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续接上文：C++学习---cstdio的源码学习分析03-文件重命名函数rename
cstdio中的文件操作函数
stdio.h中定义了文件删除函数remove，文件重命名函数rename，创建临时文件函数tmpfile，生成临时文件名函数tmpnam。接下来我们一起来分析一下tmpfile对应的源码实现。
创建临时文件函数tmpfile
FILE * tmpfile ( void );
GLibc中还定义了tmpfile64，针对新的FILE结构定义，本文中以tmpfile函数进行分析。
183 /* Create a temporary file and open it read/write.
184
185 This function is a possible cancellation point and therefore not
186 marked with __THROW. */
187 #ifndef __USE_FILE_OFFSET64
188 extern FILE *tmpfile (void)
189 __attribute_malloc__ __attr_dealloc_fclose __wur;
190 #else
191 # ifdef __REDIRECT
192 extern FILE *__REDIRECT (tmpfile, (void), tmpfile64)
193 __attribute_malloc__ __attr_dealloc_fclose __wur;
194 # else
195 # define tmpfile tmpfile64
196 # endif
197 #endif
实现方式
源码如下，基本流程是通过tmpnam函数生成一个文件名，然后以"w+b"(二进制读写)打开，返回，下面进行具体流程分析：
//glibc/stdio-common/tmpfile.c
30 /* This returns a new stream opened on a temporary file (generated
31 by tmpnam). The file is opened with mode "w+b" (binary read/write).
32 If we couldn't generate a unique filename or the file couldn't
33 be opened, NULL is returned. */
34 FILE *
35 tmpfile (void)
36 {
37 int fd;
38 FILE *f;
39 int flags = 0;
40 #ifdef FLAGS
41 flags = FLAGS;
42 #endif
43
44 /* First try a system specific method. */
45 fd = __gen_tempfd (flags);
46
47 if (fd < 0)
48 {
49 char buf[FILENAME_MAX];
50
51 if (__path_search (buf, sizeof buf, NULL, "tmpf", 0))
52 return NULL;
53
54 fd = __gen_tempname (buf, 0, flags, __GT_FILE);
55 if (fd < 0)
56 return NULL;
57
58 /* Note that this relies on the Unix semantics that
59 a file is not really removed until it is closed. */
60 (void) __unlink (buf);
61 }
62
63 if ((f = __fdopen (fd, "w+b")) == NULL)
64 __close (fd);
65
66 return f;
67 }
__gen_tempfd获取临时fd
首先通过系统特定方法__gen_tempfd获取临时fd FLAGS定义，大文件size打开的标志，这里tmpfile和tmpfile64实际上是一种处理方式,所以__gen_tempfd的入参是0,除非是特定的架构中对O_LARGEFILE宏定义不同，这也提现了Glibc代码的兼容性，没有直接使用数字0，给后面的平台留足了兼容空间，同时的，代码也将会变得比较复杂难懂。
//glibc/bits/fcntl.h
52 /* All opens support large file sizes, so there is no flag bit for this. */
53 #ifdef __USE_LARGEFILE64
54 # define O_LARGEFILE 0
55 #endif
//glibc/stdio-common/tmpfile64.c
21 /* If there is no O_LARGEFILE, then the plain tmpfile definition
22 does the job and it gets tmpfile64 as an alias. */
23
24 #if defined O_LARGEFILE && O_LARGEFILE != 0
25 # define FLAGS O_LARGEFILE
26 # define tmpfile tmpfile64
27 # include <tmpfile.c>
28 #endif
__gen_tempfd的逻辑
这里我们就不细节展开__open的流程了，后续有专门的文章进行分析。关注打开的文件节点和flags信息。
先尝试打开P_tmpdir(默认是/tmp路径)，如果失败返回fd小于0，且错误码为ENOENT(/* No such file or directory */)，而且之前打开的P_tmpdir不是"/tmp"，那就尝试打开"/tmp"路径，这也是为了兼容其它平台可以提前定义P_tmpdir的情况。
flags信息为：读写+临时文件+如果文件存在返回失败
文件控制信息为：被当前用户读或写
//glibc/libio/stdio.h
118 #if defined __USE_MISC || defined __USE_XOPEN
119 /* Default path prefix for `tempnam' and `tmpnam'. */
120 # define P_tmpdir "/tmp"
121 #endif
//glibc/sysdeps/unix/sysv/linux/gentempfd.c
24 int
25 __gen_tempfd (int flags)
26 {
27 int fd = __open (P_tmpdir, O_RDWR | O_TMPFILE | O_EXCL | flags,
28 S_IRUSR | S_IWUSR);
29 if (fd < 0 && errno == ENOENT && strcmp (P_tmpdir, "/tmp") != 0)
30 fd = __open ("/tmp", O_RDWR | O_TMPFILE | O_EXCL | flags,
31 S_IRUSR | S_IWUSR);
32
33 return fd;
34 }
__open的函数原型 第一次参数是打开的文件名，第二个参数是open flag，当O_CREAT和O_TMPFILE在OFLAG中时，增加第三个参数表示文件的保护，即由谁进行读写。
//glibc/sysdeps/unix/sysv/linux/open.c
28 /* Open FILE with access OFLAG. If O_CREAT or O_TMPFILE is in OFLAG,
29 a third argument is the file protection. */
30 int
31 __libc_open (const char *file, int oflag, ...)
32 {
33 int mode = 0;
34
35 if (__OPEN_NEEDS_MODE (oflag))
36 {
37 va_list arg;
38 va_start (arg, oflag);
39 mode = va_arg (arg, int);
40 va_end (arg);
41 }
42
43 return SYSCALL_CANCEL (openat, AT_FDCWD, file, oflag, mode);
44 }
__path_search生成临时文件路径
__path_search (buf, sizeof buf, NULL, "tmpf", 0)
根据函数原型的解释，在dir(默认可以选择/tmp)中查找可用的目录，然后生成文件路径dir/file，拷贝到buf中，中间的逻辑其实就是为了生成字符串
sprintf (tmpl, "%.*s/%.*sXXXXXX", (int) dlen, dir, (int) plen, pfx);
其中dir可以从几个路径获取，有优先级和控制考虑，pfx优先使用传入的，否则判空之后使用'file'
//glibc/sysdeps/posix/tempname.c
105 /* Path search algorithm, for tmpnam, tmpfile, etc. If DIR is
106 non-null and exists, uses it; otherwise uses the first of $TMPDIR,
107 P_tmpdir, /tmp that exists. Copies into TMPL a template suitable
108 for use with mk[s]temp. Will fail (-1) if DIR is non-null and
109 doesn't exist, none of the searched dirs exists, or there's not
110 enough space in TMPL. */
111 int
112 __path_search (char *tmpl, size_t tmpl_len, const char *dir, const char *pfx,
113 int try_tmpdir)
114 {
115 const char *d;
116 size_t dlen, plen;
117
118 if (!pfx || !pfx[0])
119 {
120 pfx = "file";
121 plen = 4;
122 }
123 else
124 {
125 plen = strlen (pfx);
126 if (plen > 5)
127 plen = 5;
128 }
129
130 if (try_tmpdir)
131 {
132 d = __secure_getenv ("TMPDIR");
133 if (d != NULL && direxists (d))
134 dir = d;
135 else if (dir != NULL && direxists (dir))
136 /* nothing */ ;
137 else
138 dir = NULL;
139 }
140 if (dir == NULL)
141 {
142 if (direxists (P_tmpdir))
143 dir = P_tmpdir;
144 else if (strcmp (P_tmpdir, "/tmp") != 0 && direxists ("/tmp"))
145 dir = "/tmp";
146 else
147 {
148 __set_errno (ENOENT);
149 return -1;
150 }
151 }
152
153 dlen = strlen (dir);
154 while (dlen > 1 && dir[dlen - 1] == '/')
155 dlen--; /* remove trailing slashes */
156
157 /* check we have room for "${dir}/${pfx}XXXXXX\0" */
158 if (tmpl_len < dlen + 1 + plen + 6 + 1)
159 {
160 __set_errno (EINVAL);
161 return -1;
162 }
163
164 sprintf (tmpl, "%.*s/%.*sXXXXXX", (int) dlen, dir, (int) plen, pfx);
165 return 0;
166 }
__gen_tempname生成临时文件fd
fd = __gen_tempname (buf, 0, flags, __GT_FILE);
__GT_FILE表示新建文件，注意上面我们获取到的临时文件路径为"/tmp/tmpfXXXXXX"（关键就在于后面的6个X）
gen_tempname_len函数转发时，增加了一个参数6，这个参数在其函数定义中有解释，表示临时路径中至少有X_SUFFIX_LEN个"X"s，这个是方面后面生成临时数字的，因为临时文件可能不止一个，它们按照/tmp/tmpfxxxxxx，/tmp/tmpfxxxxxx...排序，x是多个数字的组合
后面调用try_tempname_len获取，传入对应创建的函数指针try_file，try_dir，try_nocreate<这里我们就不深入分析是如何生成文件名的了，
看到最后的try_file函数，实际上它与我们前面看到的__gen_tempfd一样，也是调用__open打开文件的。
141 /* The __kind argument to __gen_tempname may be one of: */
142 # define __GT_FILE 0 /* create a file */
143 # define __GT_DIR 1 /* create a directory */
144 # define __GT_NOCREATE 2 /* just find a name not currently in use */
//glibc/sysdeps/posix/tempname.c
332 int
333 __gen_tempname (char *tmpl, int suffixlen, int flags, int kind)
334 {
335 return gen_tempname_len (tmpl, suffixlen, flags, kind, 6);
336 }
203 /* Generate a temporary file name based on TMPL. TMPL must match the
204 rules for mk[s]temp (i.e., end in at least X_SUFFIX_LEN "X"s,
205 possibly with a suffix).
206 The name constructed does not exist at the time of the call to
207 this function. TMPL is overwritten with the result.
208
209 KIND may be one of:
210 __GT_NOCREATE: simply verify that the name does not exist
211 at the time of the call.
212 __GT_FILE: create the file using open(O_CREAT|O_EXCL)
213 and return a read-write fd. The file is mode 0600.
214 __GT_DIR: create a directory, which will be mode 0700.
215
216 We use a clever algorithm to get hard-to-predict names. */
217 #ifdef _LIBC
218 static
219 #endif
220 int
221 gen_tempname_len (char *tmpl, int suffixlen, int flags, int kind,
222 size_t x_suffix_len)
223 {
224 static int (*const tryfunc[]) (char *, void *) =
225 {
226 [__GT_FILE] = try_file,
227 [__GT_DIR] = try_dir,
228 [__GT_NOCREATE] = try_nocreate
229 };
230 return try_tempname_len (tmpl, suffixlen, &flags, tryfunc[kind],
231 x_suffix_len);
232 }
174 static int
175 try_file (char *tmpl, void *flags)
176 {
177 int *openflags = flags;
178 return __open (tmpl,
179 (*openflags & ~O_ACCMODE)
180 | O_RDWR | O_CREAT | O_EXCL, S_IRUSR | S_IWUSR);
181 }
__unlink删除上面生成的临时文件
注意这里是比较重要的点，为什么刚生成fd之后又要删除这个临时文件呢，注释有说明，依赖于unix的语义，一个文件只有在被关闭的时候才会真正被删除，所以这也是临时文件的含义，只有调用者主动关闭tmpfile之后，它就会被直接删除，而不是像普通文件那样保存下来。
__fdopen打开上面的临时文件fd，返回file对象
具体细节我们就不做具体分析了，通过对内部_IO_new_fdopen函数的操作打开对应fd的文件，返回文件流对象。
_IO_FILE_plus是对FILE file和const struct _IO_jump_t *vtable的封装，后者是为了C++streambuf做的兼容封装，保存了一些函数跳转表
#define __fdopen _IO_fdopen
//glibc/libio/iofdopen.c
163 libc_hidden_ver (_IO_new_fdopen, _IO_fdopen)
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;
...
151 /* For append mode, set the file offset to the end of the file if we added
152 O_APPEND to the file descriptor flags. Don't update the offset cache
153 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 }
161 return &new_f->fp.file;
162 }
//glibc/libio/libioP.h
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 can
322 be used to smash to a pointer to a virtual function table. */
323
324 struct _IO_FILE_plus
325 {
326 FILE file;
327 const struct _IO_jump_t *vtable;
328 };
293 struct _IO_jump_t
294 {
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 };
