file_cat.c

#include<stdio.h>
#include <string.h>
#include<stdlib.h>
#include <sys/stat.h>
int main(){
    FILE* file_a;
    FILE* file_b;
    FILE* file_c;
    char buffer[1024];
    
    file_a = fopen("a.txt", "r");
    file_b = fopen("b.txt", "r");
    file_c = fopen("c.txt", "a");
    if(file_a == NULL || file_b == NULL || file_c == NULL){
        printf("!!!\n");
        return -1;
    }
    while(fgets(buffer, 1024, file_a)!=NULL){
        size_t len = strlen(buffer);
        printf("%lu %s\n",len,buffer);
        fprintf(file_c, "%s\n",buffer);
    }
    
    while(fgets(buffer, 1024, file_b)!=NULL){
        size_t len = strlen(buffer);
        printf("%lu %s\n",len,buffer);
        fprintf(file_c, "%s\n",buffer);
    }
    fclose(file_a);
    fclose(file_b);
    fclose(file_c);
    
    return 0;
}