Little C program to find the maximum value of select's timeout

posix_select_example.c

#include <fcntl.h>
#include <limits.h>
#include <stdint.h>
#include <stdio.h>
#include <stdlib.h>
#include <sys/fcntl.h>
#include <sys/select.h>

/* min and max integer values.  T is a signed or unsigned integer type. */

/* Returns 1 if T is signed, else 0. */
#define INTTYPE_SIGNED(T) ((T)-1 < (T)0)

/*
 * Returns (T)(maximum value of a T).
 *
 * Pains are taken (perhaps unnecessarily) to avoid integer overflow
 * with signed types.
 */
#define INTTYPE_MAX(T)                      \
    (((T)1 << (CHAR_BIT*sizeof(T)-INTTYPE_SIGNED(T)-1)) - 1 +   \
     ((T)1 << (CHAR_BIT*sizeof(T)-INTTYPE_SIGNED(T)-1)))

/*
 * Returns (T)(minimum value of a T).

 * Pains are taken (perhaps unnecessarily) to avoid integer overflow
 * with signed types.
 * assert: twos complement architecture
 */
#define INTTYPE_MIN(T) ((T)(-INTTYPE_MAX(T)-1))

/* Returns 1 if V has signed type, else 0. */
#define INT_VALUE_SIGNED(V) ((V)-(V)-1 < 0)

/*
 * Returns maximum value for V's type.
 *
 * Pains are taken (perhaps unnecessarily) to avoid integer overflow
 * with signed types.
 */
#define INT_VALUE_MAX(V)                        \
    (((V)-(V)+1 << (CHAR_BIT*sizeof(V)-INT_VALUE_SIGNED(V)-1)) - 1 +    \
     ((V)-(V)+1 << (CHAR_BIT*sizeof(V)-INT_VALUE_SIGNED(V)-1)))

/*
 * Returns minimum value for V's type.

 * Pains are taken (perhaps unnecessarily) to avoid integer overflow
 * with signed types.
 * assert: twos complement architecture
 */
#define INT_VALUE_MIN(V) (-INT_VALUE_MAX(V)-1)int: INT_MAX, unsigned int: UINT_MAX)

int
main(void)
{
    int             retval, devzero_fd;
    fd_set          rfds;
    struct timeval  tv;
    __darwin_time_t search_lower, search_upper;

    /* Read from /dev/zero, so there is always input */
    devzero_fd = open("/dev/zero", O_RDONLY);

    /* Decrement from the maximum value of tv.tv_sec to find the
     * maximum value accepted by select(); start the lower bounds
     * of the search at the max 32-bit integer, because it is a
     * known-working value. */
    // search_lower = INT_VALUE_MAX(tv.tv_sec) >> 31;
    search_lower = 0;
    search_upper = INT_VALUE_MAX(tv.tv_sec);
    tv.tv_usec = 0;

    while (search_lower <= search_upper) {
        tv.tv_sec = search_lower + ((search_upper - search_lower) / 2);

        printf("select(..., %ld)\n", tv.tv_sec);
        FD_ZERO(&rfds);
        FD_SET(devzero_fd, &rfds);

        retval = select(devzero_fd + 1, &rfds, NULL, NULL, &tv);
        if (retval > 0)
            search_lower = tv.tv_sec + 1;
        else if (retval < 0) {
            search_upper = tv.tv_sec - 1;
        }
    }

    if (retval == -1) {
        tv.tv_sec -= 1;
        if (!select(1, &rfds, NULL, NULL, &tv))
            perror("select()");
    }
    printf("\nMaximum value for tv.tv_sec=%ld\n", tv.tv_sec);

    exit(EXIT_SUCCESS);
}