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READ(2) | System Calls Manual | READ(2) |
NAME¶
read, readv, pread, preadv — read inputLIBRARY¶
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 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; /* Base address. */ size_t iov_len; /* Length. */ };
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.ERRORS¶
The read(), readv(), pread() and preadv() system calls will succeed unless:- [
EBADF
] - The d argument is not a valid file or socket descriptor 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 allocated address space.
- [
EIO
] - An I/O error occurred while reading from the file system.
- [
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.
- [
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 before the end-of-file, 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
.
- [
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 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 array points outside the process's allocated address space.
- [
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), select(2), socket(2), socketpair(2), fread(3), readdir(3)STANDARDS¶
The read() system call is expected to conform to IEEE Std 1003.1-1990 (“POSIX.1”). The readv() and pread() system calls are expected to conform to X/Open Portability Guide Issue 4, Version 2 (“XPG4.2”).HISTORY¶
The preadv() system call appeared in FreeBSD 6.0. The pread() function appeared in AT&T System V Release 4 UNIX. The readv() system call appeared in 4.2BSD. The read() function appeared in Version 6 AT&T UNIX.October 11, 2006 | Debian |