read() from stdin

Question

Consider the following line of code:

while((n = read(STDIN_FILENO, buff, BUFSIZ)) > 0)

As per my understanding read/write f

Answer 1

Your code reads:

while((n = read(0, buff, BUFSIZ) != 0))

This is flawed - the parentheses mean it is interpreted as:

while ((n = (read(0, buff, BUFSIZ) != 0)) != 0)

where the boolean condition is evaluated before the assignment, so n will only obtain the values 0 (the condition is not true) and 1 (the condition is true).

You should write:

while ((n = read(0, buff, BUFSIZ)) > 0)

This stops on EOF or a read error, and n lets you know which condition you encountered.

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Apparently, the code above was a typo in the question.

Unbuffered I/O will read up to the number of characters you read (but not more). It may read less on account of EOF or an error. It may also read less because less is available at the time of the call. Consider a terminal; typically, that will only read up to the end of line because there isn't any more available than that. Consider a pipe; if the feeding process has generated 128 unread bytes, then if BUFSIZ is 4096, you'll only get 128 bytes from the read. A non-blocking file descriptor may return because nothing is available; a socket may return fewer bytes because there isn't more information available yet; a disk read may return fewer bytes because there are fewer than the requested number of bytes left in the file when the read is performed.

In general, though, read() won't return just one byte if you request many bytes.