read(2)read(2)Name
read, readv - read from a file
Syntax
cc = read(d, buf, nbytes)
int cc, d;
char *buf;
int nbytes;
#include <sys/types.h>
#include <sys/uio.h>
cc = readv(d, iov, iovcnt)
int cc, d;
struct iovec *iov;
int iovcnt;
Arguments
d File descriptor.
buf Character pointer where information is stored.
nbytes Integer that tells you how many bytes to read.
iov Pointer to an iovec structure.
iovcnt The number of iovec structures to be processed.
Description
The system call attempts to read nbytes of data from the object refer‐
enced by the descriptor d into the buffer pointed to by buf. The sys‐
tem call performs the same action, but scatters the input data into the
iovcnt buffers specified by the members of the iovec following array:
iov[0], iov[1], ..., iov[iovcnt-1].
For the iovec structure is defined as follows:
struct iovec {
caddr_t iov_base;
int iov_len;
};
Each iovec entry specifies the base address and length of an area in
memory where data should be placed. The system call fills an area com‐
pletely before proceeding to the next area.
On objects that are capable of seeking, the starts at a position given
by the pointer associated with d. See for more information. Upon
return from 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.
When attempting to read from an empty pipe (or FIFO):
· If no process has the pipe open for writing, returns zero to indi‐
cate end-of-file.
· If some process has the pipe open for writing and O_NDELAY or
O_NONBLOCK is set, returns a -1, errno is to [EWOULDBLOCK]. If
some process has the pipe open for writing and O_NDELAY and O_NON‐
BLOCK are clear, blocks until data is written or the pipe is
closed by all processes that opened the pipe for writing.
Upon successful completion, and return the number of bytes actually
read and placed in the buffer. The system reads the number of bytes
requested if the descriptor references a file which has that many bytes
left before the end-of-file; this is not true in any other instance.
Unless the SV_INTERRUPT bit has been set for a signal, the system calls
are automatically restarted when a process receives a signal while
waiting for input. See also
Return Values
If the returned value is 0, then end-of-file has been reached.
If the read is successful, the number of bytes actually read is
returned. Otherwise, a -1 is returned and the global variable errno is
set to indicate the error.
Diagnostics
The and system calls fail if one or more of the following are true:
[EBADF] The d argument is not a valid file or socket descriptor
open for reading.
[EFAULT] The buf points outside the allocated address space.
[EINTR] A read from a slow device was interrupted before any
data arrived by the delivery of a signal.
[EIO] An I/O error occurred while reading from the file sys‐
tem.
[ESTALE] The file handle given in the argument is invalid. The
file referred to by that file handle no longer exists or
has been revoked.
[EWOULDBLOCK] The O_DELAY or O_NONBLOCK flag is set for the file
descriptor and the process would be delayed in the read
operation.
In addition, may return one of the following errors:
[EINVAL] The 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 over‐
flowed a 32-bit integer.
[EFAULT] Part of the iov points outside the process's allocated
address space.
[ETIMEDOUT] A connect request or remote file operation failed
because the connected party did not respond after a
period of time determined by the communications proto‐
col.
Environment
SYSTEM_FIVE
When you use the System V environment, note the following:
· If your program is compiled in this environment, a and system call
returns 0 if the file has been set up for non-blocking I/O and the
read would block.
· In this environment, the parameter nbytes is of type int instead
of type unsigned.
POSIX
In the POSIX environment, [EAGAIN] is returned in errno instead of
[EWOULDBLOCK].
See Alsodup(2), open(2), pipe(2), sigvec(2), socket(2), socketpair(2)read(2)
