pipe에 관한 질문.

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <ctype.h>
#include <sys/types.h>
#include <unistd.h>
 
/*
 
	 A sends: Hello, world!
A receives: hELLO, WORLD!
 
process A
close(BA[1]) at line L1; 
close(AB[1]) at line L2; 
 
process B
close(AB[1]) at line L3. 
*/
 
char *msg="Hello, world!";
/* convert upper case to lower case or vise versa */
char conv(char c)
{
     if (islower(c)) return toupper(c);
     if (isupper(c)) return tolower(c);
     return c;
}
 
int main(int argc, char **argv)
{
 
	 pid_t pid, AB[2], BA[2];
     char buf[20];
 
     /* create two pipes */
     if (pipe(AB)<0 || pipe(BA)<0)
         perror("pipe calls"), exit(1);
 
     /* create process B */
     if ((pid=fork())<0)
         perror("fork call"), exit(2);
 
 
     if (pid>0) {       /* process A */
        close(AB[0]); 
 
		if(argv[1] == NULL || argv[1] == "2" || argv[1] == "3")
			close(BA[1]);                // L1
 
 
        printf("     A sends: %s\n", msg);
        write(AB[1], msg, strlen(msg));
 
	   if(argv[1] == NULL || argv[1] == "1" || argv[1] == "3")
		close(AB[1]);                // L2
 
        printf("A receives: ");
        while (read(BA[0], buf, 1)==1)
             printf("%c", buf[0]);
 
        printf("\n"); exit(0);
     }
 
     if (pid==0) {    /* process B */
        close(BA[0]); 
 
		if(argv[1] == NULL || argv[1] == "1" || argv[1] == "2")
		close(AB[1]);                // L3
 
        while (read(AB[0], buf, 1)==1) {
            buf[0] = conv(buf[0]);
            write(BA[1], buf, 1);
        }
        exit(0);
     }
 
	return 0;
}