How to Determine Whether C Input is an 8 bit Binary Number

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I'm writing a program that will mess around with converting binary values to hex and vice versa.

The user should enter a value which will be stored in an int variable. I need to determine whether or not this user input is an eight-bit binary number like 10010010, 11110000, 10101010, etc.

I've already written a function to determine how many characters the user enters, but I'm not finding much on how to make sure the user's input is either a 1 or a 0.

Is this just a simple solution that I'm not seeing?

If your program to store user input in a char array (string) you can use strspn function that searches for the first occurrence of a not matching char from a given string.

#include <stdio.h>
#include <string.h>

int main () {
  const char user_input[] = "101th";
  const char bdigits[] = "01";

  int i = strspn (user_input, bdigits);
  printf("Not a binary digit at position %d.\n",i);
  return 0;

Signed Binary Numbers and Two's Complement Numbers, But how do we represent signed binary numbers if all we have is a bunch of one's and zero's. We know that binary digits, or bits only have two values, either a​  Input: Enter an 8 bits integer number: 31 Now, enter a bit number (from 0 to 7) to check, whether it is SET or not: 3 Output: Bit number 3 is SET in number 31 Program to check whether bit is SET or not in C language

use like this:

char st[1024];
fgets(st, sizeof(st), stdin);
int i = 0;
for (;;) {
  if (i == 8) {
    if (st[i] == '\n') {
    } else {
  if (st[i] != '0' && st[i] != '1') {
      // error

Basics of Binary for C Programming, You see the range of values that can be stored in 8 bits, or 1 byte. Just as decimal places in a base 10 number increase by powers of 10, bits in a binary int input; printf("Type a value 0 to 255: "); scanf("%d",&input); printf("%d is binary %sn"  Problem Statement: You have given an integer number. You have to write a C Program to Count Number of Ones in Binary representation of a given integer number. Note: One’s bit is also called as the set bit. Suppose you have integer value 13. Its binary representation is 1101. So the number of set bits in 13 is 3. Takes another example.

Rather than taking character input and then attempting to convert the resulting string into a numeric value that you can manipulate, simply require the user to enter a 8-bit numeric value. Whether entered in decimal, hexadecimal or octal, the resulting value is always stored in memory as a binary value.

Since you are wanting to limit input to a 8-bit value, C99 introduced exact width variable types that eliminate any ambiguity in type-size between architectures. Understand that char and unsigned char are defined as 1-byte/8-bits on virtually every system, where types such as long can be 4-bytes on some systems (e.g. x86) and 8-bytes on others (e.g. x86_64). Use of the exact-width types removes all ambiguity.

The exact-width types are defined in stdint.h and the macros for use with printf or scanf are defined in inttypes.h (note: inttypes.h is required to include stdint.h, so if using the macros, inttypes.h is all that is needed) See: man inttypes.h

In requiring an 8-bit value, it is useful to take entry into a temporary variable of larger type (e.g. as a 64-bit value) and then test whether the value entered is within the range of an 8-bit value. A simple conditional checking that the unsigned value is 0-255 or a signed value is -128 to 127 is sufficient. If the value entered is not with those ranges for unsigned/signed types, it isn't an 8-bit value.

After reading user input and validating the conversion and that the value is within the range of 8-bit, you can output the value in any base you like using the macros provided in inttypes.h for unsigned and hexadecimal types, and a simple function to output a padded binary representation.

Putting it altogether, you could do something like the following:

#include <stdio.h>
#include <limits.h>     /* for CHAR_BIT */
#include <inttypes.h>   /* for exact width macros, includes <stdint.h> */

/** binary representation of 'v' padded to 'sz' bits.
 *  the padding amount is limited to the number of
 *  bits in 'v'. valid range: 0 - sizeof v * CHAR_BIT.
void binprnpad (const uint64_t v, size_t sz)
    if (!sz) {      /* validate sz not zero */
        fputs ("error: invalid sz.\n", stderr);
    if (!v)  {      /* handle v = 0 case */
        while (sz--)
            putchar ('0');

    if (sz > sizeof v * CHAR_BIT)   /* validate size not greater than bits */
        sz = sizeof v * CHAR_BIT;

    while (sz--)    /* output char representation of binary value */
        putchar ((v >> sz & 1) ? '1' : '0');

int main (void) {

    uint64_t tmp = 0;   /* temporary value to store user input value */

    fputs ("enter a value: ", stdout);          /* prompt */
    if (scanf ("%" SCNu64, &tmp) == 1) {        /* read/validate input */
        if (tmp <= UINT8_MAX) {                 /* check 8-bit value */
            uint8_t val = (uint8_t)tmp;         /* assign 8-bit value */
            printf ("\nunsigned : %" PRIu8      /* output dec, hex */
                    "\nhex      : %" PRIx8
                    "\nbinary   : ", val, val);
            binprnpad (val, sizeof val * CHAR_BIT); /* output binary */
            putchar ('\n');     /* tidy up with newline */
        else    /* handle error: not an 8-bit value */
            fputs ("error: entry exceeds 8-bit value.\n", stderr);
    else    /* handle error: invalid input */
        fputs ("error: invalid entry.\n", stderr);

Example Use/Output

$ ./bin/8bitentry
enter a value: 127

unsigned : 127
hex      : 7f
binary   : 01111111

If the user attempts to enter something outside the range of an 8-bit value:

$ ./bin/8bitentry
enter a value: 256
error: entry exceeds 8-bit value.

Taking Input as a String Representation of a Binary Value

The only scenario that wouldn't be covered by requiring input as above, is if you actually want the user to enter the binary representation as a string which you then want to convert to a numeric value. The simplest way to handle a string containing a binary representation of a value is to use the base-2 conversion provided by strtoul. This allows complete error checking of input by making use of the endptr parameter to validate only '0's and '1's were entered. See man strtoul

A simple implementation taking string entry of '0's and '1's could be:

#include <stdio.h>
#include <stdlib.h>     /* for strtoul */
#include <errno.h>      /* for errno */

#define MAXC 1024

int main (void) {

    char buf[MAXC],
        *endptr;        /* end pointer for use with strtoul */
    unsigned long tmp;
    unsigned char val;

    fputs ("enter an 8-bit binary no.: ", stdout);  /* prompt */
    if (fgets (buf, MAXC, stdin) == NULL) {
        fputs ("(user canceled input)\n", stderr);
        return 1;

    errno = 0;                          /* reset errno 0 */
    tmp = strtoul (buf, &endptr, 2);    /* convert base2 string to value */
    if (buf == endptr) {
        fputs ("error: invalid binary entry.\n", stderr);
        return 1;
    else if (errno) {
        fputs ("error: over/underflow occured.\n", stderr);
        return 1;
    else if (tmp > 255) {
        fputs ("error: input exceeds 8-bit value.\n", stderr);
        return 1;
    else if (*endptr != '\n' && *endptr != 0)
        fprintf (stderr, "warning: conversion ended at invalid char '%c'\n",

    val = (unsigned char)tmp;
    printf ("unsigned: %hhu\nhex     : %hhx\n", val, val);

Example Use/Output

$ ./bin/8bitentry_string
enter an 8-bit binary no.: 1111111
unsigned: 127
hex     : 7f

Or user enters more than an 8-bit binary value:

$ ./bin/8bitentry_string
enter an 8-bit binary no.: 111111111
error: input exceeds 8-bit value.

Or user enters a value beginning with an invalid character:

$ ./bin/8bitentry_string
enter an 8-bit binary no.: 310
error: invalid binary entry.

Or user enters a value containing an invalid character:

$ ./bin/8bitentry_string
enter an 8-bit binary no.: 10131
warning: conversion ended at invalid char '3'
unsigned: 5
hex     : 5

Look things over and let me know if you have further questions.

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How to check a particular bit is SET or not using C program , How to check whether a bit is SET (High) or not using C programming language? Here, we will read a number and bit and check input bit is SET or not. returns 8 (in Binary: 0000 1000), see the binary 3rd (count from 0 to 7) bit is SET here. An “n-bit” binary encoder has 2 n input lines and n-bit output lines such as 4-to-2, 8-to-3 and 16-to-4 line configurations.. The output lines of a digital encoder generate the binary equivalent of the input line whose value is equal to “1” and are available to encode either a decimal or hexadecimal input pattern to typically a binary or “BCD” (binary coded decimal) output code.

  • Why not require the user to enter only an 8-bit value by using the type unsigned char or uint8_t? Otherwise, simply check if the value is -128 to 127 (for signed 8-bit value) or 0-255 (for unsigned) If you are taking character input of '0's and '1's, then just check that the character entered is '0' or '1' (note, each character will be 8-bits)
  • The simple solution I'm not seeing is... if you take input as an unsigned char (or int) and the user enters (say 9), the value in memory will already be 00001001. A computer only knows 0 or 1. So regardless of how you take the numeric value as input, it will be stored as a binary value in memory, and you are free to manipulate the bits into whatever base representation you like.
  • I see what you're saying, thanks!