| |
READ(2) BSD System Calls Manual READ(2)
NAME
read, readv, pread -- 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
readv(int d, const struct iovec *iov, int iovcnt);
ssize_t
pread(int d, void *buf, size_t nbytes, off_t offset);
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]. Pread() performs the same function, but reads from the specified position in
the file without modifying the file pointer.
For readv(), the iovec structure is defined as:
struct iovec {
char *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. 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 associ-
ated with d (see lseek(2)). Upon return from read(), the pointer is incremented by the num-
ber 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(), and pread() 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 indi-
cate the error.
ERRORS
Read(), readv(), and pread() 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 inte-
ger.
[EFAULT] Part of the iov points outside the process's allocated address space.
The pread() call may also return the following errors:
[EINVAL] The specified file offset is invalid.
[ESPIPE] The file descriptor is associated with a pipe, socket, or FIFO.
SEE ALSO
dup(2), fcntl(2), open(2), pipe(2), select(2), socket(2), socketpair(2)
STANDARDS
The read() function call is expected to conform to ISO/IEC 9945-1:1990 (``POSIX.1''). The
readv() and pread() functions are expected to conform to X/Open Portability Guide Issue 4,
Version 2 (``XPG4.2'').
HISTORY
The pread() function call appeared in AT&T System V Release 4 UNIX. The readv() function
call appeared in 4.2BSD. A read() function call appeared in Version 6 AT&T UNIX.
BSD February 26, 1994 BSD |
|
