我简单的说明一下_flags的规则,_flags的高两位字节,这是由 libc 固定的,不同的 libc 或许不同,但是大体相同,这就像一个文件的头标示符一样,标志这是一个什么文件,正如注释所说High-order word is _IO_MAGIC; rest is flags.,而低两位字节的位数规则可以参考下面代码,不同位的功能,在注中已经标明。
if (n <= 0) return0; /* This is an optimized implementation. If the amount to be written straddles a block boundary (or the filebuf is unbuffered), use sys_write directly. */
/* First figure out how much space is available in the buffer. */ if ((f->_flags & _IO_LINE_BUF) && (f->_flags & _IO_CURRENTLY_PUTTING)) { count = f->_IO_buf_end - f->_IO_write_ptr; if (count >= n) { constchar *p; for (p = s + n; p > s; ) { if (*--p == '\n') { count = p - s + 1; must_flush = 1; break; } } } } elseif (f->_IO_write_end > f->_IO_write_ptr) count = f->_IO_write_end - f->_IO_write_ptr; /* Space available. */
/* Then fill the buffer. */ if (count > 0) { if (count > to_do) count = to_do; f->_IO_write_ptr = __mempcpy (f->_IO_write_ptr, s, count); s += count; to_do -= count; } if (to_do + must_flush > 0) { _IO_size_t block_size, do_write; /* Next flush the (full) buffer. */ if (_IO_OVERFLOW (f, EOF) == EOF) /* If nothing else has to be written we must not signal the caller that everything has been written. */ return to_do == 0 ? EOF : n - to_do;
/* Try to maintain alignment: write a whole number of blocks. */ 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; }
/* Now write out the remainder. Normally, this will fit in the buffer, but it's somewhat messier for line-buffered files, so we let _IO_default_xsputn handle the general case. */ if (to_do) to_do -= _IO_default_xsputn (f, s+do_write, to_do); } return n - to_do; } libc_hidden_ver (_IO_new_file_xsputn, _IO_file_xsputn)
总之我们的最终目的是到下面的代码中去。
/* Next flush the (full) buffer. */ if (_IO_OVERFLOW (f, EOF) == EOF)
前面先判断了一堆缓冲区还够不够输出之类的,之后调用到了_IO_OVERFLOW,也就是下面的代码。
代码来自:glibc-2.27/libio/fileops.c:744
int _IO_new_file_overflow (_IO_FILE *f, int ch) { if (f->_flags & _IO_NO_WRITES) /* SET ERROR */ { f->_flags |= _IO_ERR_SEEN; __set_errno (EBADF); return EOF; } /* If currently reading or no buffer allocated. */ if ((f->_flags & _IO_CURRENTLY_PUTTING) == 0 || f->_IO_write_base == NULL) { /* Allocate a buffer if needed. */ if (f->_IO_write_base == NULL) { _IO_doallocbuf (f); _IO_setg (f, f->_IO_buf_base, f->_IO_buf_base, f->_IO_buf_base); } /* Otherwise must be currently reading. If _IO_read_ptr (and hence also _IO_read_end) is at the buffer end, logically slide the buffer forwards one block (by setting the read pointers to all point at the beginning of the block). This makes room for subsequent output. Otherwise, set the read pointers to _IO_read_end (leaving that alone, so it can continue to correspond to the external position). */ 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; }