READ(2) BSD System Calls Manual 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
readv(int d, const struct iovec *iov, int iovcnt);
ssize_t
pread(int d, void *buf, size_t nbytes, off_t offset);
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 speci‐
fied by the members of the iov array: iov[0], iov[1], ..., iov[iovcnt-1].
The pread() and preadv() calls perform the same function, 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 {
char *iov_base; /* Base address. */
size_t iov_len; /* Length. */
};
Each iovec entry specifies the base address and length of an area in mem‐
ory where data should be placed. Readv() and preadv() 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 refer‐
ences 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 read‐
ing 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] Buf points outside the allocated 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 negative.
[EAGAIN] The file was marked for non-blocking I/O, and no data
were ready to be read.
In addition, readv() and preadv() 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 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 points outside the process's allocated
address space.
The pread() and preadv() calls 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 ALSOdup(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 preadv() function call was added in DragonFly 1.5. The pread() func‐
tion 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 April 27, 2006 BSD
