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Manual page for READ(2)

read, readv - read input

BSD SYNOPSIS

int read(int fd, char *buf, int nbytes);

#include <sys/types.h> #include <sys/uio.h>

int readv(int fd, struct iovec *iov, int iovcnt);

POSIX SYNOPSIS

size_t read(int fd, const void *buf, size_t nbytes);

DESCRIPTION

The read function attempts to read nbytes of data from the object referenced by the descriptor fd into the buffer pointed to by buf.

With objects capable of seeking, read starts at the position given by the seek pointer associated with fd (see lseek.2 Upon return, the seek pointer for fd is incremented by the number of bytes actually read.

Objects that are not capable of seeking always read from their current position. The value of the pointer associated with such an object is undefined.

Upon successful completion, read returns 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 normal file that has that many bytes left before the end-of-file, but in no other case. If the returned value is 0, then end-of-file has been reached.

When read returns greater than zero, the st_atime field of the file is updated.

The readv function performs a scattered read, puting input data into the iovcnt buffers specified by the members of the iov array: iov[0], iov[1], iov[ iovcnt - 1]. The return value of readv is the same as that for read.

The iovec structure is defined as

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. Readv will always fill an area completely before proceeding to the next. POSIX applications should not use readv.

POSIX applications may expect the following behavior when attempting to read (but not readv) from an empty pipe or FIFO:

(1): If no process has the pipe open for writing, read returns zero to indicate end-of-file.
(2): If some process has the pipe open for writing and O_NONBLOCK is set, read returns -1 and sets errno to [EAGAIN].
(3): If some process has the pipe open for writing and O_NONBLOCK is clear, read will block until some data is written or the pipe is closed by all processes that had the pipe open for writing.

POSIX applications may expect the following behavior when attempting to read a file (other than a pipe or FIFO) that supports non-blocking reads and has no data currently available:

(1): If O_NONBLOCK is set, read returns -1 and sets errno to [EAGAIN].
(2): If O_NONBLOCK is clear, read blocks until some data becomes available.

RETURN VALUE

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.

ERRORS

Read will fail if one or more of the following are true:

[EAGAIN]: (POSIX only) The O_NONBLOCK flag is set for the file descriptor and the process would be delayed in the read operation.
[EBADF]: fd 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 system; or, the process is in a background process group and is attempting to read from its controlling terminal, and either the process is ignoring or blocking the SIGTTIN signal or the process group of the process is orphaned.
[EINTR]: A read from a slow device was interrupted before any data arrived by the delivery of a signal.
[EINVAL]: The pointer associated with fd was negative; or, An attempt was made to read greater than SSIZE_MAX bytes.
[EWOULDBLOCK]: The file was marked for non-blocking I/O by fcntl.2 and no data were ready to be read.

In addition, the following errors may occur with readv: