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Reading manual pages

READ(2)                                    BSD System Calls Manual                                   READ(2)

NAME
     read, readv, pread -- read input

LIBRARY
     Standard C Library (libc, -lc)

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);

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

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].  Pread() per-forms performs
     forms the same function, but reads from the specified position in the file without modifying the file
     pointer.

     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(), readv(), and pread() 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
     Read(), readv(), and pread() 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.

     The pread() call may also return the following errors:

     [EINVAL]           The specified file offset is invalid.

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

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 ISO/IEC 9945-1:1990 (``POSIX.1'').  The readv() and
     pread() functions are expected to conform to X/Open Portability Guide Issue 4, Version 2 (``XPG4.2'').

HISTORY
     The pread() function call appeared in AT&T System V.4 UNIX.  The readv() function call appeared in
     4.2BSD.  A read() function call appeared in Version 6 AT&T UNIX.

BSD                                           February 26, 1994                                          BSD