READ(2) OpenBSD Programmer's Manual READ(2)NAME
read, readv, pread, preadv - read input
SYNOPSIS
#include <sys/types.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);
#include <sys/types.h>
#include <sys/uio.h>
#include <unistd.h>
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);
DESCRIPTIONread() 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]. pread() and preadv() perform the same functions, but read
from the specified position in the file without modifying the file
pointer.
For readv() and preadv(), 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(), 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.
Note that readv() and preadv() will fail if the value of iovcnt exceeds
the constant IOV_MAX.
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.
ERRORSread(), readv(), pread(), and preadv() will succeed unless:
[EBADF] d is not a valid file or socket descriptor open for
reading.
[EFAULT] Part of iov or buf points outside the process's allocated
address space.
[EIO] An I/O error occurred while reading from the file system,
or the process is a member of a background process
attempting to read from its controlling terminal, the
process is ignoring or blocking the SIGTTIN signal or the
process group is orphaned.
[EINTR] A read from a slow device (i.e. one that might block for an
arbitrary amount of time) was interrupted by the delivery
of a signal before any data arrived.
[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, read() and pread() may return the following error:
[EINVAL] nbytes was larger than SSIZE_MAX.
pread() and preadv() may return the following error:
[ESPIPE] d is associated with a pipe, socket, or FIFO.
readv() and preadv() may return one of the following errors:
[EINVAL] iovcnt was less than or equal to 0, or greater than
IOV_MAX.
[EINVAL] The sum of the iov_len values in the iov array overflowed
an ssize_t.
SEE ALSOdup(2), fcntl(2), open(2), pipe(2), poll(2), select(2), socket(2),
socketpair(2)STANDARDS
The read() function conforms to IEEE Std 1003.1-1990 (``POSIX''). The
readv() and pread() functions conform to X/Open Portability Guide Issue
4.2 (``XPG4.2'').
HISTORY
The preadv() function first appeared in OpenBSD 2.7. The pread()
function appeared in AT&T System V.4 UNIX. The readv() function call
appeared in 4.2BSD. The read() function call appeared in Version 2 AT&T
UNIX.
CAVEATS
Error checks should explicitly test for -1. Code such as
while ((nr = read(fd, buf, sizeof(buf))) > 0)
is not maximally portable, as some platforms allow for nbytes to range
between SSIZE_MAX and SIZE_MAX - 2, in which case the return value of an
error-free read() may appear as a negative number distinct from -1.
Proper loops should use
while ((nr = read(fd, buf, sizeof(buf))) != -1 && nr != 0)
OpenBSD 4.9 December 30, 2009 OpenBSD 4.9