/* We always allocate an extra word following an _IO_FILE. This contains a pointer to the function jump table used. This is for compatibility with C++ streambuf; the word can be used to smash to a pointer to a virtual function table. */
struct _IO_FILE { int _flags; /* High-order word is _IO_MAGIC; rest is flags. */ #define _IO_file_flags _flags
/* The following pointers correspond to the C++ streambuf protocol. */ /* Note: Tk uses the _IO_read_ptr and _IO_read_end fields directly. */ char* _IO_read_ptr; /* Current read pointer */ char* _IO_read_end; /* End of get area. */ char* _IO_read_base; /* Start of putback+get area. */ char* _IO_write_base; /* Start of put area. */ char* _IO_write_ptr; /* Current put pointer. */ char* _IO_write_end; /* End of put area. */ char* _IO_buf_base; /* Start of reserve area. */ char* _IO_buf_end; /* End of reserve area. */ /* The following fields are used to support backing up and undo. */ char *_IO_save_base; /* Pointer to start of non-current get area. */ char *_IO_backup_base; /* Pointer to first valid character of backup area */ char *_IO_save_end; /* Pointer to end of non-current get area. */
struct _IO_marker *_markers;
struct _IO_FILE *_chain;
int _fileno; #if 0 int _blksize; #else int _flags2; #endif _IO_off_t _old_offset; /* This used to be _offset but it's too small. */
#define __HAVE_COLUMN /* temporary */ /* 1+column number of pbase(); 0 is unknown. */ unsignedshort _cur_column; signedchar _vtable_offset; char _shortbuf[1];
#define _IO_MAGIC 0xFBAD0000 /* Magic number */ #define _OLD_STDIO_MAGIC 0xFABC0000 /* Emulate old stdio. */ #define _IO_MAGIC_MASK 0xFFFF0000 #define _IO_USER_BUF 1 /* User owns buffer; don't delete it on close. */ #define _IO_UNBUFFERED 2 #define _IO_NO_READS 4 /* Reading not allowed */ #define _IO_NO_WRITES 8 /* Writing not allowd */ #define _IO_EOF_SEEN 0x10 #define _IO_ERR_SEEN 0x20 #define _IO_DELETE_DONT_CLOSE 0x40 /* Don't call close(_fileno) on cleanup. */ #define _IO_LINKED 0x80 /* Set if linked (using _chain) to streambuf::_list_all.*/ #define _IO_IN_BACKUP 0x100 #define _IO_LINE_BUF 0x200 #define _IO_TIED_PUT_GET 0x400 /* Set if put and get pointer logicly tied. */ #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
要判断flag是什么状态就直接与这些常量按位与运算。
puts函数执行流程
puts函数会调用_IO_puts函数，这个函数才完成puts函数的功能，下面是源码
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#include"libioP.h" #include<string.h> #include<limits.h> int _IO_puts (constchar *str) { int result = EOF; _IO_size_t len = strlen (str); _IO_acquire_lock (_IO_stdout);
if ((_IO_vtable_offset (_IO_stdout) != 0 || _IO_fwide (_IO_stdout, -1) == -1) && _IO_sputn (_IO_stdout, str, len) == len && _IO_putc_unlocked ('\n', _IO_stdout) != EOF) result = MIN (INT_MAX, len + 1);
_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; }
