read(2)

NAME

read, readv, pread, preadv - read input

LIBRARY

Standard C Library (libc, -lc)

SYNOPSIS

#include <sys/types.h>
#include <sys/uio.h>
#include <unistd.h>
ssize_t
read(int d, void *buf, size_t nbytes);
ssize_t
pread(int d, void *buf, size_t nbytes, off_t offset);
ssize_t
readv(int d, const struct iovec *iov, int iovcnt);
ssize_t
preadv(int d, const struct iovec  *iov,  int  iovcnt,  off_t
offset);

DESCRIPTION

The read() system call attempts to read nbytes of data from

the object
referenced by the descriptor d into the buffer pointed to by

buf. The
readv() system call 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]. The pread() and

preadv() system
calls perform the same functions, but read from the speci

fied position in
the file without modifying the file pointer.

For readv() and preadv(), the iovec structure is defined as:

struct iovec {

void *iov_base; /* Base address. */
size_t iov_len; /* Length. */

};

Each iovec entry specifies the base address and length of an

area in memory where data should be placed. The readv() system call

will always
fill an area completely before proceeding to the next.

On objects capable of seeking, the read() starts at a posi

tion 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(), 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 de

scriptor 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 re

turned. Upon reading end-of-file, zero is returned. Otherwise, a -1 is re

turned and the
global variable errno is set to indicate the error.

ERRORS

The read(), readv(), pread() and preadv() system calls will

succeed
unless:

[EBADF] The d argument is not a valid file or

socket descrip

tor open for reading.

[ECONNRESET] The d argument refers to a socket, and

the remote

socket end is forcibly closed.

[EFAULT] The buf argument points outside the allo

cated address

space.

[EIO] An I/O error occurred while reading from

the file sys

tem.

[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 nega

tive.

[EAGAIN] The file was marked for non-blocking I/O,

and no data

were ready to be read.

[EISDIR] The file descriptor is associated with a

directory

residing on a file system that does not

allow regular
read operations on directories (e.g.

NFS).

[EOPNOTSUPP] The file descriptor is associated with a

file system

and file type that do not allow regular

read operations on it.

[EOVERFLOW] The file descriptor is associated with a

regular file,

nbytes is greater than 0, offset is be

fore the end-offile, and offset is greater than or equal

to the offset maximum established for this file

system.

[EINVAL] The value nbytes is greater than INT_MAX.

In addition, readv() and preadv() may return one of the fol

lowing errors:

[EINVAL] The iovcnt argument was less than or

equal to 0, or

greater than IOV_MAX.

[EINVAL] One of the iov_len values in the iov ar

ray was nega

tive.

[EINVAL] The sum of the iov_len values in the iov

array over

flowed a 32-bit integer.

[EFAULT] Part of the iov array points outside the

process's

allocated address space.

The pread() and preadv() system calls may also return the

following
errors:

[EINVAL] The offset value was negative.

[ESPIPE] The file descriptor is associated with a

pipe, socket,

or FIFO.

SEE ALSO

dup(2), fcntl(2), getdirentries(2), open(2), pipe(2), se

lect(2),
socket(2), socketpair(2), fread(3), readdir(3)

STANDARDS

The read() system call is expected to conform to ISO/IEC

9945-1:1990
(``POSIX.1''). The readv() and pread() system calls are ex

pected to conform to X/Open Portability Guide Issue 4.2 (``XPG4.2'').

HISTORY

The preadv() system call appeared in FreeBSD 6.0. The

pread() function
appeared in AT&T System V.4 UNIX. The readv() system call

appeared in
4.2BSD. The read() function appeared in Version 6 AT&T

UNIX.

BSD October 16, 2004

BSD