NAME

     read, readv, pread, preadv — read input

LIBRARY

     Standard C Library (libc, -lc)

SYNOPSIS

     #include <unistd.h>

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

     ssize_t
     pread(int fd, void *buf, size_t nbytes, off_t offset);

     #include <sys/uio.h>

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

     ssize_t
     preadv(int fd, const struct iovec *iov, int iovcnt, off_t offset);

DESCRIPTION

     The read() system call attempts to read nbytes of data from the object referenced by the
     descriptor fd 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
     iov array: iov[0], iov[1], ..., iov[iovcnt-1].  The pread() and preadv() system calls
     perform the same functions, but read from the specified position in the file without
     modifying the file pointer.

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

           struct iovec {
                   void   *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.  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 fd (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(), readv(), pread() and preadv() 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.

     In accordance with IEEE Std 1003.1-2004 (“POSIX.1”), both read(2) and write(2) syscalls are
     atomic with respect to each other in the effects on file content, when they operate on
     regular files.  If two threads each call one of the read(2) or write(2), syscalls, each call
     will see either all of the changes of the other call, or none of them.  The FreeBSD kernel
     implements this guarantee by locking the file ranges affected by the calls.

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

     The read(), readv(), pread() and preadv() system calls will succeed unless:

     [EBADF]            The fd argument is not a valid file or socket descriptor open for
                        reading.

     [ECONNRESET]       The fd argument refers to a socket, and the remote socket end is forcibly
                        closed.

     [EFAULT]           The buf argument points outside the allocated address space.

     [EIO]              An I/O error occurred while reading from the file system.

     [EINTEGRITY]       Corrupted data was detected while reading from the file system.

     [EBUSY]            Failed to read from a file, e.g. /proc/<pid>/regs while <pid> is not
                        stopped

     [EINTR]            A read from a slow device (i.e. one that might block for an arbitrary
                        amount of time) was interrupted by the delivery of a signal before any
                        data arrived.

     [EINVAL]           The pointer associated with fd was negative.

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

     [EISDIR]           The file descriptor is associated with a directory.  Directories may only
                        be read directly if the filesystem supports it and the
                        security.bsd.allow_read_dir sysctl MIB is set to a non-zero value.  For
                        most scenarios, the readdir(3) function should be used instead.

     [EOPNOTSUPP]       The file descriptor is associated with a file system and file type that
                        do not allow regular read operations on it.

     [EOVERFLOW]        The file descriptor is associated with a regular file, nbytes is greater
                        than 0, offset is before the end-of-file, and offset is greater than or
                        equal to the offset maximum established for this file system.

     [EINVAL]           The value nbytes is greater than INT_MAX.

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

     [EINVAL]           The iovcnt argument was less than or equal to 0, or greater than IOV_MAX.

     [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 array points outside the process's allocated address
                        space.

     The pread() and preadv() system calls may also return the following errors:

     [EINVAL]           The offset value was negative.

     [ESPIPE]           The file descriptor is associated with a pipe, socket, or FIFO.

SEE ALSO

     dup(2), fcntl(2), getdirentries(2), open(2), pipe(2), select(2), socket(2), socketpair(2),
     fread(3), readdir(3)

STANDARDS

     The read() system call is expected to conform to ISO/IEC 9945-1:1990 (“POSIX.1”).  The
     readv() and pread() system calls are expected to conform to X/Open Portability Guide
     Issue 4, Version 2 (“XPG4.2”).

HISTORY

     The preadv() system call appeared in FreeBSD 6.0.  The pread() function appeared in AT&T
     System V Release 4 UNIX.  The readv() system call appeared in 4.2BSD.  The read() function
     appeared in Version 1 AT&T UNIX.