isalnum function > ctype.h > header files in c Programming

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Syntax	int isalnum (int c)
Header File	ctype.h
Return Value	Return non-zero if letter is either alphabet or digit Return zero if character is not alphanumeric

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void main()
{
char ch = '1';

if(isalnum(ch))

    printf("nThis is alpha-numeric character");

else
   
    printf("nThis is non alpha-numeric character");
}

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This is alpha-numeric character

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1. Uppercase Letters [ A-Z]
2. Lowercase Letters [a-z]
3. Digits [0-9]

1. character ch is passed to function isalnum 
2. if ch belongs to the set of alpha-numeric character then it returns positive value 
3. otherwise it returns zero .

1. Positive non-zero value given to if block , will result in TRUE condition
2. Zero value given to if block , will result in FALSE condition

Question	How we are passing character value to function , which contradicts to the Given Syntax ?
Answer	We are passing character to function , automatically gets converted into (int) equivalent ascii value

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