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read, readv, pread, preadv - read input

 

NAME

      read, readv, pread, preadv - 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
      pread(int d, void *buf, size_t nbytes, off_t offset);
 
      ssize_t
      readv(int d, const struct iovec *iov, int iovcnt);
 
      ssize_t
      preadv(int d, 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 d 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 mem‐
      ory 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 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(), 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 refer‐
      ences a normal file that has that many bytes left before the end-of-file,
      but in no other case.
      If successful, the number of bytes actually read is returned.  Upon read‐
      ing 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 d argument is not a valid file or socket descrip‐
                         tor open for reading.
 
      [ECONNRESET]       The d 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 sys‐
                         tem.
 
      [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 d 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
                         residing on a file system that does not allow regular
                         read operations on directories (e.g. NFS).
 
      [EOPNOTSUPP]       The file descriptor is associated with a file system
                         and file type that do not allow regular read opera‐
                         tions 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 off‐
                         set 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 nega‐
                         tive.
 
      [EINVAL]           The sum of the iov_len values in the iov array over‐
                         flowed 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.
      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 con‐
      form to X/Open Portability Guide Issue 4.2 (“XPG4.2”).
 

HISTORY

      The preadv() system call appeared in FreeBSD 6.0.  The pread() function
      appeared in AT&T System V.4 UNIX.  The readv() system call appeared in
      4.2BSD.  The read() function appeared in Version 6 AT&T UNIX.
 

Sections

Based on BSD UNIX
FreeBSD is an advanced operating system for x86 compatible (including Pentium and Athlon), amd64 compatible (including Opteron, Athlon64, and EM64T), UltraSPARC, IA-64, PC-98 and ARM architectures. It is derived from BSD, the version of UNIX developed at the University of California, Berkeley. It is developed and maintained by a large team of individuals. Additional platforms are in various stages of development.