引言
_IO_new_fdopen 是 Glibc 中 fdopen 函数的内部实现，接受 fd 和打开 mode，返回文件流 FILE 指针。是 stdio.h 中比较重要的函数，我们来一起看看它的源码实现。
入参说明
接受两个参数，fd 是对应文件的 fd 号，mode 是对应的模式，包括如下的组合：
"r":read，文件必须存在；
"w":write，文件存在则从头开始覆盖写，不存在则创建新文件写入；
"a":append，文件存在则从末尾开始追加写入，不存在则创建新文件写入；
"r+":read/update,文件必须存在，可读可写；
"w+":write/update,文件存在则从头开始读写，不存在则创建新文件读写；
"a+":append/update,文件存在则从末尾开始追加读写，不存在则创建新文件读写。
//glibc/include/stdio.h185 extern FILE *_IO_new_fdopen (int, const char*);186 # define fdopen(fd, mode) _IO_new_fdopen (fd, mode)
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对于二进制文件读写则使用"b"，则组合之后有"rb", "wb", "ab", "r+b", "w+b", "a+b","rb+", "wb+", "ab+"。C++11 中增加了一个新的"x"与"w"组合使用，如果当前文件存在，则函数强行返回 fail,避免对已有文件的 overwriting，"wx", "wbx", "w+x" or "w+bx"/"wb+x"
1.常用变量定义
定义了 locked_FILE 指针 new_f，里面包含了_IO_FILE_plus，_IO_lock_t，_IO_wide_data,主要是_IO_FILE_plus 包含了主要的记录内容。do_seek 表明当前默认是不进行文件读写指针移动的，除非 mode 指定需要 append 模式。
//glibc/libio/iofdopen.c 33 FILE * 34 _IO_new_fdopen (int fd, const char *mode) 35 { 36 int read_write; 37 struct locked_FILE 38 { 39 struct _IO_FILE_plus fp; 40 #ifdef _IO_MTSAFE_IO 41 _IO_lock_t lock; 42 #endif 43 struct _IO_wide_data wd; 44 } *new_f; 45 int i; 46 int use_mmap = 0; 47 48 /* Decide whether we modify the offset of the file we attach to and seek to 49 the end of file. We only do this if the mode is 'a' and if the file 50 descriptor did not have O_APPEND in its flags already. */ 51 bool do_seek = false;
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_IO_FILE_plus
其中实际上保存了我们最后要返回的 FILE file，再加上针对 C++ streambuf 做的兼容虚函数表。
293 struct _IO_jump_t294 {295 JUMP_FIELD(size_t, __dummy);296 JUMP_FIELD(size_t, __dummy2);297 JUMP_FIELD(_IO_finish_t, __finish); 298 JUMP_FIELD(_IO_overflow_t, __overflow);299 JUMP_FIELD(_IO_underflow_t, __underflow);300 JUMP_FIELD(_IO_underflow_t, __uflow);301 JUMP_FIELD(_IO_pbackfail_t, __pbackfail);302 /* showmany */303 JUMP_FIELD(_IO_xsputn_t, __xsputn);304 JUMP_FIELD(_IO_xsgetn_t, __xsgetn);305 JUMP_FIELD(_IO_seekoff_t, __seekoff);306 JUMP_FIELD(_IO_seekpos_t, __seekpos);307 JUMP_FIELD(_IO_setbuf_t, __setbuf);308 JUMP_FIELD(_IO_sync_t, __sync);309 JUMP_FIELD(_IO_doallocate_t, __doallocate);310 JUMP_FIELD(_IO_read_t, __read);311 JUMP_FIELD(_IO_write_t, __write);312 JUMP_FIELD(_IO_seek_t, __seek);313 JUMP_FIELD(_IO_close_t, __close);314 JUMP_FIELD(_IO_stat_t, __stat);315 JUMP_FIELD(_IO_showmanyc_t, __showmanyc);316 JUMP_FIELD(_IO_imbue_t, __imbue);317 };318 319 /* We always allocate an extra word following an _IO_FILE.320 This contains a pointer to the function jump table used.321 This is for compatibility with C++ streambuf; the word can322 be used to smash to a pointer to a virtual function table. */323 324 struct _IO_FILE_plus325 {326 FILE file;327 const struct _IO_jump_t *vtable;328 }
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FILE 结构体里面包含的内容参考https://xie.infoq.cn/article/e7f8771d4f0ed770105041f52的说明
_IO_lock_t
参考https://xie.infoq.cn/article/53aced666a17f52c15b40d0c2
_IO_wide_data
宽字节数据的读写控制信息
120 /* Extra data for wide character streams. */121 struct _IO_wide_data122 {123 wchar_t *_IO_read_ptr; /* Current read pointer */124 wchar_t *_IO_read_end; /* End of get area. */125 wchar_t *_IO_read_base; /* Start of putback+get area. */126 wchar_t *_IO_write_base; /* Start of put area. */127 wchar_t *_IO_write_ptr; /* Current put pointer. */128 wchar_t *_IO_write_end; /* End of put area. */129 wchar_t *_IO_buf_base; /* Start of reserve area. */130 wchar_t *_IO_buf_end; /* End of reserve area. */131 /* The following fields are used to support backing up and undo. */132 wchar_t *_IO_save_base; /* Pointer to start of non-current get area. */133 wchar_t *_IO_backup_base; /* Pointer to first valid character of134 backup area */135 wchar_t *_IO_save_end; /* Pointer to end of non-current get area. */136 137 __mbstate_t _IO_state;138 __mbstate_t _IO_last_state;139 struct _IO_codecvt _codecvt;140 141 wchar_t _shortbuf[1];142 143 const struct _IO_jump_t *_wide_vtable; 144 };
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2.mode 信息解析
通过解析传入的 mode 字符串，对 int read_write 进行赋值，基本逻辑与上面提到的字符组合一致。首先查看第一个字符，只能是"r"/"w"/"a"中的一个，进行组合赋值 70 #define _IO_NO_READS 0x0004 /* Reading not allowed. /71 #define _IO_NO_WRITES 0x0008 / Writing not allowed. */80 #define _IO_IS_APPENDING 0x1000 然后检查第二个到第五个(最长的情况为"wb+x",加上最后的\0),注意这里只针对"+"和"m"做了特殊标记，分别为_IO_IS_APPENDING(注意这里是与，所以即使前面有 rwa 填充，+都只会保留为 0x1000);use_mmap 标记为 1。
53 switch (*mode) 54 { 55 case 'r': 56 read_write = _IO_NO_WRITES; 57 break; 58 case 'w': 59 read_write = _IO_NO_READS; 60 break; 61 case 'a': 62 read_write = _IO_NO_READS|_IO_IS_APPENDING; 63 break; 64 default: 65 __set_errno (EINVAL); 66 return NULL; 67 } 68 for (i = 1; i < 5; ++i) 69 { 70 switch (*++mode) 71 { 72 case '\0': 73 break; 74 case '+': 75 read_write &= _IO_IS_APPENDING; 76 break; 77 case 'm': 78 use_mmap = 1; 79 continue; 80 case 'x': 81 case 'b': 82 default: 83 /* Ignore */ 84 continue; 85 } 86 break; 87 }
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3.调用__fcntl 获取 FD 的状态信息
这里获取 FD 的状态信息（获取信息失败也返回 NULL）后进行检查，以下两种情况都视为 EINVAL，参数异常，返回 NULL。
88 int fd_flags = __fcntl (fd, F_GETFL); 89 if (fd_flags == -1) 90 return NULL; 91 92 if (((fd_flags & O_ACCMODE) == O_RDONLY && !(read_write & _IO_NO_WRITES)) 93 || ((fd_flags & O_ACCMODE) == O_WRONLY && !(read_write & _IO_NO_READS))) 94 { 95 __set_errno (EINVAL); 96 return NULL; 97 } 86 #define F_GETFL 3 /* Get file status flags. */ 24 /* File access modes for `open' and `fcntl'. */ 25 #define O_RDONLY 0 /* Open read-only. */ 26 #define O_WRONLY 1 /* Open write-only. */ 27 #define O_RDWR 2 /* Open read/write. */ 78 /* Mask for file access modes. This is system-dependent in case 79 some system ever wants to define some other flavor of access. */ 80 #define O_ACCMODE (O_RDONLY|O_WRONLY|O_RDWR)
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4.解析 append 信息并更新 fd 的 status flags
这里判断 read_write 是否有设定"a"，即追加模式，而且当前查询的文件 status flags 没有设置 append，那么将 do_seek(进行文件读写指针移动)置为 true，
同时调用__fcntl，F_SETFL 模式将文件的 status flags 设置为原有 flags 与 O_APPEND 的或。
115 if ((read_write & _IO_IS_APPENDING) && !(fd_flags & O_APPEND))116 {117 do_seek = true;118 if (__fcntl (fd, F_SETFL, fd_flags | O_APPEND) == -1)119 return NULL;120 }
87 #define F_SETFL 4 /* Set file status flags. */
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5.为 new_f 分配堆内存空间
分配大小为 sizeof (struct locked_FILE)，如果分配失败，则返回 NULL。
122 new_f = (struct locked_FILE *) malloc (sizeof (struct locked_FILE));123 if (new_f == NULL)124 return NULL;
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6._IO_no_init 初始化
使用 &new_f->lock 初始化 new_f->fp.file._lock，即_IO_FILE 中的_lock；
125 #ifdef _IO_MTSAFE_IO126 new_f->fp.file._lock = &new_f->lock;127 #endif128 _IO_no_init (&new_f->fp.file, 0, 0, &new_f->wd,129 #if _G_HAVE_MMAP130 (use_mmap && (read_write & _IO_NO_WRITES))131 ? &_IO_wfile_jumps_maybe_mmap :132 #endif133 &_IO_wfile_jumps);
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调用_IO_no_init 初始化 &new_f->fp.file 指针，其中 flags 和 orientation 置 0,其余参数基本都是置空，注意，这里还调用了_IO_old_init 对一些基础参数做了置空操作，同时，值得关注的一点：_IO_lock_init 初始化了*fp->_lock，便于后续的使用。
其中_IO_jump_t 根据_G_HAVE_MMAP 宏和当前模式中有"m"且"r"，则使用_IO_wfile_jumps_maybe_mmap，否则使用_IO_wfile_jumps，两者区别就在于 mmap 和 munmap 在读写文件时的使用，这里就不细展开了。
//glibc/libio/genops.c 560 void 561 _IO_no_init (FILE *fp, int flags, int orientation, 562 struct _IO_wide_data *wd, const struct _IO_jump_t *jmp) 563 { 564 _IO_old_init (fp, flags); 565 fp->_mode = orientation; 566 if (orientation >= 0) 567 { 568 fp->_wide_data = wd; 569 fp->_wide_data->_IO_buf_base = NULL; 570 fp->_wide_data->_IO_buf_end = NULL; 571 fp->_wide_data->_IO_read_base = NULL; 572 fp->_wide_data->_IO_read_ptr = NULL; 573 fp->_wide_data->_IO_read_end = NULL; 574 fp->_wide_data->_IO_write_base = NULL; 575 fp->_wide_data->_IO_write_ptr = NULL; 576 fp->_wide_data->_IO_write_end = NULL; 577 fp->_wide_data->_IO_save_base = NULL; 578 fp->_wide_data->_IO_backup_base = NULL; 579 fp->_wide_data->_IO_save_end = NULL; 580 581 fp->_wide_data->_wide_vtable = jmp; 582 } 583 else 584 /* Cause predictable crash when a wide function is called on a byte 585 stream. */ 586 fp->_wide_data = (struct _IO_wide_data *) -1L; 587 fp->_freeres_list = NULL; 588 } 529 void 530 _IO_old_init (FILE *fp, int flags) 531 { 532 fp->_flags = _IO_MAGIC|flags; 533 fp->_flags2 = 0; 534 if (stdio_needs_locking) 535 fp->_flags2 |= _IO_FLAGS2_NEED_LOCK; 536 fp->_IO_buf_base = NULL; 537 fp->_IO_buf_end = NULL; 538 fp->_IO_read_base = NULL; 539 fp->_IO_read_ptr = NULL; 540 fp->_IO_read_end = NULL; 541 fp->_IO_write_base = NULL; 542 fp->_IO_write_ptr = NULL; 543 fp->_IO_write_end = NULL; 544 fp->_chain = NULL; /* Not necessary. */ 545 546 fp->_IO_save_base = NULL; 547 fp->_IO_backup_base = NULL; 548 fp->_IO_save_end = NULL; 549 fp->_markers = NULL; 550 fp->_cur_column = 0; 551 #if _IO_JUMPS_OFFSET 552 fp->_vtable_offset = 0; 553 #endif 554 #ifdef _IO_MTSAFE_IO 555 if (fp->_lock != NULL) 556 _IO_lock_init (*fp->_lock); 557 #endif 558 }
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7._IO_JUMPS 初始化 &new_f->fp
宏展开后，可以看到，实际上是初始化 &new_f->fp->vtable，即前文提到的兼容 C++ streambuf 的虚函数表，根据当前模式中有"m"且"r"置为_IO_file_jumps_maybe_mmap，否则为_IO_file_jumps。
98 #define _IO_JUMPS(THIS) (THIS)->vtable 134 _IO_JUMPS (&new_f->fp) =135 #if _G_HAVE_MMAP136 (use_mmap && (read_write & _IO_NO_WRITES)) ? &_IO_file_jumps_maybe_mmap :137 #endif138 &_IO_file_jumps;
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8._IO_new_file_init_internal 初始化
将_offset 初始化为-1;_flags 或上（_IO_IS_FILEBUF+_IO_NO_READS+_IO_NO_WRITES+_IO_TIED_PUT_GET）即，文件 buffer+只写+只读+put/get 指针移动一致指向同一个位置；调用_IO_link_in 链接 fp;_fileno 初始化为-1；
139 _IO_new_file_init_internal (&new_f->fp);
//glibc/libio/fileops.c 104 void 105 _IO_new_file_init_internal (struct _IO_FILE_plus *fp) 106 { 107 /* POSIX.1 allows another file handle to be used to change the position 108 of our file descriptor. Hence we actually don't know the actual 109 position before we do the first fseek (and until a following fflush). */ 110 fp->file._offset = _IO_pos_BAD; 111 fp->file._flags |= CLOSED_FILEBUF_FLAGS; 112 113 _IO_link_in (fp); 114 fp->file._fileno = -1; 115 } 93 /* _IO_pos_BAD is an off64_t value indicating error, unknown, or EOF. */ 94 #define _IO_pos_BAD ((off64_t) -1) 100 #define CLOSED_FILEBUF_FLAGS \ 101 (_IO_IS_FILEBUF+_IO_NO_READS+_IO_NO_WRITES+_IO_TIED_PUT_GET) 78 #define _IO_TIED_PUT_GET 0x0400 /* Put and get pointer move in unison. */
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_IO_link_in (fp)
这里实际上就是链接文件的过程,_IO_lock_lock 加锁解锁都是配对的，这里主要是要将_chain 赋值为_IO_list_all，然后将_IO_list_all 赋值为当前的 fp，代表当前要打开的文件链接到_IO_list_all 上了。
35 #ifdef _IO_MTSAFE_IO 36 static _IO_lock_t list_all_lock = _IO_lock_initializer; 37 #endif 85 void 86 _IO_link_in (struct _IO_FILE_plus *fp) 87 { 88 if ((fp->file._flags & _IO_LINKED) == 0) 89 { 90 fp->file._flags |= _IO_LINKED; 91 #ifdef _IO_MTSAFE_IO 92 _IO_cleanup_region_start_noarg (flush_cleanup); 93 _IO_lock_lock (list_all_lock); 94 run_fp = (FILE *) fp; 95 _IO_flockfile ((FILE *) fp); 96 #endif 97 fp->file._chain = (FILE *) _IO_list_all; 98 _IO_list_all = fp; 99 #ifdef _IO_MTSAFE_IO 100 _IO_funlockfile ((FILE *) fp); 101 run_fp = NULL; 102 _IO_lock_unlock (list_all_lock); 103 _IO_cleanup_region_end (0); 104 #endif 105 } 106 }
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9._IO_mask_flags 设置 mask
将在_IO_new_file_init_internal 中置为-1 的_fileno 置为 fd 自身；
_flags 先与~_IO_DELETE_DONT_CLOSE,表示后续可调用关闭 close,再清空_flags 后置为 read_write 的值。
140 /* We only need to record the fd because _IO_file_init_internal will141 have unset the offset. It is important to unset the cached142 offset because the real offset in the file could change between143 now and when the handle is activated and we would then mislead144 ftell into believing that we have a valid offset. */145 new_f->fp.file._fileno = fd;146 new_f->fp.file._flags &= ~_IO_DELETE_DONT_CLOSE;147 148 _IO_mask_flags (&new_f->fp.file, read_write,149 _IO_NO_READS+_IO_NO_WRITES+_IO_IS_APPENDING);
74 #define _IO_DELETE_DONT_CLOSE 0x0040 /* Don't call close(_fileno) on close. */ 518 #define _IO_mask_flags(fp, f, mask) \ 519 ((fp)->_flags = ((fp)->_flags & ~(mask)) | ((f) & (mask)))
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10.针对 append 模式特殊处理
如果是 append 模式，那么我们要将对应的读写位置移动到文件末尾。
151 /* For append mode, set the file offset to the end of the file if we added152 O_APPEND to the file descriptor flags. Don't update the offset cache153 though, since the file handle is not active. */154 if (do_seek && ((read_write & (_IO_IS_APPENDING | _IO_NO_READS))155 == (_IO_IS_APPENDING | _IO_NO_READS)))156 {157 off64_t new_pos = _IO_SYSSEEK (&new_f->fp.file, 0, _IO_seek_end);158 if (new_pos == _IO_pos_BAD && errno != ESPIPE)159 return NULL;160 }
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_IO_SYSSEEK
_IO_SYSSEEK (&new_f->fp.file, 0, _IO_seek_end)即相对当前文件末尾，跳动 0,即移动到末尾。
257 #define _IO_SYSSEEK(FP, OFFSET, MODE) JUMP2 (__seek, FP, OFFSET, MODE)125 #define JUMP2(FUNC, THIS, X1, X2) (_IO_JUMPS_FUNC(THIS)->FUNC) (THIS, X1, X2)
252 /* The 'sysseek' hook is used to re-position an external file.253 It generalizes the Unix lseek(2) function.254 It matches the streambuf::sys_seek virtual function, which is255 specific to this implementation. */256 typedef off64_t (*_IO_seek_t) (FILE *, off64_t, int); 57 #define _IO_seek_end 2
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11.最后返回文件对象
161 return &new_f->fp.file;
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