V6/usr/sys/file.h

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including files from this version of Unix. 
/*
 * One file structure is allocated
 * for each open/creat/pipe call.
 * Main use is to hold the read/write
 * pointer associated with each open
 * file.
 */
struct	file
{
	char	f_flag;
	char	f_count;	/* reference count */
	int	f_inode;	/* pointer to inode structure */
	char	*f_offset[2];	/* read/write character pointer */
} file[NFILE];

/* flags */
#define	FREAD	01
#define	FWRITE	02
#define	FPIPE	04