C converting char to int

Question

I'm trying to build a program in C that:

1. Gets age of user
2. Checks if age is between 18 to 120
3. Checks if age doesn't contain other characters such as letters, dots and so.
4. If it isn't between 18-120 or contains other characters go back to section 1.

To check number 3 I believe that I need to scan the age from the user as a char but then i cant check if it's between 18 to 120. I can't use arrays or strings. This is the code that I have for now which checks that code doesn't contain other characters:

void main() {
char age;
int error = 0;
do
{
    error = 0;
    printf("Please enter your age:");
    scanf("%c", &name);
    while (name != '\n')
    {
        if ((name<'0') || (name>'9')){
            error++;
        }
        name = getchar();
    }
} while (error != 0);
}  

Answer 1

You should use the long int strtol(const char *nptr, char **endptr, int base) function, read the manual.

Answer 2

Keep age and input in separate variables like age and ch.
As each ch is read, test for EOF, '\n' and if it is a digit. OP's code has done some of that.
If it passes, accumulate the age. Test for the maximum value (120). This will also detect overflow.
After the input line is processed, check for the minimum value (18).

#include <ctype.h>
#include <stdio.h>

int main(void) {
  int error;
  int ch;
  do {
    int age = 0;
    error = 0;
    printf("Please enter your age:");
    fflush(stdout);
    while ((ch = fgetc(stdin)) != EOF && ch != '\n') {
      // Maybe also throw out white-space
      if (isspace(ch))
        continue;
      // or if (!isdigit(ch)) {
      if ((ch < '0') || (ch > '9')) {
        error = 1;
      } else {
        age = age * 10 + ch - '0';
        if (age > 120)
          error = 2;
      }
    }
    if (age < 18)
      error = 3;
    printf("Age: %d Error: %d\n", age, error);
  } while (error && ch != EOF);
  return 0;
}

Important to use int ch rather than char ch.

Minor bits: Use correct signature. Example:

 // void main()
 int main(void)

When using scan(), always good to check return value. In this situation, code can simple use fgetc() or getchar().

Code posted does not compile. (name is not defined.) Unless you are having compilation problem, always post compile-able code.