fgetpos() -- STDIO Function (libc)

Get value of file-position indicator
#include <stdio.h>
int
fgetpos(fp, position)
FILE *fp; fpos_t *position;

fgetpos()  copies the  value of  the file-position  indicator for  the file
stream pointed to  by fp into the area pointed  to by position. position is
of type fpos_t, which is defined in the header stdio.h.

The function  fsetpos() can  use the  information written into  position to
return  the file-position  indicator  to where  it was  when fgetpos()  was
called.

fgetpos() returns zero if all went  well.  If an error occurred, it returns
nonzero and sets errno to an appropriate value.

Example

This example seeks to a random line in a very large file.

#include <math.h>
#include <stdarg.h>
#include <stddef.h>
#include <stdio.h>
#include <stdlib.h>
#include <time.h>

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

main(argc, argv)
int argc; char *argv[];
{
    int c;
    long count;
    FILE *ifp, *tmp;
    fpos_t loc;

    if (argc != 2)
        fatal("usage: fscanf inputfile\n");
    if ((ifp = fopen(argv[1], "r")) == NULL)
        fatal("Cannot open %s\n", argv[1]);
    if((tmp = tmpfile()) == NULL)
        fatal("Cannot build index file");

    /* seed random-number generator */
    srand ((unsigned int)time(NULL));

    for (count = 1;!feof(ifp); count++) {
        /* for monster files */
        if (fgetpos(ifp, &loc))
            fatal ("fgetpos error");

        if (fwrite(&loc, sizeof(loc), 1, tmp) != 1)
            fatal("Write fail on index");
        rand();
        while('\n' != (c = fgetc(ifp)) && EOF != c)
            ;
    }

    count = rand() % count;
    fseek(tmp, count * sizeof(loc), SEEK_SET);

    if(fread(&loc, sizeof(loc), 1, tmp) != 1)
        fatal("Read fail on index");

    fsetpos(ifp, &loc);
    while((c = fgetc(ifp)) != EOF) {
        if(c =='@')
            putchar('\n');
        else
            putchar(c);

        if(c == '\n')
            break;
    }
}

See Also

fseek(),
fsetpos(),
ftell(),
libc,
rewind()
ANSI Standard, §7.9.9.1

Notes

The ANSI  Standard introduced fgetpos() and fsetpos()  to manipulate a file
whose  file-position  indicator  cannot  be  stored  within a  long.  Under
COHERENT fgetpos() behaves the same as the function ftell().

© 2012 by Stephen A. Ness