使用教材

《“笨办法” 学C语言（Learn C The Hard Way）》
https://www.jianshu.com/p/b0631208a794

ex14.c

打印属于字母或者空格的字符及其对应的ASCII值

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

// forward declaration
void print_letters(char arg[], int len);

void print_arguments(int argc, char *argv[]) 
{
    int i = 0 ;
    int len = 0;

    for(i = 0; i < argc; i++) {
        len = strlen(argv[i]);
        print_letters(argv[i], len);
    }
}

void print_letters(char arg[], int len) 
{
    int i = 0 ;

    for(i = 0 ; i< len; i++) {
        char ch = arg[i];

        if(isalpha(ch) || isblank(ch)) {
            printf("'%c' == %d \n", ch, ch);
        }
    }

    printf("\n");
}


int main(int argc, char *agrv[]) 
{
    print_arguments(argc, agrv);
    return 0;
}

Makefile

CC=clang
CFLAGS=-Wall -g

clean:
    rm -f ex14

run

anno@anno-m:~/Documents/mycode/ex14$ make ex14
clang -Wall -g    ex14.c   -o ex14
anno@anno-m:~/Documents/mycode/ex14$ ./ex14
'e' == 101 
'x' == 120 

anno@anno-m:~/Documents/mycode/ex14$ ./ex14 "I go 3 space"
'e' == 101 
'x' == 120 

'I' == 73 
' ' == 32 
'g' == 103 
'o' == 111 
' ' == 32 
' ' == 32 
's' == 115 
'p' == 112 
'a' == 97 
'c' == 99 
'e' == 101 

anno@anno-m:~/Documents/mycode/ex14$ ./ex14 hell1o worl2d !3
./ex14 hell1o worl2d ls
'e' == 101 
'x' == 120 

'h' == 104 
'e' == 101 
'l' == 108 
'l' == 108 
'o' == 111 

'w' == 119 
'o' == 111 
'r' == 114 
'l' == 108 
'd' == 100 

'l' == 108 
's' == 115

note

前置声明 forward declaration：解决未定义函数之前就可以使用这个函数；
isalpha(ch)、isblank(ch)来自头文件<ctype.h>

SYNOPSIS
       #include <ctype.h>

       int isalnum(int c);
       int isalpha(int c);
       int isascii(int c);
       int isblank(int c);
       int iscntrl(int c);
       int isdigit(int c);
       int isgraph(int c);
       int islower(int c);
       int isprint(int c);
       int ispunct(int c);
       int isspace(int c);
       int isupper(int c);
       int isxdigit(int c);

strlen来自<string.h> ：返回字符串长度，不包括最后的\0

NAME
       strlen - calculate the length of a string

SYNOPSIS
       #include <string.h>

       size_t strlen(const char *s);

DESCRIPTION
       The  strlen() function calculates the length of the string s, excluding
       the terminating null byte ('\0').

RETURN VALUE
       The strlen() function returns the number of bytes in the string s.