# read

## NAME



           read - read from a file descriptor
    


## LIBRARY



           Standard C library (_libc_ , _-lc_)
    


## SYNOPSIS



           **#include <unistd.h>**
    
           **ssize_t read(** size_t count;
                        **int** _fd_**, void** _buf_**[**_count_**], size_t** _count_**);**
    


## DESCRIPTION



           **read**() attempts to read up to _count_ bytes from file descriptor _fd_
           into the buffer starting at _buf_.
    
           On files that support seeking, the read operation commences at the
           file offset, and the file offset is incremented by the number of
           bytes read.  If the file offset is at or past the end of file, no
           bytes are read, and **read**() returns zero.
    
           If _count_ is zero, **read**() _may_ detect the errors described below.
           In the absence of any errors, or if **read**() does not check for
           errors, a **read**() with a _count_ of 0 returns zero and has no other
           effects.
    
           According to POSIX.1, if _count_ is greater than **SSIZE_MAX** , the
           result is implementation-defined; see NOTES for the upper limit on
           Linux.
    


## RETURN VALUE



           On success, the number of bytes read is returned (zero indicates
           end of file), and the file position is advanced by this number.
           It is not an error if this number is smaller than the number of
           bytes requested; this may happen for example because fewer bytes
           are actually available right now (maybe because we were close to
           end-of-file, or because we are reading from a pipe, or from a
           terminal), or because **read**() was interrupted by a signal.  See
           also NOTES.
    
           On error, -1 is returned, and _errno_ is set to indicate the error.
           In this case, it is left unspecified whether the file position (if
           any) changes.
    


## ERRORS



           **EAGAIN** The file descriptor _fd_ refers to a file other than a socket
                  and has been marked nonblocking (**O_NONBLOCK**), and the read
                  would block.  See open(2) for further details on the
                  **O_NONBLOCK** flag.
    
           **EAGAIN** or **EWOULDBLOCK**
                  The file descriptor _fd_ refers to a socket and has been
                  marked nonblocking (**O_NONBLOCK**), and the read would block.
                  POSIX.1-2001 allows either error to be returned for this
                  case, and does not require these constants to have the same
                  value, so a portable application should check for both
                  possibilities.
    
           **EBADF** _fd_ is not a valid file descriptor or is not open for
                  reading.
    
           **EFAULT** _buf_ is outside your accessible address space.
    
           **EINTR** The call was interrupted by a signal before any data was
                  read; see signal(7).
    
           **EINVAL** _fd_ is attached to an object which is unsuitable for
                  reading; or the file was opened with the **O_DIRECT** flag, and
                  either the address specified in _buf_ , the value specified in
                  _count_ , or the file offset is not suitably aligned.
    
           **EINVAL** _fd_ was created via a call to timerfd_create(2) and the
                  wrong size buffer was given to **read**(); see
                  timerfd_create(2) for further information.
    
           **EIO** I/O error.  This will happen for example when the process
                  is in a background process group, tries to read from its
                  controlling terminal, and either it is ignoring or blocking
                  **SIGTTIN** or its process group is orphaned.  It may also
                  occur when there is a low-level I/O error while reading
                  from a disk or tape.  A further possible cause of **EIO** on
                  networked filesystems is when an advisory lock had been
                  taken out on the file descriptor and this lock has been
                  lost.  See the _Lost locks_ section of fcntl(2) for further
                  details.
    
           **EISDIR** _fd_ refers to a directory.
    
           Other errors may occur, depending on the object connected to _fd_.
    


## STANDARDS



           POSIX.1-2008.
    


## NOTES



           On Linux, **read**() (and similar system calls) will transfer at most
           0x7ffff000 (2,147,479,552) bytes, returning the number of bytes
           actually transferred.  (This is true on both 32-bit and 64-bit
           systems.)
    
           On NFS filesystems, reading small amounts of data will update the
           timestamp only the first time, subsequent calls may not do so.
           This is caused by client side attribute caching, because most if
           not all NFS clients leave _st_atime_ (last file access time) updates
           to the server, and client side reads satisfied from the client's
           cache will not cause _st_atime_ updates on the server as there are
           no server-side reads.  UNIX semantics can be obtained by disabling
           client-side attribute caching, but in most situations this will
           substantially increase server load and decrease performance.
    


## BUGS



           According to POSIX.1-2008/SUSv4 Section XSI 2.9.7 ("Thread
           Interactions with Regular File Operations"):
    
               All of the following functions shall be atomic with respect to
               each other in the effects specified in POSIX.1-2008 when they
               operate on regular files or symbolic links: ...
    
           Among the APIs subsequently listed are **read**() and readv(2).  And
           among the effects that should be atomic across threads (and
           processes) are updates of the file offset.  However, before Linux
           3.14, this was not the case: if two processes that share an open
           file description (see open(2)) perform a **read**() (or readv(2)) at
           the same time, then the I/O operations were not atomic with
           respect to updating the file offset, with the result that the
           reads in the two processes might (incorrectly) overlap in the
           blocks of data that they obtained.  This problem was fixed in
           Linux 3.14.
    


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