linux 文件操作write详解

以下，我是搬运工，来自man 2文件的write

WRITE(2)                                                                                Linux Programmer's Manual              

NAME
       write - write to a file descriptor

SYNOPSIS
       #include <unistd.h>

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

DESCRIPTION
       write() writes up to count bytes from the buffer starting at buf to the file referred to by the file descriptor fd.

       The  number  of bytes written may be less than count if, for example, there is insufficient space on the underlying physical medium, or the RLIMIT_FSIZE resource limit is encountered (see setr‐
       limit(2)), or the call was interrupted by a signal handler after having written less than count bytes.  (See also pipe(7).)

       For a seekable file (i.e., one to which lseek(2) may be applied, for example, a regular file) writing takes place at the file offset, and the file offset is incremented by the number  of  bytes
       actually  written.   If  the file was open(2)ed with O_APPEND, the file offset is first set to the end of the file before writing.  The adjustment of the file offset and the write operation are
       performed as an atomic step.

       POSIX requires that a read(2) that can be proved to occur after a write() has returned will return the new data.  Note that not all filesystems are POSIX conforming.

       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 written is returned (zero indicates nothing was written).  It is not an error if this number is smaller than the number of bytes requested; this may  happen  for
       example because the disk device was filled.  See also NOTES.

       On error, -1 is returned, and errno is set appropriately.

       If  count  is  zero  and fd refers to a regular file, then write() may return a failure status if one of the errors below is detected.  If no errors are detected, or error detection is not per‐
       formed, 0 will be returned without causing any other effect.  If count is zero and fd refers to a file other than a regular file, the results are not specified.

ERRORS
       EAGAIN The file descriptor fd refers to a file other than a socket and has been marked nonblocking (O_NONBLOCK), and the write 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 write 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 writing.

       EDESTADDRREQ
              fd refers to a datagram socket for which a peer address has not been set using connect(2).

       EDQUOT The user's quota of disk blocks on the filesystem containing the file referred to by fd has been exhausted.

       EFAULT buf is outside your accessible address space.

       EFBIG  An attempt was made to write a file that exceeds the implementation-defined maximum file size or the process's file size limit, or to write at a position past the maximum allowed offset.

       EINTR  The call was interrupted by a signal before any data was written; see signal(7).

       EINVAL fd is attached to an object which is unsuitable for writing; 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.

       EIO    A  low-level I/O error occurred while modifying the inode.  This error may relate to the write-back of data written by an earlier write(2), which may have been issued to a different file
              descriptor on the same file.  Since Linux 4.13, errors from write-back come with a promise that they may be reported by subsequent.  write(2) requests, and will be reported by  a  subse‐
              quent  fsync(2) (whether or not they were also reported by write(2)).  An alternate cause of EIO on networked filesystems is when an advisory lock had been taken out on the file descrip‐
              tor and this lock has been lost.  See the Lost locks section of fcntl(2) for further details.

       ENOSPC The device containing the file referred to by fd has no room for the data.

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

       EPIPE  fd is connected to a pipe or socket whose reading end is closed.  When this happens the writing process will also receive a SIGPIPE signal.  (Thus, the write return value is seen only if
              the program catches, blocks or ignores this signal.)

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

CONFORMING TO
       SVr4, 4.3BSD, POSIX.1-2001.

       Under SVr4 a write may be interrupted and return EINTR at any point, not just before any data is written.

NOTES
       The types size_t and ssize_t are, respectively, unsigned and signed integer data types specified by POSIX.1.

       A  successful  return  from write() does not make any guarantee that data has been committed to disk.  On some filesystems, including NFS, it does not even guarantee that space has successfully
       been reserved for the data.  In this case, some errors might be delayed until a future write(2), fsync(2), or even close(2).  The only way to be sure is to call  fsync(2)  after  you  are  done
       writing all your data.

       If  a  write() is interrupted by a signal handler before any bytes are written, then the call fails with the error EINTR; if it is interrupted after at least one byte has been written, the call
       succeeds, and returns the number of bytes written.

       On Linux, write() (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.)