NAME
read, readv
— read input
SYNOPSIS
#include
<sys/types.h>
#include <sys/uio.h>
#include <unistd.h>
ssize_t
read(int
d, void *buf,
size_t nbytes);
ssize_t
readv(int
d, const struct iovec
*iov, int
iovcnt);
DESCRIPTION
Read()
attempts to read nbytes of data from the object
referenced by the descriptor d into the buffer pointed
to by buf. Readv() performs the
same action, but scatters the input data into the iovcnt
buffers specified by the members of the iov array:
iov[0], iov[1], ..., iov[iovcnt-1].
For
readv(), the
iovec structure is defined as:
struct iovec {
void *iov_base;
size_t 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.
On objects capable of seeking, the
read() starts
at a position given by the pointer associated with d
(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() and
readv() 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.
RETURN VALUES
If successful, the number of bytes actually read is returned. Upon reading end-of-file, zero is returned. Otherwise, a -1 is returned and the global variable errno is set to indicate the error.
ERRORS
Read() and readv()
will succeed unless:
- [
EBADF] - D 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.
- [
EINTR] - A read from a slow device was interrupted before any data arrived by the delivery of a signal.
- [
EINVAL] - The pointer associated with d was negative.
- [
EAGAIN] - The file was marked for non-blocking I/O, and no data were ready to be read.
In addition, readv() may return one of the
following errors:
- [
EINVAL] - Iovcnt was less than or equal to 0, or greater than 16.
- [
EINVAL] - One of the iov_len values in the iov array was negative.
- [
EINVAL] - The sum of the iov_len values in the iov array overflowed a 32-bit integer.
- [
EFAULT] - Part of the iov points outside the process's allocated address space.
SEE ALSO
dup(2), fcntl(2), open(2), pipe(2), select(2), socket(2), socketpair(2)
STANDARDS
Read() is expected to conform to IEEE Std
1003.1-1988 (“POSIX”).
HISTORY
The readv() function call appeared in
4.2BSD. A read function call
appeared in Version 6 AT&T UNIX.