[英]Under what circumstances does the read() syscall return 0?
I'm looking at the read
syscall in Unix, which (at least in Linux) has this signature: [1]我正在查看 Unix 中的
read
系统调用,它(至少在 Linux 中)具有以下签名:[1]
ssize_t read(int fd, void* buf, size_t count);
Let's assume that the call succeeds (ie no negative return values) and that count > 0
(ie the buffer actually can store a nonzero amount of bytes).让我们假设调用成功(即没有负返回值)并且
count > 0
(即缓冲区实际上可以存储非零字节数)。 Under which circumstances would read()
return 0?在什么情况下
read()
返回 0? I can think of the following:我可以想到以下几点:
fd
refers to a regular file and the end of the file has been reached.fd
引用常规文件并且已到达文件末尾时。fd
refers to the receiving end of a pipe, socket or FIFO, the sending end has been closed and the pipe's/socket's/FIFO's own buffer has been exhausted.fd
指的是管道、套接字或 FIFO 的接收端时,发送端已经关闭并且管道/套接字/FIFO 自己的缓冲区已经耗尽。fd
refers to the slave side of a terminal device that is in ICANON
and Ctrl-D
has been sent into the master side while the line buffer was empty.fd
指的是ICANON
中的终端设备的从端并且Ctrl-D
已发送到主端时,而行缓冲区为空。 I'm curious if there are any other situations that I'm not aware of, where read()
would return with a result of 0. I'm especially interested (because of reasons) in situations like the last one in the list above, where read()
returns 0 once, but subsequent calls to read()
on the same FD could return a nonzero result.我很好奇是否还有其他我不知道的情况,其中
read()
将返回 0 结果。我对上面列表中的最后一种情况特别感兴趣(由于原因) ,其中read()
返回 0 一次,但随后在同一 FD 上调用read()
可能返回非零结果。 If an answer only applies to a certain flavor of Unix, I'm still interested in hearing it.如果答案仅适用于某种风格的 Unix,我仍然有兴趣听到它。
[1] I know this signature is for the libc wrapper, not the actual syscall, but that's not important right now. [1] 我知道这个签名是用于 libc 包装器的,而不是实际的系统调用,但这现在并不重要。
Normally a return value of 0
always means end-of-file.通常返回值
0
总是意味着文件结束。 However, if you specify 0
as the number of bytes to read, it will always return 0
unless there's an error detected.但是,如果您指定
0
作为要读取的字节数,除非检测到错误,否则它将始终返回0
。
Terminal devices are a special case.终端设备是一个特例。 If the terminal is in cooked mode, typing Control-d tells the device driver to return from any pending
read()
immediately with whatever is in the input editing buffer, rather than waiting for the user to enter a newline.如果终端处于熟模式,输入Control-d 会告诉设备驱动程序立即从任何挂起的
read()
返回输入编辑缓冲区中的任何内容,而不是等待用户输入换行符。 If the buffer is empty, this results in a zero-length read.如果缓冲区为空,则会导致读取长度为零。 This is how typing the EOF character at the beginning of a line is automatically treated as EOF by applications.
这就是在行首输入 EOF 字符被应用程序自动视为 EOF 的方式。
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