Get the maximum value of a variable in C

Question

Is there a function in C that returns the maximum value of a variable like this (I will name the function "maxvalue" in example below)?

int a;
printf("%d", maxvalue(a)); // 32767
unsigned int b;
printf("%d", maxvalue(b)); // 65535

So basically the function returns values like INT_MAX when the variable is signed INT, UINT_MAX when unsigned int, etc.

Answer 1

Such a function is not defined by the C standard library. You can try defining a macro that calculates it:

#define MAX_VALUE(a) (((unsigned long long)1 << (sizeof(a) * CHAR_BIT)) - 1)

When using it, be careful it is assigned to a type large enough. For example:

#include <stdlib.h>
#include <stdio.h>
#include <limits.h>

#define IS_TYPE_SIGNED(a) ((a-1) < 0)
#define MAX_VALUE_UNSIGNED(a) (((unsigned long long)1 << \
        (sizeof(a) * CHAR_BIT)) - 1)
#define MAX_VALUE_SIGNED(a) (MAX_VALUE_UNSIGNED(a) >> 1)
#define MAX_VALUE(a) (IS_TYPE_SIGNED(a) ? \
        MAX_VALUE_SIGNED(a) : MAX_VALUE_UNSIGNED(a))

int main(void)
{
    unsigned int i = 0;
    signed int j = 0;

    printf("%llu\n", MAX_VALUE(i));
    printf("%llu\n", MAX_VALUE(j));
    return EXIT_SUCCESS;
}

This prints out:

4294967295
2147483647

Answer 2

You can do it quite easily with a C11 type-generic expression:

#define maxvalue(type) _Generic(type, int: INT_MAX, \
                                      unsigned int: UINT_MAX)

It's not a function, but I think it does what you want. Here's a simple example program:

#include <stdio.h>
#include <limits.h>

#define maxvalue(type) _Generic(type, int: INT_MAX, \
                                      unsigned int: UINT_MAX)

int main(void)
{
    int i;
    unsigned int ui;

    printf("%u\n", maxvalue(i));
    printf("%u\n", maxvalue(ui));

    return 0;
}

And its output:

$ clang -Wall -Werror -Wextra -pedantic -std=c11 example.c -o example
$ ./example 
2147483647
4294967295

My answers are larger than yours because my system has 32-bit integers. You appear to have a 16-bit machine.

Answer 3

Here are macros from my library that work for types, rather than variables:

/* 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))

Edit: Adapting these to the question:

/* min and max integer values.  V is a signed or unsigned integer value. */

/* 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)

Afterthought: These invoke UB if V is a variable, or an expression containing variables, that have not been assigned a value ... which is the case in the question as asked. They are likely to work on many implementations, but the C standard doesn't guarantee it, and they will certainly fail on an implementation that initializes uninitialized variables with trap values.

Answer 4

Could easily be done using ANSI C 89:

    #include<stdio.h>
    #include<limits.h>

int main(void) {

printf("Max value of char: %d\n", CHAR_MAX);
printf("Min value of char: %d\n", CHAR_MIN);

printf("Max value of short: %d\n", SHRT_MAX);
printf("Min value of short: %d\n", SHRT_MIN);

printf("Max value of int: %d\n", INT_MAX);
printf("Min value of int: %d\n", INT_MIN);

printf("\n\n");
return 0;
}

Notice that you can include float.h and then use:

printf("Max value of Double: %d\n", DBL_MAX);

Though, it is less recommended.

Good luck, Ron