問題描述
C中的文件描述符分配 (File Descriptor Assignment in C)
當在 C 中創建套接字或打開/創建文件時，分配給套接字/文件的文件描述符是否保證是可用的最低值描述符？C 規範在這方面對文件描述符分配有什麼看法？
參考解法
方法 1:
It's not guaranteed to be the lowest, and is implementation dependent (1). In general, however, the routine that assigns open file descriptors uses a method that gives you the first open on. It could be that immediately after several lower ones free, leaving you with a higher descriptor than you might expect though.
The only reason I can think of to know this, though, is for the select function, which is sped up if you pass it the highest file descriptor you need to check for.
(1) Note that those implementations that follow the IEEE standard do guarantee the lowest unused descriptor for files, but this may not apply to sockets. Not every implementation follows the IEEE standard for open(), so if you're writing portable software it is best not to depend on it.
方法 2:
I don't think you'll find it in the C spec, more likely the spec for your OS. My experience in Linux has been that it's always the lowest.
方法 3:
I'll counter this with another question ‑ why does this matter? You shouldn't be comparing the file descriptor with anything (unless checking for stdin/stdout/stderr) or doing math with it. As long as it fits in an int (and its guaranteed to) that's all you really need to know.
方法 4:
Steve M is right; C has no notion of sockets, and its file I/O functions use a [pointer to a] FILE object, not a descriptor.
方法 5:
<p>@aib the open(), close(), lseek(), read(), write() all make use of file descriptors. I hardly ever use streams for I/O.</p><p>@Kyle it matters because of statements like select(). Knowing the highest descriptor can improve performance.</p>
(by Kevin P、Adam Davis、Steve M、Kyle Cronin、aib、Kevin P)