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fseek()                   STDIO Function                  fseek()




Seek on file stream

#include <stdio.h>
iinntt ffsseeeekk(_f_p, _w_h_e_r_e, _h_o_w)
FFIILLEE *_f_p; lloonngg _w_h_e_r_e; iinntt _h_o_w;

fseek changes  where the next read or  write operation will occur
within  the file  stream  fp.  It  handles any  effects the  seek
routine might  have had on  the internal buffering  strategies of
the system.  The arguments where and how specify the desired seek
position.  where indicates the new seek position in the file.  It
is measured from  the start of the file if  how equals zero, from
the current seek position if how  equals one, and from the end of
the file if how equals two.

fseek differs  from its cousin lseek in that  lseek is a COHERENT
system call  and takes  a file number,  whereas fseek is  a STDIO
function and takes a FILE pointer.

***** Example *****

This  example opens  file  argv[1] and  prints  its last  argv[2]
characters (default, 100).   It demonstrates the functions fseek,
ftell, and fclose.


#include <stdio.h>
extern long atol();



void fatal(message)
char *message;
{
        fprintf(stderr, "tail: %s\n", s);
        exit(1);
}



main(argc, argv)
int argc; char *argv[];
{
        register FILE *ifp;
        register int c;
        long nchars, size;



        if (argc < 2 || argc > 3)
                fatal("Usage: tail file [ nchars ]");
        nchars = (argc == 3) ? atol(argv[2]) : 100L;




COHERENT Lexicon                                           Page 1




fseek()                   STDIO Function                  fseek()





        if ((ifp = fopen(argv[1], "r")) == NULL)
                fatal("cannot open input file");
        /* Seek to end */
        if (fseek(ifp, 0L, 2) == -1)
                fatal("seek error");



        /* Find current size */
        size = ftell(ifp);
        size = (size < nchars) ? 0L : size - nchars;



        /* Seek to point */
        if (fseek(ifp, size, 0) == -1)
                fatal("seek error");
        while ((c = getc(ifp)) != EOF)
                /* Copy rest to stdout */
                putchar(c);
        if (fclose(ifp) == EOF)
                fatal("cannot close");
        exit(0);
}


***** See Also *****

ftell(), lseek(), STDIO

***** Diagnostics *****

For any diagnostic error, fseek returns -1; otherwise, it returns
zero.  If  fseek goes  beyond the  end of the  file, it  will not
return an error message  until the corresponding read or write is
performed.



















COHERENT Lexicon                                           Page 2

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