标准I/O的操作都是围绕流（ stream ）来进行的。在标准I/O中，流用 FILE* 来描述。而每个被使用的文件都在内存中开辟一个区域，用来存放文件的有关信息，这些信息是保存在一个结构体类型的变量中的，该结构体类型是由系统定义的，取名为 FILE 。
因此， FILE 本质就是一个结构体，一个与被操作的文件所对应的结构体（该结构体描述了该文件）。而对于标准I/O来说，如果需要对文件尽心操作，只需要操作与该文件所对应的结构体指针即可完成。也就是 FILE* 。 FILE* 通常也被称为流指针。
FILE 结构体可在 <stdio.h> 中查看。
typedef struct _IO_FILE FILE; struct _IO_FILE { int _flags; /* High-order word is _IO_MAGIC; rest is flags. */ /* The following pointers correspond to the C++ streambuf protocol. */ 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; int _flags2; __off_t _old_offset; /* This used to be _offset but it's too small. */ /* 1+column number of pbase(); 0 is unknown. */ unsigned short _cur_column; signed char _vtable_offset; char _shortbuf[1]; _IO_lock_t *_lock; #ifdef _IO_USE_OLD_IO_FILE }; struct _IO_FILE_complete { struct _IO_FILE _file; #endif __off64_t _offset; /* Wide character stream stuff. */ struct _IO_codecvt *_codecvt; struct _IO_wide_data *_wide_data; struct _IO_FILE *_freeres_list; void *_freeres_buf; size_t __pad5; int _mode; /* Make sure we don't get into trouble again. */ char _unused2[15 * sizeof (int) - 4 * sizeof (void *) - sizeof (size_t)]; };点击复制复制失败已复制
其中， _IO_buf_base 与 _IO_buf_end 类型为 char* ，表示标准I/O操作的缓存区的起始以及结尾地址。
