usr/src/libI77/backspace.c

#include "fio.h"
f_back(a) alist *a;
{       unit *b;
        int n,i;
        long x;
        char buf[32];
        if(a->aunit >= MXUNIT || a->aunit < 0)
                err(a->aerr,101,"backspace")
        b= &units[a->aunit];
        if(b->useek==0) err(a->aerr,106,"backspace")
        if(b->ufd==NULL) err(a->aerr,114,"backspace")
        if(b->uend==1)
        {       b->uend=0;
                return(0);
        }
        if(b->uwrt)
        {       t_runc(b);
                nowreading(b);
        }
        if(b->url>0)
        {
                x=ftell(b->ufd);
                x /= b->url;
                x *= b->url;
                fseek(b->ufd,x,0);
                return(0);
        }
        if(b->ufmt==0)
        {       fseek(b->ufd,-(long)sizeof(int),1);
                fread((char *)&n,sizeof(int),1,b->ufd);
                fseek(b->ufd,-(long)n-2*sizeof(int),1);
                return(0);
        }
        for(;;)
        {
                x=ftell(b->ufd);
                if(x<sizeof(buf)) x=0;
                else x -= sizeof(buf);
                fseek(b->ufd,x,0);
                n=fread(buf,1,sizeof(buf),b->ufd);
                for(i=n-1;i>=0;i--)
                {
                        if(buf[i]!='\n') continue;
                        fseek(b->ufd,(long)(i-n),1);
                        return(0);
                }
                if(x==0) return(0);
                else if(n==0) err(a->aerr,(EOF),"backspace")
                else err(a->aerr,errno,"backspace");
        }
}