How to get ASCII code of C `char` portably in ANSI C

Question

As you all know, ANSI C does not require implementations to use ASCII code points for the char type. However it does define a basic character set with alphanumeric characters, whitespace characters and other printable characters. Are there library functions to portably convert a char to its ASCII code and back?

Answer 1

Here are some functions to do the job, which return 0 if the character is not found; hopefully they are self-explanatory:

char const ascii_table[] = " !\"#$%&'()*+,-./0123456789:;<=>?@ABCDEFGHIJKLMNOPQRSTUVWXYZ[\]^_`abcdefghijklmnopqrstuvwxyz{|}~";

int char_to_ascii(int ch)
{
    char const *p = strchr(ascii_table, ch);
    return p ? p - ascii_table + 32 : 0;
}

int ascii_to_char(int a)
{
     return (a >= 32 && a < 128) ? ascii_table[a-32] : 0;
}

Expanding this to cover 0-127 instead of 32-127 is left as an exercise to the reader 😉

Answer 2

Are there library functions to portably convert a char to its ASCII code and back?

There are no such functions in the standard library.

Hovewer, let's be realistic: It's very unlikely that your code will ever be used on a platform that doesn't use ASCII.

I would do this:

char char_to_ascii(char ch)
{
    return ch;
}

char ascii_to_char(char ch)
{
    return ch;
}

And then, if you need to compile the code for an exotic platform that doesn't use ASCII, you write proper implementations for those functions.