read, readv, pread -- read input
Standard C Library (libc, -lc)
#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);
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() system call 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 {
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 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(), 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.
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.
The read(), readv(), and pread() system calls will succeed unless:
[EBADF] The d argument is not a valid file or socket descriptor
open for reading.
[EFAULT] The buf argument 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.
[EISDIR] The file descriptor is associated with a directory
residing on a filesystem that does not allow regular
read operations on directories (e.g. NFS).
[EOPNOTSUPP] The file descriptor is associated with a filesystem
and file type that do not allow regular read operations
on it.
In addition, readv() may return one of the following errors:
[EINVAL] The iovcnt argument 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.
The pread() system 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.
dup(2), fcntl(2), getdirentries(2), open(2), pipe(2), select(2),
socket(2), socketpair(2), fread(3), readdir(3)
The read() system call is expected to conform to ISO/IEC 9945-1:1990
(``POSIX.1''). The readv() and pread() system calls are expected to conform
to X/Open Portability Guide Issue 4.2 (``XPG4.2'').
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.
FreeBSD 5.2.1 February 26, 1994 FreeBSD 5.2.1 [ Back ] |
