destination buffer
On success, the number of bytes read is returned (zero indicates end of file), and the file position is advanced by this number. It is not an error if this number is smaller than the number of bytes requested; this may happen for example because fewer bytes are actu‐ ally available right now (maybe because we were close to end-of- file, or because we are reading from a pipe, or from a terminal), or because read() was interrupted by a signal. On error, -1 is returned, and errno is set appropriately. In this case it is left unspecified whether the file position (if any) changes.
Errors:
EAGAIN The file descriptor fd refers to a file other than a socket and has been marked nonblocking (O_NONBLOCK), and the read would block.
EAGAIN or EWOULDBLOCK The file descriptor fd refers to a socket and has been marked nonblocking (O_NONBLOCK), and the read would block. POSIX.1-2001 allows either error to be returned for this case, and does not require these constants to have the same value, so a portable application should check for both possi‐ bilities.
EBADF fd is not a valid file descriptor or is not open for reading.
EFAULT dst is outside your accessible address space.
EINTR The call was interrupted by a signal before any data was read; see signal(7).
EINVAL fd is attached to an object which is unsuitable for reading; or the file was opened with the O_DIRECT flag, and either the address specified in dst, the value specified in count, or the current file offset is not suitably aligned.
EINVAL fd was created via a call to timerfd_create(2) and the wrong size buffer was given to read(); see timerfd_create(2) for further information.
EIO I/O error. This will happen for example when the process is in a background process group, tries to read from its con‐ trolling tty, and either it is ignoring or blocking SIGTTIN or its process group is orphaned. It may also occur when there is a low-level I/O error while reading from a disk or tape.
EISDIR fd refers to a directory.
Other errors may occur, depending on the object connected to fd. POSIX allows a read() that is interrupted after reading some data to return -1 (with errno set to EINTR) or to return the number of bytes already read.
read() attempts to read up to dst.length bytes from the device associated with the file descriptor of this instance into dst.
If dst.length is zero, read() returns zero and has no other results. If count is greater than SSIZE_MAX, the result is unspecified.