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;
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 performs the same action, but scatters the input data into the iovcnt buffers specified by the members of the iovec array: iov[0], iov[1], ..., iov[iovcnt−1].
For readv, the iovec structure is defined as
struct iovec {
caddr_tiov_base;
intiov_len;
};
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(.). 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 a object is undefined.
Upon successful completion, read and readv 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 file which has that many bytes left before the end-of-file, but in no other cases.
RETURN VALUE
If the returned value is 0, then end-of-file has been reached.
If 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.
ENVIRONMENT
When your program is compiled using the System V environment, read and readv return 0 if the file has been set up for non-blocking I/O and the read would block.
Differs from the System V definition in that the parameter nbytes is of type int rather than type unsigned.
DIAGNOSTICS
The read and readv system calls will 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 system.
[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 The file referred to by that file handle no longer exists or has been revoked.
In addition, readv 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 overflowed 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 properly respond after a period of time which is dependent on the communications protocol.