READ(2)

NAME
read, readv - read input

SYNOPSIS
#include <sys/types.h>
#include <sys/uio.h>
#include <unistd.h>
ssize_t
read(int d, void *buf, size_t nbytes);
ssize_t
readv(int d, const struct iovec *iov, int iovcnt);

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

For readv(), the iovec structure is defined as:

    struct iovec {
    	char   *iov_base;  /* Base address. */
    	size_t iov_len;    /* Length. */
    };

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.

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

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 normal file that has that many bytes left before the end-of-file, but in no other case.

IMPLEMENTATION NOTES

In the non-threaded library read() is implemented as the read syscall.

In the threaded library, the read syscall is assembled to _thread_sys_read() and read() is implemented as a function which locks d for read, then calls _thread_sys_read(). If the call to _thread_sys_read() would block, a context switch is performed. Before returning, read() unlocks d.

In the non-threaded library readv() is implemented as the readv syscall.

In the threaded library, the readv syscall is assembled to _thread_sys_readv() and readv() is implemented as a function which locks d for read, then calls _thread_sys_readv(). If the call to _thread_sys_readv() would block, a context switch is performed. Before returning, readv() unlocks d.

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
Read() and readv() will succeed unless:

[EBADF]
    D 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.

[EINTR]
    A read from a slow device was interrupted before any data arrived by the delivery of a signal.

[EINVAL]
    The pointer associated with d was negative.

[EAGAIN]
    The file was marked for non-blocking I/O, and no data were ready to be read.

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

[EINVAL]
    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.

SEE ALSO
dup(2), fcntl(2), open(2), pipe(2), select(2), socket(2), socketpair(2)

STANDARDS
The read() function call is expected to conform to p1003.1-90.

HISTORY
The readv() function call appeared in 4.2BSD. A read() function call appeared in AT&T UNIX.