man read – Man page for read

June 1, 2007 – 1:22 am

READ


Section: Linux Programmer’s Manual (2)
Updated: 1997–07–12
Index
Return to Main Contents

 

NAME

read – read from a file descriptor
 

SYNOPSIS


#include <unistd.h>

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

 

DESCRIPTION

read()

attempts to read up to
count

bytes from file descriptor
fd

into the buffer starting at
buf.

If
count

is zero, read() returns zero and has no other results.
If
count

is greater than SSIZE_MAX, the result is unspecified.

 

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.
On error, –1 is returned, and
errno

is set appropriately. In this case it is left unspecified whether
the file position (if any) changes.
 

ERRORS


EAGAIN


Non–blocking I/O has been selected using
O_NONBLOCK

and no data was immediately available for reading.

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.
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 current file offset is not suitably aligned.

EIO


I/O error. This will happen for example when the process is in a
background process group, tries to read from its controlling tty,
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.
EISDIR


fd

refers to a directory.


Other errors may occur, depending on the object connected to
fd.

POSIX allows a
read()

that is interrupted after reading some data
to return –1 (with
errno

set to EINTR) or to return the number of bytes already read.
 

CONFORMING TO

SVr4, 4.3BSD, POSIX.1–2001.
 

RESTRICTIONS

On NFS file systems, reading small amounts of data will only update the
time stamp 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.

Many filesystems and disks were considered to be fast enough that the
implementation of
O_NONBLOCK

was deemed unnecessary. So, O_NONBLOCK may not be available on files
and/or disks.
 

SEE ALSO

close(2),

fcntl(2),

ioctl(2),

lseek(2),

open(2),

pread(2),

readdir(2),

readlink(2),

readv(2),

select(2),

write(2),

fread(3)



 

Index



NAME

SYNOPSIS

DESCRIPTION

RETURN VALUE

ERRORS

CONFORMING TO

RESTRICTIONS

SEE ALSO



Related posts:

  1. man sendfile – Man page for sendfile
  2. man stapprobes.nfs – Man page for stapprobes.nfs
  3. man getmsg – Man page for getmsg
  4. man spufs – Man page for spufs
  5. man XStoreBytes – Man page for XStoreBytes
  6. man sendto – Man page for sendto
  7. man sendmsg – Man page for sendmsg
  8. man XInternAtom – Man page for XInternAtom
  9. man Net::HTTP – Man page for Net::HTTP
  10. man fseek – Man page for fseek

Post a Comment