READ

Section: System Calls (2)
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BSD mandoc
BSD 4

NAME

read readv - read input

SYNOPSIS

Fd #include <sys/types.h> Fd #include <sys/uio.h> Fd #include <unistd.h> Ft ssize_t Fn read int d void *buf size_t nbytes Ft ssize_t Fn readv int d const struct iovec *iov int iovcnt

DESCRIPTION

Fn Read attempts to read Fa nbytes of data from the object referenced by the descriptor Fa d into the buffer pointed to by Fa buf . Fn Readv performs the same action, but scatters the input data into the Fa iovcnt buffers specified by the members of the Fa iov array: iov[0], iov[1], ..., iov[iovcnt-1].

For Fn readv , the Fa iovec structure is defined as:

struct iovec {
        void *iov_base;
        size_t iov_len;
};

Each Fa iovec entry specifies the base address and length of an area in memory where data should be placed. Fn Readv will always fill an area completely before proceeding to the next.

On objects capable of seeking, the Fn read starts at a position given by the pointer associated with Fa d (see lseek(2)). Upon return from Fn 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.

Upon successful completion, Fn read and Fn 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 normal file that has that many bytes left before the end-of-file, but in no other case.

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

Fn Read and Fn readv will succeed unless:

Bq Er EBADF: Fa D is not a valid file or socket descriptor open for reading.
Bq Er EFAULT: Fa Buf points outside the allocated address space.
Bq Er EIO: An I/O error occurred while reading from the file system.
Bq Er EINTR: A read from a slow device was interrupted before any data arrived by the delivery of a signal.
Bq Er EINVAL: The pointer associated with Fa d was negative.
Bq Er EAGAIN: The file was marked for non-blocking I/O, and no data were ready to be read.

In addition, Fn readv may return one of the following errors:

Bq Er EINVAL: Fa Iovcnt was less than or equal to 0, or greater than {UIO_MAXIOV}
Bq Er EINVAL: One of the Fa iov_len values in the Fa iov array was negative.
Bq Er EINVAL: The sum of the Fa iov_len values in the Fa iov array overflowed a 32-bit integer.
Bq Er EFAULT: Part of the Fa iov points outside the process's allocated address space.

STANDARDS

The Fn read function is expected to conform to St -p1003.1-88 .

HISTORY

The Fn readv function call appeared in BSD 4.2 The Fn read function call appeared in AT&T System v6 .

This document was created by man2html, using the manual pages.
Time: 16:28:59 GMT, April 18, 2022