由于未初始化的数据指针，无法插入SET/P吗? [英] Piping into SET /P fails due to uninitialised data pointer?

问题描述

假设我们有一个文本文件sample.txt:

Supposing we have got a text file sample.txt:

one
two
...

现在我们要删除第一行:

Now we want to remove the first line:

two
...

一种快速的方法是使用输入重定向set /P和findstr ¹ (我知道还有其他使用more或for /F的方法，但是让我们忘记关于它们的信息):

A quick way to do that is to use input redirection, set /P and findstr¹ (I know there are other ways using more or for /F, but let us forget about them for now):

@echo off
< "sample.txt" (
    set /P =""
    findstr "^"
)

输出将与预期的一样.

但是，当我用type和管道|替换输入重定向<时，为什么输出为空:

However, why is the output empty when I replace the input redirection < by type and a pipe | :

@echo off
type "sample.txt" | (
    set /P =""
    findstr "^"
)

当我用pause > nul替换set /P =""时，输出是我所期望的-输出输出文件，但缺少第一行的第一个字符(因为它被pause占用).但是，为什么set /P似乎消耗了所有内容，而不是像重定向<方法那样只消耗了第一行?那是个错误吗?

When I replace set /P ="" by pause > nul, the output is what I expect -- the input file is output but with the first character of the first line missing (as it is consumed by pause). But why does set /P seem to consume everything instead of only the first line like it does with the redirection < approach? Is that a bug?

在我看来，set /P未能充分初始化指向管道数据的读取指针.

To me it looks like set /P fails to adequately initialise the reading pointer to the piped data.

我在Windows 7和Windows 10上看到了这种奇怪的行为.

I watched that strange behaviour on Windows 7 and on Windows 10.

它变得更加不可思议:当多次调用包含管道的脚本时，例如通过像for /L %I in (1,1,1000) do @pipe.bat这样的循环，而输入文件包含约十五行或更多行，有时(千分之几)返回输入文件的片段；每次该片段都是完全相同的；似乎一开始总是缺少80个字节.

It becomes even more weird: when calling the script containing the pipe multiple times, for instance by a loop like for /L %I in (1,1,1000) do @pipe.bat, and the input file contains about fifteen lines or more, sometimes (a few times out of thousand) a fragment of the input file is returned; that fragment is exactly the same each time; it seems that there are always 80 bytes missing at the beginning.

_{1)findstr挂起，以防最后一行没有被换行符终止，因此让我们假设在那里.}

_{1) findstr hangs in case the last line is not terminated by a line-break, so let us assume such is there.}

推荐答案

检索数据时，set /p尝试用stdin的数据填充1023字符缓冲区(如果有).读操作结束后，将搜索行的第一行，一旦找到行的第一行(或到达缓冲区的末尾)，将调用SetFilePointer API以在输入的行末之后重新定位输入流指针.读行.这样，下一次读取操作将在读取行之后开始检索数据.

When retrieving data, the set /p tries to fill a 1023 character buffer (if they are available) with data from stdin. Once this read operation has ended, the first end of line is searched and once it has been found (or the end of the buffer has been reached), the SetFilePointer API is called to reposition the input stream pointer after the end of the read line. This way the next read operation will start to retreive data after the read line.

当磁盘文件与输入流相关联时，此方法可完美运行，但正如Microsoft在

This works flawlessly when a disk file is associated with the input stream, but as Microsoft states in the SetFilePointer documentation

hFile 参数必须引用存储在搜寻设备上的文件； 例如磁盘卷.调用 SetFilePointer 函数，使用 处理非寻找设备，例如管道或通讯设备 即使 SetFilePointer 函数可能不支持该设备 不返回错误. SetFilePointer 函数的行为 这种情况是不确定的.

The hFile parameter must refer to a file stored on a seeking device; for example, a disk volume. Calling the SetFilePointer function with a handle to a non-seeking device such as a pipe or a communications device is not supported, even though the SetFilePointer function may not return an error. The behavior of the SetFilePointer function in this case is undefined.

正在发生的事情是，虽然没有产生任何错误，但在将stdin与管道相关联，指针未移回并且1023个字节(或可用的读取字节数)的情况下，重新定位读取指针的调用失败继续阅读.

What is happening is that, while not generating any error, the call to reposition the read pointer fails when stdin is associated with a pipe, the pointer is not moved back and the 1023 bytes (or the number of available read bytes) keep read.

编辑以响应Aacini的请求

edited in response to Aacini request

set命令由eSet函数处理，该函数调用SetWork以确定将执行哪种类型的set命令.

The set command is processed by the eSet function, who calls SetWork to determine which type of set command will be executed.

由于它是set /p，因此调用SetPromptUser函数，并从该函数中调用ReadBufFromInput函数

As it is a set /p the SetPromptUser function is called and from this function the ReadBufFromInput function is called

add     esp, 0Ch
lea     eax, [ebp+var_80C]
push    eax             ; int
push    3FFh            ; int
lea     eax, [ebp+Value]
push    eax             ; int
xor     esi, esi
push    0FFFFFFF6h      ; nStdHandle
mov     word ptr [ebp+Value], si
call    edi ; GetStdHandle(x) ; GetStdHandle(x)
push    eax             ; hFile
call    _ReadBufFromInput@16 ; ReadBufFromInput(x,x,x,x)

它从标准输入句柄(0FFFFFFF6h = -10 =

it requests 3FFh (1023) characters from standard input handle (0FFFFFFF6h = -10 = STD_INPUT_HANDLE)

ReadBufFromInput使用GetFileType API来确定是从控制台还是从文件读取

ReadBufFromInput uses the GetFileType API to determine if it should read from the console or from a file

; Attributes: bp-based frame

; int __stdcall ReadBufFromInput(HANDLE hFile, int, int, int)
_ReadBufFromInput@16 proc near

hFile= dword ptr  8

; FUNCTION CHUNK AT .text:4AD10D3D SIZE 00000006 BYTES

mov     edi, edi
push    ebp
mov     ebp, esp
push    [ebp+hFile]     ; hFile
call    ds:__imp__GetFileType@4 ; GetFileType(x)
and     eax, 0FFFF7FFFh
cmp     eax, 2
jz      loc_4AD10D3D

，并且在这种情况下，它是管道(

and, as in this case it is a pipe (GetFileType returns 3) the code jumps to the ReadBufFromFile function

; Attributes: bp-based frame

; int __stdcall ReadBufFromFile(HANDLE hFile, LPWSTR lpWideCharStr, DWORD cchWideChar, LPDWORD lpNumberOfBytesRead)
_ReadBufFromFile@16 proc near

var_C= dword ptr -0Ch
cchMultiByte= dword ptr -8
NumberOfBytesRead= dword ptr -4
hFile= dword ptr  8
lpWideCharStr= dword ptr  0Ch
cchWideChar= dword ptr  10h
lpNumberOfBytesRead= dword ptr  14h

此函数将调用ReadFile API函数以检索指示的字符数

This function will call the ReadFile API function to retrive the indicated number of characters

push    ebx             ; lpOverlapped
push    [ebp+lpNumberOfBytesRead] ; lpNumberOfBytesRead
mov     [ebp+var_C], eax
push    [ebp+cchWideChar] ; nNumberOfBytesToRead
push    edi             ; lpBuffer
push    [ebp+hFile]     ; hFile
call    ds:__imp__ReadFile@20 ; ReadFile(x,x,x,x,x)

迭代返回的缓冲区以寻找行尾，一旦找到行末，输入流中的指针将在找到位置后移动

The returned buffer is iterated in search of an end of line, and once it is found, the pointer in the input stream is moved after the found poisition

.text:4AD06A15 loc_4AD06A15:                           
.text:4AD06A15                 cmp     [ebp+NumberOfBytesRead], 3
.text:4AD06A19                 jl      short loc_4AD06A2D
.text:4AD06A1B                 mov     al, [esi]
.text:4AD06A1D                 cmp     al, 0Ah
.text:4AD06A1F                 jz      loc_4AD06BCF
.text:4AD06A25
.text:4AD06A25 loc_4AD06A25:                           
.text:4AD06A25                 cmp     al, 0Dh
.text:4AD06A27                 jz      loc_4AD06D14
.text:4AD06A2D
.text:4AD06A2D loc_4AD06A2D:                           
.text:4AD06A2D                 movzx   eax, byte ptr [esi]
.text:4AD06A30                 cmp     byte ptr _DbcsLeadCharTable[eax], bl
.text:4AD06A36                 jnz     loc_4AD12018
.text:4AD06A3C                 dec     [ebp+NumberOfBytesRead]
.text:4AD06A3F                 inc     esi
.text:4AD06A40
.text:4AD06A40 loc_4AD06A40:                           
.text:4AD06A40                 cmp     [ebp+NumberOfBytesRead], ebx
.text:4AD06A43                 jg      short loc_4AD06A15

.text:4AD06BCF loc_4AD06BCF:                          
.text:4AD06BCF                 cmp     byte ptr [esi+1], 0Dh
.text:4AD06BD3                 jnz     loc_4AD06A25
.text:4AD06BD9                 jmp     loc_4AD06D1E

.text:4AD06D14 loc_4AD06D14:                           
.text:4AD06D14                 cmp     byte ptr [esi+1], 0Ah
.text:4AD06D18                 jnz     loc_4AD06A2D
.text:4AD06D1E
.text:4AD06D1E loc_4AD06D1E:                          
.text:4AD06D1E                 mov     eax, [ebp+var_C]
.text:4AD06D21                 mov     [esi+2], bl
.text:4AD06D24                 sub     esi, edi
.text:4AD06D26                 inc     esi
.text:4AD06D27                 inc     esi
.text:4AD06D28                 push    ebx             ; dwMoveMethod
.text:4AD06D29                 push    ebx             ; lpDistanceToMoveHigh
.text:4AD06D2A                 mov     [ebp+cchMultiByte], esi
.text:4AD06D2D                 add     esi, eax
.text:4AD06D2F                 push    esi             ; lDistanceToMove
.text:4AD06D30                 push    [ebp+hFile]     ; hFile
.text:4AD06D33                 call    ds:__imp__SetFilePointer@16 ; SetFilePointer(x,x,x,x)

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