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 read 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 iovec following array: iov[0], iov[1], ..., iov[iovcnt−1].

For readv, the iovec structure is defined as follows:

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 fills an area completely before proceeding to the next area.

On objects that are capable of seeking, the read starts at a position given by the pointer associated with d. See lseek() for more information. 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. 

When attempting to read from an empty pipe (or FIFO):

•If no process has the pipe open for writing, read returns zero to indicate end-of-file.

•If some process has the pipe open for writing and O_NDELAY or O_NONBLOCK is set, read returns a −1, errno is to [EWOULDBLOCK].  If some process has the pipe open for writing and O_NDELAY and O_NONBLOCK are clear, read blocks until data is written or the pipe is closed by all processes that opened the pipe for writing.

Upon successful completion, read and readv 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 read system calls are automatically restarted when a process receives a signal while waiting for input. See also sigvec(.).

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 read and readv 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 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. 

[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, 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 respond after a period of time determined by the communications protocol. 

Environment

SYSTEM_FIVE

When you use the System V environment, note the following:

•If your program is compiled in this environment, a read and readv 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 Also

dup(2), open(2), pipe(2), sigvec(2), socket(2), socketpair(2)