【操蛋的“a+”】file access permission 文件写入权限

昨天七八节课写了两个小时的样子，零警告，零错误。算是把hash table给堆出来了

悲剧的事情在后面，回寝室debug绝对不止5个小时。。。。老是文件的写入fwrite有问题。问题简直就是诡异。。。。

不能理解。今天晚上把学长扯上，在港饮之都坐了一个半小时的样子。最后电脑没电报警，距离自动关机只有10分钟了。

first blood！kill the fucking bug。

下面是男人说的话。。。重点关注一下红色部分就可以了，不重要的部分也没全贴上来

FOPEN(3)                                                  Linux Programmer's Manual                                                 FOPEN(3)


NAME
       fopen, fdopen, freopen - stream open functions


SYNOPSIS
       #include <stdio.h>
       FILE *fopen(const char *path, const char *mode);
       FILE *fdopen(int fd, const char *mode);
       FILE *freopen(const char *path, const char *mode, FILE *stream);
   Feature Test Macro Requirements for glibc (see feature_test_macros(7)):

       fdopen(): _POSIX_C_SOURCE >= 1 || _XOPEN_SOURCE || _POSIX_SOURCE
DESCRIPTION
       The fopen() function opens the file whose name is the string pointed to by path and associates a stream with it.

       The  argument  mode  points to a string beginning with one of the following sequences (possibly followed by additional characters, as
       described below):

       r      Open text file for reading.  The stream is positioned at the beginning of the file.
       r+     Open for reading and writing.  The stream is positioned at the beginning of the file.
       w      Truncate file to zero length or create text file for writing.  The stream is positioned at the beginning of the file.
       w+     Open for reading and writing.  The file is created if it does not exist, otherwise it is truncated.  The stream is  positioned
              at the beginning of the file.
       a      Open  for  appending (writing at end of file).  The file is created if it does not exist.  The stream is positioned at the end
              of the file.
       a+     Open for reading and appending (writing at end of file).  The file is created if it does not exist.  The initial file position
              for reading is at the beginning of the file, but output is always appended to the end of the file.
       The  mode  string  can  also include the letter 'b' either as a last character or as a character between the characters in any of the

 two-character strings described above.  This is strictly for compatibility with C89 and has no effect; the  'b'  is  ignored  on  all

       POSIX  conforming systems, including Linux.  (Other systems may treat text files and binary files differently, and adding the 'b' may
       be a good idea if you do I/O to a binary file and expect that your program may be ported to non-UNIX environments.)

       See NOTES below for details of glibc extensions for mode.

       Any created files will have mode S_IRUSR | S_IWUSR | S_IRGRP | S_IWGRP | S_IROTH | S_IWOTH (0666), as modified by the process's umask
       value (see umask(2)).

       Reads  and  writes  may be intermixed on read/write streams in any order.  Note that ANSI C requires that a file positioning function
       intervene between output and input, unless an input operation encounters end-of-file.  (If this condition is not met, then a read  is
       allowed  to  return  the result of writes other than the most recent.)  Therefore it is good practice (and indeed sometimes necessary
       under Linux) to put an fseek(3) or fgetpos(3) operation between write and read operations on such a stream.  This operation may be an
       apparent no-op (as in fseek(..., 0L, SEEK_CUR) called for its synchronizing side effect.

       Opening  a  file  in append mode (a as the first character of mode) causes all subsequent write operations to this stream to occur at
       end-of-file, as if preceded the call:

           fseek(stream,0,SEEK_END);

       The fdopen() function associates a stream with the existing file descriptor, fd.  The mode of the stream  (one  of  the  values  "r",
       "r+",  "w",  "w+", "a", "a+") must be compatible with the mode of the file descriptor.  The file position indicator of the new stream
       is set to that belonging to fd, and the error and end-of-file indicators are cleared.  Modes "w" or "w+" do not cause  truncation  of
       the file.  The file descriptor is not dup'ed, and will be closed when the stream created by fdopen() is closed.  The result of apply‐
       ing fdopen() to a shared memory object is undefined.

       The freopen() function opens the file whose name is the string pointed to by path and associates the stream pointed to by stream with
       it.   The  original  stream (if it exists) is closed.  The mode argument is used just as in the fopen() function.  The primary use of
       the freopen() function is to change the file associated with a standard text stream (stderr, stdin, or stdout).

RETURN VALUE
       Upon successful completion fopen(), fdopen() and freopen() return a FILE pointer.  Otherwise, NULL is returned and errno  is  set  to
       indicate the error.

ERRORS
       EINVAL The mode provided to fopen(), fdopen(), or freopen() was invalid.

       The fopen(), fdopen() and freopen() functions may also fail and set errno for any of the errors specified for the routine malloc(3).

       The fopen() function may also fail and set errno for any of the errors specified for the routine open(2).
 Manual page fopen(3) line 43 (press h for help or q to quit)

好吧我承认一开始没认真看红色部分的manual。然后，无邪的以为w+能很聪明的帮我把offset调整到文件头，然后给予写入权限。天真了。

The file is created if it does not exist, otherwise it is truncated.

老大。。。你说文件不存在就创建一个，好，我事先touch了一个test.txt

然后，然后悲剧了，bug是每次写入完毕之后只有最后一个数据被保存下来。上帝啊，5个小时啊！

otherwise it is truncated、、、、、、、

之后试了a+

嗯，a+，all 嘛。我想怎么办就怎么办。最高权限模式。就像root一样，这样就不存在file access permission的问题了

还认真看了a+的说明。但是它也没说only for。。。好吧。刚才又看了一遍之前debug的时候时间太久了，头晕，没注意那个always。。。

时间长了，人累了注意力就不集中鸟。。。惯性思维完全占据我的大脑 T_T

but output is always appended to the end of the file. 其实我觉得这个写说明文档的人应该注意下，不是output always append to eof

而是data to be writed。。。。。。。。。。。

仅此纪念逝去的6.5个小时

jasonleaster

2014.03.07 于XTU.