Check if a number is an integer or not in C language

Testing whether input is a valid int decimal numeral or is a decimal numeral in [-2³¹, 2³¹) is actually a bit complicated. The C standard does not provide a direct way to do this. What we can do is:

• Read characters and check to see whether they are in the expected form: spaces, an optional minus sign (hyphen), and digits. (Any non-digits after the digits will be allowed and ignored.)
• Try using strtol to convert the numeral to a long. We use strtol because there is no C-standard library routine for converting to an int (or your fixed bounds using 2³¹) that provides error indications.
• Compare the long produced by strtol to the int bounds.

Example code for int bounds follows. If you want bounds of -2147483648 and 2147483647 instead, substitute those for INT_MIN and INT_MAX. To be completely safe, the code should actually use long long and strtoll, since the C standard does not require long to be able to represent −2147483648.

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


int main(void)
{
    //  Prepare a buffer.
    size_t BufferSize = 100, BufferUsed = 0;
    char *Buffer = malloc(BufferSize * sizeof *Buffer);

    //  Skip white space.
    int c;
    do
        c = getchar();
    while (isspace(c));

    if (c == EOF)
    {
        printf("Input is not an int:  EOF before \"-\" or digit seen.\n");
        exit(EXIT_SUCCESS);
    }

    //  Accept a hyphen as a minus sign.
    if (c == '-')
    {
        Buffer[BufferUsed++] = c;
        c = getchar();
    }

    //  Accept digits.
    while (isdigit(c))
    {
        Buffer[BufferUsed++] = c;
        if (BufferSize <= BufferUsed)
        {
            BufferSize *= 2;
printf("Realloc:  size = %zu, used = %zu.\n", BufferSize, BufferUsed);
            char *NewBuffer = realloc(Buffer, BufferSize * sizeof *NewBuffer);
            if (!NewBuffer)
            {
                fprintf(stderr, "Error, unable to allocate %zu bytes.\n",
                    BufferSize);
                exit(EXIT_FAILURE);
            }
            Buffer = NewBuffer;
        }
        c = getchar();
    }

    //  Ensure we saw at least one digit (input is not blank or just a hyphen).
    if (BufferUsed == 0 || BufferUsed == 1 && Buffer[0] == '-')
    {
        printf("Input is not an int:  No digits present.\n");
        exit(EXIT_SUCCESS);
    }

    //  Put back the unaccepted character, if any.
    if (c != EOF)
        ungetc(c, stdin);

    //  Terminate the string.
    Buffer[BufferUsed] = 0;

    //  Attempt to convert the numeral to long.
    char *End;
    errno = 0;
    long x = strtol(Buffer, &End, 10);

    //  Test whether strtol succeeded.
    if (*End)
    {
        /*  I do not expect this to occur since we already tested the input
            characters.
        */
        printf("Input is not an int:  strtol rejected %c.\n", *End);
        exit(EXIT_SUCCESS);
    }
    if (errno == ERANGE)
    {
        printf("Input is not an int:  strtol reported out of range.\n");
        exit(EXIT_SUCCESS);
    }

    if (x < INT_MIN || INT_MAX < x)
    {
        printf("Input is not an int:  Value is outside bounds.\n");
        exit(EXIT_SUCCESS);
    }

    printf("Input is an int, %ld.\n", x);

    free(Buffer);
}