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#include <stdio.h>
char *my_strcat(char *str_a, char *str_b);
 
char *my_strcat(char *str_a, char *str_b)
{
 
    /*まずaの'\0'を見つける*/
    while(*str_a){
        str_a++;
    }
 
    /*aの'\0'の位置へbをコピー*/
    while(*str_b){
        *str_a = *str_b;
        str_b++;
        str_a++;
    }
    *str_a = '\0';
    return str_a;
}
 
 
int main(void){
    char a[800];
    char b[800];
 
	printf("文字を入力\na:");
	scanf("%s",a);
	printf("\nb:");
	scanf("%s",b);
 
 
    printf("\na:%s\t", a);
    printf("b:%s\n", b);
    my_strcat(a, b);
    printf("結果\n%s", a);
 
	return 0;
}

prog.c: In function ‘main’:
prog.c:28:2: warning: implicit declaration of function ‘scanf_s’ [-Wimplicit-function-declaration]
  scanf_s("%s",a);
  ^
/home/Xey1wS/cc4xVzXU.o: In function `main':
prog.c:(.text.startup+0x31): undefined reference to `scanf_s'
prog.c:(.text.startup+0x4d): undefined reference to `scanf_s'
collect2: error: ld returned 1 exit status

Standard output is empty