Manual Pages  — READ

struct iovec {
        void   *iov_base;  /* Base address. */
        size_t iov_len;    /* Length. */
};

On objects capable of seeking, the read() starts at a position given by the pointer associated with fd (see lseek(2)). Upon return from read(), the pointer is incremented by the number of bytes actually read.

Objects that are not capable of seeking always read from the current position. The value of the pointer associated with such an object is undefined.

Upon successful completion, read(), readv(), pread() and preadv() return the number of bytes actually read and placed in the buffer. The system guarantees to read the number of bytes requested if the descriptor references a normal file that has that many bytes left before the end-of-file, but in no other case.

In accordance with IEEE Std 1003.1-2004 ("POSIX.1"), both read(2) and write(2) syscalls are atomic with respect to each other in the effects on file content, when they operate on regular files. If two threads each call one of the read(2) or write(2), syscalls, each call will see either all of the changes of the other call, or none of them. The FreeBSD kernel implements this guarantee by locking the file ranges affected by the calls.