在分析puts代码之前先看一些基本的知识:
一些flag:
#define _IO_USER_BUF 1
#define _IO_UNBUFFERED 2
#define _IO_NO_READS 4
#define _IO_NO_WRITES 8
#define _IO_EOF_SEEN 0x10
#define _IO_ERR_SEEN 0x20
#define _IO_DELETE_DONT_CLOSE 0x40
#define _IO_LINKED 0x80
#define _IO_IN_BACKUP 0x100
#define _IO_LINE_BUF 0x200
#define _IO_TIED_PUT_GET 0x400
#define _IO_CURRENTLY_PUTTING 0x800
#define _IO_IS_APPENDING 0x1000
#define _IO_IS_FILEBUF 0x2000
#define _IO_BAD_SEEN 0x4000
#define _IO_USER_LOCK 0x8000
_IO_FILE结构 :
struct _IO_FILE {
int _flags;
char* _IO_read_ptr;
char* _IO_read_end;
char* _IO_read_base;
char* _IO_write_base;
char* _IO_write_ptr;
char* _IO_write_end;
char* _IO_buf_base;
char* _IO_buf_end;
char *_IO_save_base;
char *_IO_backup_base;
char *_IO_save_end;
struct _IO_marker *_markers;
struct _IO_FILE *_chain;
int _fileno;
#if 0
int _blksize;
#else
int _flags2;
#endif
_IO_off_t _old_offset;
#define __HAVE_COLUMN
unsigned short _cur_column;
signed char _vtable_offset;
char _shortbuf[1];
_IO_lock_t *_lock;
#ifdef _IO_USE_OLD_IO_FILE
};
vtable:
#ifdef _LIBC
versioned_symbol (libc, _IO_new_do_write, _IO_do_write, GLIBC_2_1);
versioned_symbol (libc, _IO_new_file_attach, _IO_file_attach, GLIBC_2_1);
versioned_symbol (libc, _IO_new_file_close_it, _IO_file_close_it, GLIBC_2_1);
versioned_symbol (libc, _IO_new_file_finish, _IO_file_finish, GLIBC_2_1);
versioned_symbol (libc, _IO_new_file_fopen, _IO_file_fopen, GLIBC_2_1);
versioned_symbol (libc, _IO_new_file_init, _IO_file_init, GLIBC_2_1);
versioned_symbol (libc, _IO_new_file_setbuf, _IO_file_setbuf, GLIBC_2_1);
versioned_symbol (libc, _IO_new_file_sync, _IO_file_sync, GLIBC_2_1);
versioned_symbol (libc, _IO_new_file_overflow, _IO_file_overflow, GLIBC_2_1);
versioned_symbol (libc, _IO_new_file_seekoff, _IO_file_seekoff, GLIBC_2_1);
versioned_symbol (libc, _IO_new_file_underflow, _IO_file_underflow, GLIBC_2_1);
versioned_symbol (libc, _IO_new_file_write, _IO_file_write, GLIBC_2_1);
versioned_symbol (libc, _IO_new_file_xsputn, _IO_file_xsputn, GLIBC_2_1);
#endif
const struct _IO_jump_t _IO_file_jumps =
{
JUMP_INIT_DUMMY,
JUMP_INIT(finish, _IO_file_finish),
JUMP_INIT(overflow, _IO_file_overflow),
JUMP_INIT(underflow, _IO_file_underflow),
JUMP_INIT(uflow, _IO_default_uflow),
JUMP_INIT(pbackfail, _IO_default_pbackfail),
JUMP_INIT(xsputn, _IO_file_xsputn),
JUMP_INIT(xsgetn, _IO_file_xsgetn),
JUMP_INIT(seekoff, _IO_new_file_seekoff),
JUMP_INIT(seekpos, _IO_default_seekpos),
JUMP_INIT(setbuf, _IO_new_file_setbuf),
JUMP_INIT(sync, _IO_new_file_sync),
JUMP_INIT(doallocate, _IO_file_doallocate),
JUMP_INIT(read, _IO_file_read),
JUMP_INIT(write, _IO_new_file_write),
JUMP_INIT(seek, _IO_file_seek),
JUMP_INIT(close, _IO_file_close),
JUMP_INIT(stat, _IO_file_stat),
JUMP_INIT(showmanyc, _IO_default_showmanyc),
JUMP_INIT(imbue, _IO_default_imbue)
};
puts函数
接着puts函数进入了_IO_sputn,看一下它的定义
是调用了vtable里面的 xsputn函数,从上面的那个结构体初始化可以看出,就是_IO_file_xsputn函数,也就是_IO_new_file_xsputn函数
转到_IO_new_file_xsputn函数看一下:
_IO_size_t _IO_new_file_xsputn (_IO_FILE *f, const void *data, _IO_size_t n)
{
const char *s = (const char *) data;
_IO_size_t to_do = n;
int must_flush = 0;
_IO_size_t count = 0;
if (n <= 0)
return 0;
if ((f->_flags & _IO_LINE_BUF) && (f->_flags & _IO_CURRENTLY_PUTTING))
{
count = f->_IO_buf_end - f->_IO_write_ptr;
if (count >= n)
{
const char *p;
for (p = s + n; p > s; ){
if (*--p == '\n'){
count = p - s + 1;
must_flush = 1;
break;
}
}
}
}
else if (f->_IO_write_end > f->_IO_write_ptr)
count = f->_IO_write_end - f->_IO_write_ptr;
if (count > 0)
{
if (count > to_do)
count = to_do;
#ifdef _LIBC
f->_IO_write_ptr = __mempcpy (f->_IO_write_ptr, s, count);
#else
memcpy (f->_IO_write_ptr, s, count);
f->_IO_write_ptr += count;
#endif
s += count;
to_do -= count;
}
if (to_do + must_flush > 0)
{
_IO_size_t block_size, do_write;
if (_IO_OVERFLOW (f, EOF) == EOF)
return to_do == 0 ? EOF : n - to_do;
block_size = f->_IO_buf_end - f->_IO_buf_base;
do_write = to_do - (block_size >= 128 ? to_do % block_size : 0);
if (do_write)
{
count = new_do_write (f, s, do_write);
to_do -= count;
if (count < do_write)
return n - to_do;
}
if (to_do)
to_do -= _IO_default_xsputn (f, s+do_write, to_do);
}
return n - to_do;
}
代码逻辑比较简单,这里就不多说明了。
该函数里面调用了三个函数:
_IO_OVERFLOW (f, EOF) == EOF
new_do_write (f, s, do_write);
_IO_default_xsputn (f, s+do_write, to_do);
继续分析这三个函数:
_IO_OVERFLOW :
这个实际上是调用了_IO_file_overflow函数,代码里面的名称是_IO_new_file_overflow,
可以由函数名大概猜出,该函数的就是用于当输出缓冲区满了的时候,继续写会溢出,所以要flush输出缓冲区,同样还有_IO_file_underflow函数是read时候缓冲区内容不够,重新往缓冲区读入数据
int _IO_new_file_overflow (_IO_FILE *f, int ch)
{
if (f->_flags & _IO_NO_WRITES)
{
f->_flags |= _IO_ERR_SEEN;
__set_errno (EBADF);
return EOF;
}
if ((f->_flags & _IO_CURRENTLY_PUTTING) == 0 || f->_IO_write_base == NULL)
{
if (f->_IO_write_base == NULL)
{
_IO_doallocbuf (f);
_IO_setg (f, f->_IO_buf_base, f->_IO_buf_base, f->_IO_buf_base);
}
if (__glibc_unlikely (_IO_in_backup (f)))
{
size_t nbackup = f->_IO_read_end - f->_IO_read_ptr;
_IO_free_backup_area (f);
f->_IO_read_base -= MIN (nbackup,f->_IO_read_base - f->_IO_buf_base);
f->_IO_read_ptr = f->_IO_read_base;
}
if (f->_IO_read_ptr == f->_IO_buf_end)
f->_IO_read_end = f->_IO_read_ptr = f->_IO_buf_base;
f->_IO_write_ptr = f->_IO_read_ptr;
f->_IO_write_base = f->_IO_write_ptr;
f->_IO_write_end = f->_IO_buf_end;
f->_IO_read_base = f->_IO_read_ptr = f->_IO_read_end;
f->_flags |= _IO_CURRENTLY_PUTTING;
if (f->_mode <= 0 && f->_flags & (_IO_LINE_BUF | _IO_UNBUFFERED))
f->_IO_write_end = f->_IO_write_ptr;
}
if (ch == EOF)
return _IO_do_write (f, f->_IO_write_base,f->_IO_write_ptr - f->_IO_write_base);
if (f->_IO_write_ptr == f->_IO_buf_end )
if (_IO_do_flush (f) == EOF)
return EOF;
*f->_IO_write_ptr++ = ch;
if ((f->_flags & _IO_UNBUFFERED)
|| ((f->_flags & _IO_LINE_BUF) && ch == '\n'))
if (_IO_do_write (f, f->_IO_write_base,f->_IO_write_ptr - f->_IO_write_base) == EOF)
return EOF;
return (unsigned char) ch;
}
_IO_do_write代码:
int _IO_new_do_write (_IO_FILE *fp, const char *data, _IO_size_t to_do)
{
return (to_do == 0
|| (_IO_size_t) new_do_write (fp, data, to_do) == to_do) ? 0 : EOF;
}
static _IO_size_t new_do_write (_IO_FILE *fp, const char *data, _IO_size_t to_do)
{
_IO_size_t count;
if (fp->_flags & _IO_IS_APPENDING)
fp->_offset = _IO_pos_BAD;
else if (fp->_IO_read_end != fp->_IO_write_base)
{
_IO_off64_t new_pos = _IO_SYSSEEK (fp, fp->_IO_write_base - fp->_IO_read_end, 1);
if (new_pos == _IO_pos_BAD) return 0;
fp->_offset = new_pos;
}
count = _IO_SYSWRITE (fp, data, to_do);
if (fp->_cur_column && count)
fp->_cur_column = _IO_adjust_column (fp->_cur_column - 1, data, count) + 1;
_IO_setg (fp, fp->_IO_buf_base, fp->_IO_buf_base, fp->_IO_buf_base);
fp->_IO_write_base = fp->_IO_write_ptr = fp->_IO_buf_base;
fp->_IO_write_end = (fp->_mode <= 0
&& (fp->_flags & (_IO_LINE_BUF | _IO_UNBUFFERED))
? fp->_IO_buf_base : fp->_IO_buf_end);
return count;
}
_IO_default_xsputn 代码 (这个函数会把数据拷贝到缓冲区,然后在缓冲区满的时候flush缓冲区,直到把全部数据拷贝到缓冲区):
_IO_size_t _IO_default_xsputn (_IO_FILE *f, const void *data, _IO_size_t n)
{
const char *s = (char *) data;
_IO_size_t more = n;
if (more <= 0)
return 0;
for (;;)
{
if (f->_IO_write_ptr < f->_IO_write_end)
{
_IO_size_t count = f->_IO_write_end - f->_IO_write_ptr;
if (count > more)
count = more;
if (count > 20)
{
#ifdef _LIBC
f->_IO_write_ptr = __mempcpy (f->_IO_write_ptr, s, count);
#else
memcpy (f->_IO_write_ptr, s, count);
f->_IO_write_ptr += count;
#endif
s += count;
}
else if (count)
{
char *p = f->_IO_write_ptr;
_IO_ssize_t i;
for (i = count; --i >= 0; )
*p++ = *s++;
f->_IO_write_ptr = p;
}
more -= count;
}
if (more == 0 || _IO_OVERFLOW (f, (unsigned char) *s++) == EOF)
break;
more--;
}
return n - more;
}
emmmmmm貌似只是把代码抄了一次…突然发现C语言的文件操作有很多不明白的地方…还是去看一下的C语言的文件操作,之后应该就能看懂代码为什么要这样写了…
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