unzip 使用什么方法在存档中查找单个文件？

Question

假设我创建了 100 个文件，每个文件的大小为 30MB 的随机文本数据。现在我创建一个压缩率为 0 的 zip 存档，即zip dataset.zip -r -0 *.txt. 现在我只想从这个存档中提取一个文件。

如上所述这里，也有解压/从归档中提取文件的方法有两种：

1. 查找文件末尾并查找中央目录。然后使用它来快速随机访问要提取的文件。（摊销O(1)复杂度）
2. 查看每个本地标头并提取匹配的标头。（O(n)复杂性）

unzip 使用哪种方法？从我的实验来看，它似乎使用了方法 2？

Answer 1

在大型存档中搜索单个文件时，它使用方法 1，您可以使用以下方法查看strace：

open("dataset.zip", O_RDONLY)           = 3
ioctl(1, TIOCGWINSZ, 0x7fff9a895920)    = -1 ENOTTY (Inappropriate ioctl for device)
write(1, "Archive:  dataset.zip\n", 22Archive:  dataset.zip
) = 22
lseek(3, 943718400, SEEK_SET)           = 943718400
read(3, "\340P\356(s\342\306\205\201\27\360U[\250/2\207\346<\252+u\234\225\1[<\2310E\342\274"..., 4522) = 4522
lseek(3, 943722880, SEEK_SET)           = 943722880
read(3, "\3\f\225P\\ux\v\0\1\4\350\3\0\0\4\350\3\0\0", 20) = 20
lseek(3, 943718400, SEEK_SET)           = 943718400
read(3, "\340P\356(s\342\306\205\201\27\360U[\250/2\207\346<\252+u\234\225\1[<\2310E\342\274"..., 8192) = 4522
lseek(3, 849346560, SEEK_SET)           = 849346560
read(3, "D\262nv\210\343\240C\24\227\344\367q\300\223\231\306\330\275\266\213\276M\7I'&35\2\234J"..., 8192) = 8192
stat("rand-28.txt", 0x559f43e0a550)     = -1 ENOENT (No such file or directory)
lstat("rand-28.txt", 0x559f43e0a550)    = -1 ENOENT (No such file or directory)
stat("rand-28.txt", 0x559f43e0a550)     = -1 ENOENT (No such file or directory)
lstat("rand-28.txt", 0x559f43e0a550)    = -1 ENOENT (No such file or directory)
open("rand-28.txt", O_RDWR|O_CREAT|O_TRUNC, 0666) = 4
ioctl(1, TIOCGWINSZ, 0x7fff9a895790)    = -1 ENOTTY (Inappropriate ioctl for device)
write(1, " extracting: rand-28.txt        "..., 37 extracting: rand-28.txt             ) = 37
read(3, "\275\3279Y\206\223\217}\355W%:\220YNT\0\257\260z^\361T\242\2\370\21\336\372+\306\310"..., 8192) = 8192

unzip打开dataset.zip，查找到结尾，然后查找存档中所请求文件的开头（rand-28.txt偏移量 849346560）并从那里读取。

通过扫描档案的最后 65557 字节找到中心目录；请参阅从这里开始的代码：

/*---------------------------------------------------------------------------
    Find and process the end-of-central-directory header.  UnZip need only
    check last 65557 bytes of zipfile:  comment may be up to 65535, end-of-
    central-directory record is 18 bytes, and signature itself is 4 bytes;
    add some to allow for appended garbage.  Since ZipInfo is often used as
    a debugging tool, search the whole zipfile if zipinfo_mode is true.
  ---------------------------------------------------------------------------*/