windows 上這個值是 MAX_PATH，一般不能超過 260；在 Linux 上這個值是 PATH_MAX，一般不能超過 4096 (或者通過 pathconf (_PC_PATH_MAX, ...) 來獲取，但是一般也是 4096)，就像下面這段典型的代碼：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?#else11?????char?buf[PATH_MAX?+?1]?=?{?0?};12?????if?(readlink("/proc/self/exe",?buf,?PATH_MAX)?

21?}

它用來獲取當前可執行文件的完整路徑。在 Windows 與 Linux 上分別調用了 GetModuleFileName 與 readlink 系統調用，且事先在棧上分配了接收完整路徑的內存區 buf。最后將成功獲取的路徑打印到控制臺。在兩種系統上，輸出分別如下：

Windowscurrent?executable?file?path:?E:\code\apue\02.chapter\Release\path_max.exe

Linuxcurrent?executable?file?path:?/home/yunhai/code/apue/02.chapter/path_max

其它與路徑相關的調用也是類似的。那么問題來了，這個最大路徑長度是為了方便程序編寫 (不然需要動態分配內存，且需要兩次調用，其中一次用于獲取最終的路徑長度)，還是說底層的文件系統就只能支持這么長的路徑呢？

問題的驗證

為了弄清楚這個問題，我專門寫了一個測試程序：

path_max.c1?#ifdef?WIN32??2?#??ifndef?_WIN32_WINNT????????????//?指定要求的最低平臺是?Windows?Vista。??3?#??define?_WIN32_WINNT?0x0600?????//?將此值更改為相應的值，以適用于?Windows?的其他版本。??4?#??endif??5?#include??6?#include??7?#else???8?#include??9?#include?10?#include?11?#include?12?#include?13?#endif??14?#include??15?#include?16?#include?17?#include?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

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?WIN32101?????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?}

這個小程序很簡單，每次創建一個目錄，切換進去，創建一個文件。這樣周而復始，直到出錯退出。目錄名長度固定為 9，這樣加上目錄分隔符(/ 或 \)就正好湊成 10，方便之后根據目錄深度計算路徑總長度；文件名長度固定為 100，也是為了方便計算路徑總長度。每個名字都是由 26 個字母隨機組成的，同時將目錄深度 (level)記錄在名字開始。例如第 10 級目錄可能長這樣：“10jmvrfqv”，前面的數字 10 明確的標識了目錄層級，同理，文件名前面也是層級數字，也是為了方便計算。好了，我們分別在兩個系統上跑一下這個測試程序。

1. Windows 上的結果 ( Win10 x64)NAME_MAX?=?260,?PATH_MAX?=?260mkdir?1tdobxciu

open?1gybhgvdtngozbkzvcu***itcpntxgtjhasljfycfowuuqmcvvitsthzdxpckttobaqtccxeabfmnvhoeknyjifabnynkrjwzhiq?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?=?3mkdir?17xljxtht

open?17umkwtdbxiayekrljssfnxgvcmytthwevlmrxptivqmvwrkpqkvxoircuwadhkyidoaydlvnnafejpssqdewcymluzmrwrrkwmn?failed,?errno?=?3mkdir?18aywtihk

open?18ynydjmncmakzoezcupoeqfiahsunxhqvczbuapntehglrrhubpbydnnpnbpjdschmrkzkkpvgfajemsfsvofsjoclijforjkex?failed,?errno?=?3mkdir?19qsvtxwm

open?19qpfdrjgnxthljturhvuymuoqctsmixxrircoievqpgrnxpzufozwvpsjocdxfhjodpfzljbgegutfsquaehjoaswqhwtsqbgmh?failed,?errno?=?3mkdir?20vzvuovy

open?20zqqxsfnxrccsubxwevfffunrxfugkovltzidalrmppjjyrbnohxgmwnmwcseijuhzabngyfodgodnblmvhmvnmkvqfijucfqfz?failed,?errno?=?3mkdir?21idgwupz

open?21vlkqahypwulaaykizzvqviqztswdwikvloswluydfyqmgjcaixqqrilsrjireprfrvbgfvspulfmnuksbvsnfkdncybuhxddtw?failed,?errno?=?3mkdir?22plbyhbe

open?22rbekcixmxcohnvqlyhlppmgqtpsgsjwxhuwlmhnzspkbvdrnzechjbjsglzajiadiqguutgfjfkaqdtfqeevpcrsejiiotytse?failed,?errno?=?3mkdir?23vggwvra

open?23tlbobatwbdpaxouojluctxqyddllqedqbeotpkkxgddristpprablooxlomgvneixubhxhemtlvipqwzzfgqcxjtdltytpialw?failed,?errno?=?3mkdir?24dkpkfup

open?24fujnicyzpudgnsuzcqkkgihsousejefcuepnkkxdpdxbxxfryeglrmikgxxckazfokmdrgttnqxavphpoekagfbenjyuxpfntu?failed,?errno?=?3CreateDirectory?25aajjgcq?failed,?errno?=?206

我是在 G:\ 根目錄運行這個程序的，跑到第 16 級目錄時，CreateFile 失敗返回了，錯誤碼為 3 (ERROR_PATH_NOT_FOUND：系統找不到指定的路徑)，對于文件創建失敗，這里忽略，所以程序繼續運行；跑到第 25 級目錄時，CreateDirectory 也失敗返回了，錯誤碼為 206 (ERROR_FILENAME_EXCED_RANGE：文件名或擴展名太長)，因為不能再繼續了，所以整個程序終止。看了一下，到 16 級目錄時整個路徑是這樣：

總長度達到 162，此時如果想再創建文件的話，總長度會升至 162 + 1 + 100 = 263? > MAX_PATH，所以接下來的文件創建失敗了。然后再看下到 24 級目錄時路徑的樣子：

總長度達到 243，如果再加上一個目錄的長度的話，應該是253 < MAX_PATH，按理說應該還可以容納一個目錄的創建，但是我實地在這個目錄下面嘗試創建目錄，得到了這樣的錯誤：

如果是創建文件的話，會發現輸入一定長度的文件名之后，就輸入不了了：

這個長度目前是 16 (算上后綴 .txt 4個字符)，加上之前目錄的長度 243，總長度為 243 + 1 + 16 = 260，正好等于 MAX_PATH。可見，對于 Windows 而言，這個 MAX_PATH 就是底層文件系統的限制，再怎么玩也玩不出這個杠杠。嘗試切換 CreateDirectory / SetCurrentDirectory? 實現為 _mkdir / _chdir，看看有什么不同 ：

line 62-65ret?=?_chdir?(name);

if?(ret?==?-1)

//ret?=?SetCurrentDirectory(name);

//if?(!ret)

line 81-84ret?=?_mkdir?(name);

if?(ret?==?-1)

//ret?=?CreateDirectory(name,?NULL);

//if?(!ret)

重跑上面的例子，會得到完全一樣的結果。

2. Linux 上的結果 (neokylin x64)

一開始我是在 CentOS i686 環境上跑的，沒想到一陣跑之后再回來看，虛擬機桌面居然重置了，所有 shell 都退出了，沒有截到任何輸出，無奈只好采用另一個平臺了。在這個中標麒麟的環境跑完了全程：$?./path_max

NAME_MAX?=?255,?PATH_MAX?=?4096mkdir?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

中間輸出比較多，就省略了，只顯示前后十幾行內容。我是在?/home/vmware/code/apue/02.chapter 目錄下運行這個程序的，跑到 1341942 級目錄時，mkdir 失敗返回了，錯誤碼為 28 (ENOSPC：No space left on device)。總的目錄路徑長度達到 34 + 1341941 * 10 = 13419444，最深層的一個文件總路徑長度達到 34 + 1341941 * 10 + 100 = 13419544，都遠遠大于 PATH_MAX (4096)了。此時我在運行程序的目錄新建文件或目錄，都會失敗。$?mkdir?abc

mkdir:?無法創建目錄"abc":?設備上沒有空間

$?touch?abc

touch:?無法創建"abc":?設備上沒有空間

所以可以肯定的一點是，Linux 上的 PATH_MAX 并不是底層文件系統對路徑最大長度的限制，只是一種便于程序編寫的常量。那路徑的最大長度到底由什么決定呢？看了上面 ENOSPC 的錯誤碼，我第一反應是硬盤空間滿了，然而在查看了磁盤剩余空間后，我否決了這個可能性。下面是我在執行測試程序前后分別記錄的 df 輸出。

可以看到所在的 /home 掛載點使用比例激增 (8%~22%)，但是也沒有達到 100%，所以磁盤空間還是比較充足的，很可能只是 inode 用光了(?)。不管怎樣，本質上都是一種資源限制，這使得程序員可以在 Linux 上創建比較長的文件路徑，比 Windows 提供了更大的靈活性。但是這樣長的路徑并不被所有程序所識別，例如，我在剛才創建測試目錄的地方，遞歸列出所有文件，輸出如下：$?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

……

410dntuztls:?無法打開目錄./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/70isglkct/71fwkjdms/72mhygulk/73nlnwbrx/74sroynrd/75dimglds/76ulettre/77cvdbchx/78imrnssw/79uqfezsr/80lkhipxs/81rbdpqqz/82lvyzxqt/83sgxgrxx/84tsgcvwa/85jaamdba/86zoneybo/87glbanpe/88tkzaefg/89rrwsack/90uowxuoq/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/130mgfxpr/131kekuik/132fjlbpy/133wogrgw/134eotsoo/135fquige/136smcyju/137obzwup/138fewkay/139dgizof/140gqrvpw/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/171jqobhe/172syewyj/173fbdzgu/174bufnki/175qasxqn/176rigmyh/177zwoexf/178bdrxbg/179ggdmzx/180ljyrtm/181qbywid/182wsjhtx/183moknmr/184haaeaa/185jdpxza/186eerxfa/187kpfjiq/188wrdbbh/189uuhvct/190ofpwct/191qnmtmy/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/272soljdu/273jcdwrq/274wvrusy/275noyexu/276oksjfe/277wzfwvr/278tcsfue/279uhzgjs/280lfpypw/281cyjibv/282jwnlgu/283nnktxs/284udmuma/285xzbwnj/286jemqma/287xnedax/288dcvfle/289ddnbqe/290jheimg/291rscgbr/292zlyhgw/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/373afocqc/374agluqq/375jlrfsu/376gtbemx/377lqbrlg/378xmxxvu/379pwznpd/380gdryol/381cgzdfe/382egzxfd/383ciccll/384mpoifm/385ygdqiy/386ahilzb/387bhmyia/388cmfhah/389kuxsqg/390winlgk/391jnjlim/392vurqut/393yehkub/394vkgknd/395pxnmgr/396bsnizx/397bzbbbs/398kdfjjs/399ugopik/400klxzic/401kaqhcd/402iyrruo/403d

tncvg/404feosyw/405cbohnn/406hfirse/407tqbohz/408udrjbo/409yhmpsz/410dntuzt:?文件名過長

中間輸出比較多，就省略了。跑到 410 級目錄時，ls 報錯了，上一級目錄路徑的總長度達到 34 + 409 * 10 = 4124，其實已經比 PATH_MAX 大了，不過還是因為緩沖區不足而中斷退出了。再嘗試查找文件，輸出如下：$?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

……

最后有沒有完成我沒有得到結論，因為整個虛擬機歷時 N 天這個命令還沒跑完 (N > 10)，運行中的 find 截圖為證：

我是按內存占用從高到低排序的，可以看到經過 N 天的運行 find 命令的內存占用已經超過了整個圖形界面(Xorg)，另外與 find 命令關聯的終端 (mate-terminal) 內存、CPU也在高位運行。不過至少可以說明 find 并沒有使用 PATH_MAX 來簡單限制路徑長度，可能是通過動態分配內存來實現對長路徑支持的。另外個人比較好奇這個命令的句柄開支，于是看了下 lsof 的輸出：$?lsof?-p?`pidof?find`

COMMAND????PID???USER???FD??????TYPE?DEVICE??SIZE/OFF????NODE?NAMEfind????113339?vmware??cwd???????DIR????8,5??????4096?2095005?/home/vmware/code/apue/02.chapterfind????113339?vmware??rtd???????DIR????8,3??????4096???????2?/find????113339?vmware??txt???????REG????8,3????203296?1180356?/usr/bin/findfind????113339?vmware??mem???????REG????8,3?106374736?1187939?/usr/lib/locale/locale-archivefind????113339?vmware??mem???????REG????8,3????141880?1191334?/usr/lib64/libpthread-2.20.sofind????113339?vmware??mem???????REG????8,3?????19512?1190784?/usr/lib64/libdl-2.20.sofind????113339?vmware??mem???????REG????8,3????447240?1191294?/usr/lib64/libpcre.so.1.2.3find????113339?vmware??mem???????REG????8,3???2082456?1190695?/usr/lib64/libc-2.20.sofind????113339?vmware??mem???????REG????8,3???1167000?1191118?/usr/lib64/libm-2.20.sofind????113339?vmware??mem???????REG????8,3????154784?1191415?/usr/lib64/libselinux.so.1find????113339?vmware??mem???????REG????8,3????163184?1190512?/usr/lib64/ld-2.20.sofind????113339?vmware??mem???????REG????8,3?????26254?1448972?/usr/lib64/gconv/gconv-modules.cachefind????113339?vmware??mem???????REG????8,3?????30239?3015834?/usr/share/locale/zh_CN/LC_MESSAGES/findutils.mofind????113339?vmware????0u??????CHR??136,0???????0t0???????3?/dev/pts/0find????113339?vmware????1u??????CHR??136,0???????0t0???????3?/dev/pts/0find????113339?vmware????2u??????CHR??136,0???????0t0???????3?/dev/pts/0find????113339?vmware????3r??????DIR????8,5??????4096?2095005?/home/vmware/code/apue/02.chapterfind????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)

出人意料的是并沒有很多打開中的文件句柄。按我的理解，每遍歷一層目錄應該有一個目錄文件的句柄被打開，但是現在看沒有，是因為我造的例子比較特殊，每個目錄下只有一個子目錄？還是 find 另辟蹊徑不用占用許多句柄？暫時不得而知。總的來講，find 的表現要好于 ls，頓時對這個神奇的命令產生了敬仰。最后插一句題外話，從上面的輸出我們可以看到另一個現象，就是因為路徑太長了，readlink 返回了錯誤，可見對長路徑，很多系統 api 也是不支持的。最后你可以使用 rm -rf xxxx 來嘗試清理這些目錄與文件：

不出所料 rm 占用了很多的內存和 CPU，不過好歹可以工作 (而且耗時沒有 find 那么夸張，也就十幾分鐘)，所以我的虛擬機可以恢復到測試前的工況。下面是 lsof 針對 rm 的一些輸出$?lsof?-p?`pidof?rm`

COMMAND???PID???USER???FD??????TYPE?DEVICE??SIZE/OFF????NODE?NAMErm??????70867?vmware??cwd???????DIR????8,5??????4096?2095005?/home/vmware/code/apue/02.chapterrm??????70867?vmware??rtd???????DIR????8,3??????4096???????2?/rm??????70867?vmware??txt???????REG????8,3?????62872?1181241?/usr/bin/rmrm??????70867?vmware??mem???????REG????8,3?106374736?1187939?/usr/lib/locale/locale-archiverm??????70867?vmware??mem???????REG????8,3???2082456?1190695?/usr/lib64/libc-2.20.sorm??????70867?vmware??mem???????REG????8,3????163184?1190512?/usr/lib64/ld-2.20.sorm??????70867?vmware????0u??????CHR??136,0???????0t0???????3?/dev/pts/0rm??????70867?vmware????1u??????CHR??136,0???????0t0???????3?/dev/pts/0rm??????70867?vmware????2u??????CHR??136,0???????0t0???????3?/dev/pts/0rm??????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 一樣，它沒有多少打開中的文件句柄(否則打開文件句柄數早超限了)。另外從 lsof 的輸出看，有一些節點已經被 rm 刪除了，所以 readlink 報錯。

結論

對超長的文件路徑來說，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。而且考慮到傳遞給進程的命令行總長度也是有限制的，除了一些支持遞歸目錄處理的命令外，普通的命令是不會支持這么長的路徑的。另一方面，從程序員的角度考慮，對于大路徑的支持也是比較麻煩的，首先是系統支不支持輸入、輸出超長路徑；如果支持，輸入的場景還比較好辦，輸出的場景就比較麻煩了。最簡單的辦法是自己定義一個大于 PATH_MAX 值的常量并使用它分配內存，但是這樣也存在問題，一方面日常處理比較浪費內存；另一方面如果路徑超過你自己定義的這個值，還是會出現接收截斷的問題。而且程序員沒有辦法去預知要得到的路徑長度，從而提前動態分配內存。

總結一下，這個最大路徑限制并不是系統不能支持超長的路徑，而且作為一種系統與應用之間的一個約定，有了這個約定之后，程序對路徑的處理將得到簡化。況且我們沒有對超長路徑的硬需求，絕大部分 Windows 應用在 260 個字符路徑的限制下就跑得很好，Linux 4096 的限制就更不用說了。

這個測試程序的一個額外用處就是耗盡所在分區的 inode(?)，并觀察系統在這種極限情況下的一些表現，例如，當我試圖截取屏幕時不能成功，得到下面的提示框：

當你需要驗證程序在這種極限狀態下的行為時，這個程序不失為一種有用工具。

總結

以上是生活随笔為你收集整理的linux _max_path,[apue] Linux / Windows 系统上只能建立不超过 PATH_MAX / MAX_PATH 长度的路径吗?...的全部內容，希望文章能夠幫你解決所遇到的問題。

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