linux C语言open() read()函数

man 2 open

OPEN(2)                                                              Linux Programmer's Manual                                                             OPEN(2)

NAME
       open, openat, creat - open and possibly create a file	//打开并可能创建一个文件

SYNOPSIS
       #include <sys/types.h>
       #include <sys/stat.h>
       #include <fcntl.h>

       int open(const char *pathname, int flags);
       int open(const char *pathname, int flags, mode_t mode);

       int creat(const char *pathname, mode_t mode);

       int openat(int dirfd, const char *pathname, int flags);
       int openat(int dirfd, const char *pathname, int flags, mode_t mode);

   Feature Test Macro Requirements for glibc (see feature_test_macros(7)):

       openat():
           Since glibc 2.10:
               _XOPEN_SOURCE >= 700 || _POSIX_C_SOURCE >= 200809L
           Before glibc 2.10:
               _ATFILE_SOURCE

DESCRIPTION
       Given a pathname for a file, open() returns a file descriptor, a small, nonnegative integer for use in subsequent system calls (read(2), write(2), lseek(2), fcntl(2), etc.).  The file descriptor returned by a successful call will be the lowest-numbered file descriptor not currently open for the process.

       By default, the new file descriptor is set to remain open across an execve(2) (i.e., the FD_CLOEXEC file descriptor flag described in fcntl(2) is initially disabled); the O_CLOEXEC flag, described below, can be used to change this default.  The file offset is set to the beginning of the file (see lseek(2)).

       A call to open() creates a new open file description, an entry in the system-wide table of open files.  The open file description records the file offset and the file status flags (see below).  A file descriptor is a reference to an open file description; this reference is unaffected if pathname is subsequently removed or modified to refer to a different file.  For further details on open file descriptions, see NOTES.

       The argument flags must include one of the following access modes: O_RDONLY, O_WRONLY, or O_RDWR.  These request opening the file read-only, write-only, or read/write, respectively.

       In addition, zero or more file creation flags and file status flags can be bitwise-or'd in flags.  The file creation flags are O_CLOEXEC, O_CREAT, O_DIRECTORY, O_EXCL, O_NOCTTY, O_NOFOLLOW, O_TMPFILE, and O_TRUNC.  The file status flags are all of the remaining flags listed below.  The distinction between these two groups of flags is that the file status flags can be retrieved and (in some cases) modified; see fcntl(2) for details.

       The full list of file creation flags and file status flags is as follows:

       O_APPEND
              The file is opened in append mode.  Before each write(2), the file offset is positioned at the end of the file, as if with lseek(2).  O_APPEND may lead to corrupted files on NFS filesystems if more than one process appends data to a file at once.  This is because NFS does not support appending to a file, so the client kernel has to simulate it, which can't be done without a race condition.

       O_ASYNC
              Enable signal-driven I/O: generate a signal (SIGIO by default, but this can be changed via fcntl(2)) when input or output becomes possible on this file descriptor.  This feature is available only for terminals, pseudoterminals, sockets, and (since Linux 2.6) pipes and FIFOs.  See fcntl(2) for further details.  See also BUGS, below.

       O_CLOEXEC (since Linux 2.6.23)
              Enable the close-on-exec flag for the new file descriptor.  Specifying this flag permits a program to avoid additional fcntl(2) F_SETFD operations to set the FD_CLOEXEC flag.

              Note  that  the  use  of  this flag is essential in some multithreaded programs, because using a separate fcntl(2) F_SETFD operation to set the FD_CLOEXEC flag does not suffice to avoid race conditions where one thread opens a file descriptor and attempts to set its close-on-exec flag using fcntl(2) at the same time as another thread does a fork(2) plus execve(2).  Depending on the order of execution, the race may lead to the file
              descriptor returned by open() being unintentionally leaked to the program executed by the child process created by fork(2).  (This kind of race is in principle possible for any system call that creates a file descriptor whose close-on-exec flag should be set, and various other Linux system calls provide an equivalent of the O_CLOEXEC flag to deal with this problem.)

       O_CREAT
              If the file does not exist, it will be created.  The owner (user ID) of the file is set to the effective user ID of the process.  The group ownership (group ID) is set either to the effective group ID of the process or to the group ID of the parent directory (depending on filesystem type and mount options, and the mode of the parent directory; see the mount options bsdgroups and sysvgroups described in mount(8)).

              mode specifies the mode to use in case a new file is created.  This argument must be supplied when O_CREAT or O_TMPFILE is specified in flags; if neither O_CREAT nor O_TMPFILE is specified, then mode is ignored.  The effective mode is modified by the process's umask in the usual way: in the absence of a default ACL, the mode of the created file is (mode & ~umask).  Note that this mode applies only to future accesses of  the  newly
              created file; the open() call that creates a read-only file may well return a read/write file descriptor.

              The following symbolic constants are provided for mode:

              S_IRWXU  00700 user (file owner) has read, write, and execute permission

              S_IRUSR  00400 user has read permission

              S_IWUSR  00200 user has write permission

              S_IXUSR  00100 user has execute permission

              S_IRWXG  00070 group has read, write, and execute permission

              S_IRGRP  00040 group has read permission

              S_IWGRP  00020 group has write permission

              S_IXGRP  00010 group has execute permission

              S_IRWXO  00007 others have read, write, and execute permission

              S_IROTH  00004 others have read permission

              S_IWOTH  00002 others have write permission

              S_IXOTH  00001 others have execute permission

              According to POSIX, the effect when other bits are set in mode is unspecified.  On Linux, the following bits are also honored in mode:

              S_ISUID  0004000 set-user-ID bit

              S_ISGID  0002000 set-group-ID bit (see stat(2))

              S_ISVTX  0001000 sticky bit (see stat(2))

       O_DIRECT (since Linux 2.4.10)
              Try  to minimize cache effects of the I/O to and from this file.  In general this will degrade performance, but it is useful in special situations, such as when applications do their own caching.  File I/O is done directly to/from user-space buffers.  The O_DIRECT flag on its own makes an effort to transfer data synchronously, but does not give the guarantees of the O_SYNC flag that data and necessary metadata are transferred.  To
              guarantee synchronous I/O, O_SYNC must be used in addition to O_DIRECT.  See NOTES below for further discussion.

              A semantically similar (but deprecated) interface for block devices is described in raw(8).

       O_DIRECTORY
              If pathname is not a directory, cause the open to fail.  This flag was added in kernel version 2.1.126, to avoid denial-of-service problems if opendir(3) is called on a FIFO or tape device.

       O_DSYNC
              Write operations on the file will complete according to the requirements of synchronized I/O data integrity completion.

              By the time write(2) (and similar) return, the output data has been transferred to the underlying hardware, along with any file metadata that would be required to retrieve that data (i.e., as though each write(2) was followed by a call to fdatasync(2)).  See NOTES below.

       O_EXCL Ensure that this call creates the file: if this flag is specified in conjunction with O_CREAT, and pathname already exists, then open() will fail.

              When these two flags are specified, symbolic links are not followed: if pathname is a symbolic link, then open() fails regardless of where the symbolic link points to.

              In general, the behavior of O_EXCL is undefined if it is used without O_CREAT.  There is one exception: on Linux 2.6 and later, O_EXCL can be used without O_CREAT if pathname refers to a block device.  If the block device is in use by the system (e.g., mounted), open() fails with the error EBUSY.

              On NFS, O_EXCL is supported only when using NFSv3 or later on kernel 2.6 or later.  In NFS environments where O_EXCL support is not provided, programs that rely on it for performing locking tasks will contain a race condition.  Portable programs that want to perform atomic file locking using a lockfile, and need to avoid reliance on NFS support for O_EXCL, can create a unique file on the same filesystem (e.g., incorporating  host‐
              name and PID), and use link(2) to make a link to the lockfile.  If link(2) returns 0, the lock is successful.  Otherwise, use stat(2) on the unique file to check if its link count has increased to 2, in which case the lock is also successful.

       O_LARGEFILE
              (LFS) Allow files whose sizes cannot be represented in an off_t (but can be represented in an off64_t) to be opened.  The _LARGEFILE64_SOURCE macro must be defined (before including any header files) in order to obtain this definition.  Setting the _FILE_OFFSET_BITS feature test macro to 64 (rather than using O_LARGEFILE) is the preferred method of accessing large files on 32-bit systems (see feature_test_macros(7)).

       O_NOATIME (since Linux 2.6.8)
              Do not update the file last access time (st_atime in the inode) when the file is read(2).  This flag is intended for use by indexing or backup programs, where its use can significantly reduce the amount of disk activity.  This flag may not be effective on all filesystems.  One example is NFS, where the server maintains the access time.

       O_NOCTTY
              If pathname refers to a terminal device—see tty(4)—it will not become the process's controlling terminal even if the process does not have one.

       O_NOFOLLOW
              If pathname is a symbolic link, then the open fails.  This is a FreeBSD extension, which was added to Linux in version 2.1.126.  Symbolic links in earlier components of the pathname will still be followed.  See also O_PATH below.

       O_NONBLOCK or O_NDELAY
              When possible, the file is opened in nonblocking mode.  Neither the open() nor any subsequent operations on the file descriptor which is returned will cause the calling process to wait.

              Note that this flag has no effect for regular files and block devices; that is, I/O operations will (briefly) block when device activity is required, regardless of whether O_NONBLOCK is set.  Since O_NONBLOCK semantics might eventually be implemented, applications should not depend upon blocking behavior when specifying this flag for regular files and block devices.

              For the handling of FIFOs (named pipes), see also fifo(7).  For a discussion of the effect of O_NONBLOCK in conjunction with mandatory file locks and with file leases, see fcntl(2).

       O_PATH (since Linux 2.6.39)
              Obtain a file descriptor that can be used for two purposes: to indicate a location in the filesystem tree and to perform operations that act purely at the file descriptor level.  The file itself is not opened, and other file operations (e.g., read(2), write(2), fchmod(2), fchown(2), fgetxattr(2), mmap(2)) fail with the error EBADF.

              The following operations can be performed on the resulting file descriptor:

              *  close(2); fchdir(2) (since Linux 3.5); fstat(2) (since Linux 3.6).

              *  Duplicating the file descriptor (dup(2), fcntl(2) F_DUPFD, etc.).

              *  Getting and setting file descriptor flags (fcntl(2) F_GETFD and F_SETFD).

              *  Retrieving open file status flags using the fcntl(2) F_GETFL operation: the returned flags will include the bit O_PATH.

              *  Passing the file descriptor as the dirfd argument of openat(2) and the other "*at()" system calls.  This includes linkat(2) with AT_EMPTY_PATH (or via procfs using AT_SYMLINK_FOLLOW) even if the file is not a directory.

              *  Passing the file descriptor to another process via a UNIX domain socket (see SCM_RIGHTS in unix(7)).

              When O_PATH is specified in flags, flag bits other than O_CLOEXEC, O_DIRECTORY, and O_NOFOLLOW are ignored.

              If pathname is a symbolic link and the O_NOFOLLOW flag is also specified, then the call returns a file descriptor referring to the symbolic link.  This file descriptor can be used as the dirfd argument in calls to fchownat(2), fstatat(2), linkat(2), and readlinkat(2) with an empty pathname to have the calls operate on the symbolic link.

       O_SYNC Write operations on the file will complete according to the requirements of synchronized I/O file integrity completion (by contrast with the synchronized I/O data integrity completion provided by O_DSYNC.)

              By the time write(2) (and similar) return, the output data and associated file metadata have been transferred to the underlying hardware (i.e., as though each write(2) was followed by a call to fsync(2)).  See NOTES below.

       O_TMPFILE (since Linux 3.11)
              Create an unnamed temporary file.  The pathname argument specifies a directory; an unnamed inode will be created in that directory's filesystem.  Anything written to the resulting file will be lost when the last file descriptor is closed, unless the file is given a name.

              O_TMPFILE must be specified with one of O_RDWR or O_WRONLY and, optionally, O_EXCL.  If O_EXCL is not specified, then linkat(2) can be used to link the temporary file into the filesystem, making it permanent, using code like the following:

                  char path[PATH_MAX];
                  fd = open("/path/to/dir", O_TMPFILE | O_RDWR,
                                          S_IRUSR | S_IWUSR);

                  /* File I/O on 'fd'... */

                  snprintf(path, PATH_MAX,  "/proc/self/fd/%d", fd);
                  linkat(AT_FDCWD, path, AT_FDCWD, "/path/for/file",
                                          AT_SYMLINK_FOLLOW);

              In this case, the open() mode argument determines the file permission mode, as with O_CREAT.

              Specifying O_EXCL in conjunction with O_TMPFILE prevents a temporary file from being linked into the filesystem in the above manner.  (Note that the meaning of O_EXCL in this case is different from the meaning of O_EXCL otherwise.)

              There are two main use cases for O_TMPFILE:

              *  Improved tmpfile(3) functionality: race-free creation of temporary files that (1) are automatically deleted when closed; (2) can never be reached via any pathname; (3) are not subject to symlink attacks; and (4) do not require the caller to devise unique names.

              *  Creating a file that is initially invisible, which is then populated with data and adjusted to have appropriate filesystem attributes (chown(2), chmod(2), fsetxattr(2), etc.)  before being atomically linked into the filesystem in a fully formed state (using linkat(2) as described above).

              O_TMPFILE requires support by the underlying filesystem; only a subset of Linux filesystems provide that support.  In the initial implementation, support was provided in the ext2, ext3, ext4, UDF, Minix, and shmem filesystems.  XFS support was added in Linux 3.15.

       O_TRUNC
              If the file already exists and is a regular file and the access mode allows writing (i.e., is O_RDWR or O_WRONLY) it will be truncated to length 0.  If the file is a FIFO or terminal device file, the O_TRUNC flag is ignored.  Otherwise, the effect of O_TRUNC is unspecified.

   creat()
       creat() is equivalent to open() with flags equal to O_CREAT|O_WRONLY|O_TRUNC.

   openat()
       The openat() system call operates in exactly the same way as open(), except for the differences described here.

       If the pathname given in pathname is relative, then it is interpreted relative to the directory referred to by the file descriptor dirfd (rather than relative to the current working directory of the calling process, as is done by open() for a relative pathname).

       If pathname is relative and dirfd is the special value AT_FDCWD, then pathname is interpreted relative to the current working directory of the calling process (like open()).

       If pathname is absolute, then dirfd is ignored.

RETURN VALUE
       open(), openat(), and creat() return the new file descriptor, or -1 if an error occurred (in which case, errno is set appropriately).

ERRORS
       open(), openat(), and creat() can fail with the following errors:

       EACCES The requested access to the file is not allowed, or search permission is denied for one of the directories in the path prefix of pathname, or the file did not exist yet and write access to the parent directory is not allowed.  (See also path_resolution(7).)

       EDQUOT Where O_CREAT is specified, the file does not exist, and the user's quota of disk blocks or inodes on the filesystem has been exhausted.

       EEXIST pathname already exists and O_CREAT and O_EXCL were used.

       EFAULT pathname points outside your accessible address space.

       EFBIG  See EOVERFLOW.

       EINTR  While blocked waiting to complete an open of a slow device (e.g., a FIFO; see fifo(7)), the call was interrupted by a signal handler; see signal(7).

       EINVAL The filesystem does not support the O_DIRECT flag.  See NOTES for more information.

       EINVAL Invalid value in flags.

       EINVAL O_TMPFILE was specified in flags, but neither O_WRONLY nor O_RDWR was specified.

       EISDIR pathname refers to a directory and the access requested involved writing (that is, O_WRONLY or O_RDWR is set).

       EISDIR pathname refers to an existing directory, O_TMPFILE and one of O_WRONLY or O_RDWR were specified in flags, but this kernel version does not provide the O_TMPFILE functionality.

       ELOOP  Too many symbolic links were encountered in resolving pathname.

       ELOOP  pathname was a symbolic link, and flags specified O_NOFOLLOW but not O_PATH.

       EMFILE The per-process limit on the number of open file descriptors has been reached (see the description of RLIMIT_NOFILE in getrlimit(2)).

       ENAMETOOLONG
              pathname was too long.

       ENFILE The system-wide limit on the total number of open files has been reached.

       ENODEV pathname refers to a device special file and no corresponding device exists.  (This is a Linux kernel bug; in this situation ENXIO must be returned.)

       ENOENT O_CREAT is not set and the named file does not exist.  Or, a directory component in pathname does not exist or is a dangling symbolic link.

       ENOENT pathname refers to a nonexistent directory, O_TMPFILE and one of O_WRONLY or O_RDWR were specified in flags, but this kernel version does not provide the O_TMPFILE functionality.

       ENOMEM Insufficient kernel memory was available.

       ENOSPC pathname was to be created but the device containing pathname has no room for the new file.

       ENOTDIR
              A component used as a directory in pathname is not, in fact, a directory, or O_DIRECTORY was specified and pathname was not a directory.

       ENXIO  O_NONBLOCK | O_WRONLY is set, the named file is a FIFO, and no process has the FIFO open for reading.  Or, the file is a device special file and no corresponding device exists.

       EOPNOTSUPP
              The filesystem containing pathname does not support O_TMPFILE.

       EOVERFLOW
              pathname refers to a regular file that is too large to be opened.  The usual scenario here is that an application compiled on a 32-bit platform without -D_FILE_OFFSET_BITS=64 tried to open a file whose size exceeds (1<<31)-1 bytes; see also O_LARGEFILE above.  This is the error specified by POSIX.1; in kernels before 2.6.24, Linux gave the error EFBIG for this case.

       EPERM  The O_NOATIME flag was specified, but the effective user ID of the caller did not match the owner of the file and the caller was not privileged (CAP_FOWNER).

       EPERM  The operation was prevented by a file seal; see fcntl(2).

       EROFS  pathname refers to a file on a read-only filesystem and write access was requested.

       ETXTBSY
              pathname refers to an executable image which is currently being executed and write access was requested.

       EWOULDBLOCK
              The O_NONBLOCK flag was specified, and an incompatible lease was held on the file (see fcntl(2)).

       The following additional errors can occur for openat():

       EBADF  dirfd is not a valid file descriptor.

       ENOTDIR
              pathname is a relative pathname and dirfd is a file descriptor referring to a file other than a directory.

VERSIONS
       openat() was added to Linux in kernel 2.6.16; library support was added to glibc in version 2.4.

CONFORMING TO
       open(), creat() SVr4, 4.3BSD, POSIX.1-2001, POSIX.1-2008.

       openat(): POSIX.1-2008.

       The O_DIRECT, O_NOATIME, O_PATH, and O_TMPFILE flags are Linux-specific.  One must define _GNU_SOURCE to obtain their definitions.

       The O_CLOEXEC, O_DIRECTORY, and O_NOFOLLOW flags are not specified in POSIX.1-2001, but are specified in POSIX.1-2008.  Since glibc 2.12, one can obtain their definitions by defining either _POSIX_C_SOURCE with a value greater than or equal to 200809L or _XOPEN_SOURCE with a value greater than or equal to 700.  In glibc 2.11 and earlier, one obtains the definitions by defining _GNU_SOURCE.

       As noted in feature_test_macros(7), feature test macros such as _POSIX_C_SOURCE, _XOPEN_SOURCE, and _GNU_SOURCE must be defined before including any header files.

NOTES
       Under Linux, the O_NONBLOCK flag indicates that one wants to open but does not necessarily have the intention to read or write.  This is typically used to open devices in order to get a file descriptor for use with ioctl(2).

       The (undefined) effect of O_RDONLY | O_TRUNC varies among implementations.  On many systems the file is actually truncated.

       Note that open() can open device special files, but creat() cannot create them; use mknod(2) instead.

       If the file is newly created, its st_atime, st_ctime, st_mtime fields (respectively, time of last access, time of last status change, and time of last modification; see stat(2)) are set to the current time, and so are the st_ctime and st_mtime fields of the parent directory.  Otherwise, if the file is modified because of the O_TRUNC flag, its st_ctime and st_mtime fields are set to the current time.

   Open file descriptions
       The term open file description is the one used by POSIX to refer to the entries in the system-wide table of open files.  In other contexts, this object is variously also called an "open file object", a "file handle", an "open file table entry", or—in kernel-developer parlance—a struct file.

       When a file descriptor is duplicated (using dup(2) or similar), the duplicate refers to the same open file description as the original file descriptor, and the two file descriptors consequently share the file offset and file status flags.  Such sharing can also occur between processes: a child process created via fork(2) inherits duplicates of its parent's file descriptors, and those duplicates refer to the same open file descriptions.

       Each open(2) of a file creates a new open file description; thus, there may be multiple open file descriptions corresponding to a file inode.

   Synchronized I/O
       The POSIX.1-2008 "synchronized I/O" option specifies different variants of synchronized I/O, and specifies the open() flags O_SYNC, O_DSYNC, and O_RSYNC for controlling the behavior.  Regardless of whether an implementation supports this option, it must at least support the use of O_SYNC for regular files.

       Linux implements O_SYNC and O_DSYNC, but not O_RSYNC.  (Somewhat incorrectly, glibc defines O_RSYNC to have the same value as O_SYNC.)

       O_SYNC  provides  synchronized  I/O  file  integrity completion, meaning write operations will flush data and all associated metadata to the underlying hardware.  O_DSYNC provides synchronized I/O data integrity completion, meaning write operations will flush data to the underlying hardware, but will only flush metadata updates that are required to allow a subsequent read operation to complete successfully.  Data integrity completion can
       reduce the number of disk operations that are required for applications that don't need the guarantees of file integrity completion.

       To understand the difference between the two types of completion, consider two pieces of file metadata: the file last modification timestamp (st_mtime) and the file length.  All write operations will update the last file modification timestamp, but only writes that add data to the end of the file will change the file length.  The last modification timestamp is not needed to ensure that a read completes successfully, but the  file  length
       is.  Thus, O_DSYNC would only guarantee to flush updates to the file length metadata (whereas O_SYNC would also always flush the last modification timestamp metadata).

 Manual page open(2) line 221 (press h for help or q to quit)

man 2 read

READ(2)                                                                                Linux Programmer's Manual                                                                               READ(2)

NAME
       read - read from a file descriptor	//从文件描述符中读取

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.
       //read() 尝试将文件描述符 fd 中的 count 个字节读入缓冲区，从 buf 开始。

       On  files  that  support seeking, the read operation commences at the current file offset, and the file offset is incremented by the number of bytes read.  If the current file offset is at or
       past the end of file, no bytes are read, and read() returns zero.
       //在支持查找的文件上，读取操作从当前文件偏移量开始，文件偏移量增加读取的字节数。 
       //如果当前文件偏移位于或超过文件末尾，则不读取任何字节，并且 read() 返回零。

       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.
       //如果 count 为零，read() 可能会检测到下面描述的错误。 
       //在没有任何错误的情况下，或者如果 read() 不检查错误，则计数为 0 的 read() 返回零并且没有其他效果。

       If count is greater than SSIZE_MAX, the result is unspecified.
       //如果 count 大于 SSIZE_MAX，则未指定结果。

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.
       //成功时，返回读取的字节数（零表示文件结束），并且文件位置按此数前进。 
       //如果此数字小于请求的字节数，则不是错误； 
       //这可能会发生，例如因为现在实际可用的字节数较少
       //（可能是因为我们接近文件结尾，或者因为我们正在从管道或终端读取），或者因为 read() 被中断 信号。 另见注释。

       On error, -1 is returned, and errno is set appropriately.  In this case, it is left unspecified whether the file position (if any) changes.
       //出错时，返回 -1，并适当设置 errno。 
       //在这种情况下，未指定文件位置（如果有）是否更改。

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 current 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.

       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.

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,  on  Linux
       before  version  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 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.

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)

COLOPHON
       This  page  is  part  of  release  4.04  of  the Linux man-pages project.  A description of the project, information about reporting bugs, and the latest version of this page, can be found at
       http://www.kernel.org/doc/man-pages/.

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