unexpected output

**crash88** · 05-16-2006

I'm having some trouble with this. It seems basic enough but I keep getting a large integer in the range of 1000-2000 instead of what it looks to me like should be 1-10 or return a negative number.

in main I call it like this:

Code:

 case '5': if ((findfree(roomnum)) < 0)
                                                  printf("No available rooms\n");
                                          else
                                                  printf("%d is available\n", roomnum);
                                          break;

About every time I get a large value that's about 1500 or so

Code:

 #include "hotel.h"
#include <unistd.h>
#include <sys/types.h>
#include <stdlib.h>
#include <stdio.h>
#include <ctype.h>
#include <string.h>
#include <stdbool.h>

int fd;
char buf[NLINES];

int
findfree(int room)
{
    ssize_t nread;
    int i, j;
    _Bool vacant;

    if (lseek(fd, (off_t)0, SEEK_SET) < 0)
        perror("problem with lseek in findfree: line 23");

    for (i = 1; i <= MAXROOMS; ++i) {
        if ( (nread = read(fd, buf, sizeof(buf))) < 0)
            perror("problem with read in findfree function: line 21
        vacant = true;
        for (j = 0; j < nread; ++j) {
            if (isalpha(buf[j])) {
                vacant = false;
                break;
            }
        }
        if (vacant == true) {
            room = i;
            return 1;
        }
    }

    return -1;
}

It looks to me like if buf is empty (and it should be just spaces) vacant should remain true and have the value of i passed to it (i being a number between 1 and 10 since MAXROOMS s defined as 10.

**spydoor** · 05-16-2006

You need to pass roomnum as a pointer if you want findfree to change the value of it.

Code:

findfree(&roomnum);

int findfree(int *room)
{
   ...
   *room = i;
   ...
}

if roomnum is already a pointer in main then you don't need the ampersand, but you do need to dereference it in your printf

Example.

Code:

#include <stdio.h>

void change(int *x, int val)
{
    *x = val;
}

int main(void)
{
    int i = 0;
    int *p = &i;

    change(&i, 3);
    printf("%d\n", i);

    change(p, 6);
    printf("%d\n", i);

    return 0;
}

You don't need to use pointers at all. You could just return room
You can set room to -1 if no room available, before you return it.

Code:

if((roomnum = findfree()) < 0)

**crash88** · 05-16-2006

Thanks. I still get confused with pointers.

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