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Date: Thu, 02 Sep 2004 04:52:00 -0000
From: Chakravarthy Nelluri <chakri_in5@yahoo.com>
Subject: Why fseeko always ends up calling _llseek?
To: glibc-bugs@sources.redhat.com
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Hi,

  Why fseeko always ends up calling _llseek on linux(
not sure how it behaves in other OS'es)?

Why can't it just return setting the file position
indicator for the stream.

I have a small test program.

#include <stdio.h>
#include <errno.h>

#define BUF_SIZE 1024*256 //256kb

int main(int argc, char *argv[])
{
    unsigned char buf[BUF_SIZE];
    unsigned char setbuf[BUF_SIZE];
    FILE *fp=NULL;

    if( 2!= argc) {
        printf("Invalid usage. Usage <fseektest>
<filename>.\n");
        exit(1);
    }

    fp = fopen(argv[1],"rb");
    if(NULL==fp) {
        printf("Error: filename: %s, error:
%s.\n",argv[1],strerror(errno));
        exit(1);
    }

    if( 0 != setvbuf(fp,setbuf,_IOFBF,BUF_SIZE)) {
        printf("Error setting buff, error:
%s.\n",strerror(errno));
        exit(2);
    }

    while( fread(buf,1,256,fp) ) {
        fseeko(fp,10L,SEEK_CUR);
        fseeko(fp,10L,SEEK_CUR);
        fseeko(fp,10L,SEEK_CUR);
        fseeko(fp,10L,SEEK_CUR);
        fseeko(fp,10L,SEEK_CUR);
        fseeko(fp,10L,SEEK_CUR);
        fseeko(fp,10L,SEEK_CUR);
        fseeko(fp,10L,SEEK_CUR);
        fseeko(fp,10L,SEEK_CUR);
        fseeko(fp,10L,SEEK_CUR);
        fseeko(fp,10L,SEEK_CUR);
        fseeko(fp,10L,SEEK_CUR);
        fseeko(fp,10L,SEEK_CUR);
        fseeko(fp,10L,SEEK_CUR);
        fseeko(fp,10L,SEEK_CUR);
    }

    exit(0);
}

In the above program the call to setvbuf makes sure
that 1024*256 bytes are read when fread is called for
the first time, with 256 bytes. I could see that it
always results in only read calls with 1024*256 bytes.

But when I do fseek, just after my first fread, it
results in a _llseek call. Why cannot the libc just
position itself in the already read 1024*256 bytes
buffer and return? Calling _llseek results in a very
high overhead, where there are large number of seeks.



=====
chakri




=====
chakri


		
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