READ(2)READ(2)NAME
read, readv - read input
BSD SYNOPSIS
int read(int fd, char *buf, int nbytes);
#include <sys/types.h>
#include <sys/uio.h>
int readv(int fd, struct iovec *iov, int iovcnt);
POSIX SYNOPSIS
size_t read(int fd, const void *buf, size_t nbytes);
DESCRIPTION
The read function attempts to read nbytes of data from the object
referenced by the descriptor fd into the buffer pointed to by buf.
With objects capable of seeking, read starts at the position given by
the seek pointer associated with fd (see lseek(2)). Upon return, the
seek pointer for fd is incremented by the number of bytes actually
read.
Objects that are not capable of seeking always read from their current
position. The value of the pointer associated with such an object is
undefined.
Upon successful completion, read returns 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. If the returned value is 0, then end-of-file has been reached.
When read returns greater than zero, the st_atime field of the file is
updated.
The readv function performs a scattered read, puting input data into
the iovcnt buffers specified by the members of the iov array: iov[0],
iov[1], iov[ iovcnt - 1]. The return value of readv is the same as
that for read.
The iovec structure is defined as
struct iovec {
caddr_t iov_base;
int iov_len;
};
Each iovec entry specifies the base address and length of an area in
memory where data should be placed. Readv will always fill an area
completely before proceeding to the next. POSIX applications should
not use readv.
POSIX applications may expect the following behavior when attempting to
read (but not readv) from an empty pipe or FIFO:
(1) If no process has the pipe open for writing, read returns zero to
indicate end-of-file.
(2) If some process has the pipe open for writing and O_NONBLOCK is
set, read returns -1 and sets errno to [EAGAIN].
(3) If some process has the pipe open for writing and O_NONBLOCK is
clear, read will block until some data is written or the pipe is
closed by all processes that had the pipe open for writing.
POSIX applications may expect the following behavior when attempting to
read a file (other than a pipe or FIFO) that supports non-blocking
reads and has no data currently available:
(1) If O_NONBLOCK is set, read returns -1 and sets errno to [EAGAIN].
(2) If O_NONBLOCK is clear, read blocks until some data becomes
available.
RETURN VALUE
If successful, the number of bytes actually read is returned.
Otherwise, a -1 is returned and the global variable errno is set to
indicate the error.
ERRORS
Read will fail if one or more of the following are true:
[EAGAIN] (POSIX only) The O_NONBLOCK flag is set for the file
descriptor and the process would be delayed in the read
operation.
[EBADF] fd is not a valid file or socket descriptor open for
reading.
[EFAULT] buf points outside the allocated address space.
[EIO] An I/O error occurred while reading from the file
system; or, the process is in a background process group
and is attempting to read from its controlling terminal,
and either the process is ignoring or blocking the
SIGTTIN signal or the process group of the process is
orphaned.
[EINTR] A read from a slow device was interrupted before any
data arrived by the delivery of a signal.
[EINVAL] The pointer associated with fd was negative; or, An
attempt was made to read greater than SSIZE_MAX bytes.
[EWOULDBLOCK] The file was marked for non-blocking I/O by fcntl(2),
and no data were ready to be read.
In addition, the following errors may occur with readv:
[EFAULT] Part of the iov points outside the process's allocated
address space.
[EINVAL] Iovcnt was less than or equal to 0, or greater than 16.;
or, one of the iov_len values in the iov array was
negative; or, the sum of the iov_len values in the iov
array overflowed a 32-bit integer.
SEE ALSOcreat(2), dup(2), fcntl(2), lseek(2), open(2), pipe(2)
August 1, 1992 READ(2)
