使用strcat的错误字符串连接

Question

I'm writing a program that reads strings from file, saves them to 'string buffer' and then concatenates those string and writes them to another file.

#define _CRT_SECURE_NO_WARNINGS
#include <cstdlib>
#include <iostream>
#include <string.h>
#include <stdio.h>

int main() {
    FILE *f = fopen("Read.txt", "r");
    char line[20];
    char buff[15][20];
    int i = 0;
    while (fgets(line, 18, f)) {
        strcpy(buff[i], line);
        i++;
    }
    FILE *h = fopen("Out.txt", "w+");
    for (int j = 0; j < i; ++j) {
        char ct[4] = "smt";
        strcat(buff[j], ct);
        fputs(buff[j], h);
    }
    return 0;
}

Contents of file Read.txt:

Lorem ipsum 
dolor sit 
amet

Expected output (File Out.txt):

Lorem ipsumsmt 
dolor sitsmt 
ametsmt

But what I get in Out.txt:

Lorem ipsum 
smtdolor sit 
smtamet
smt

So how to get expected result?

PS I think that the problem occurs when I use function fgets() .

Answer 1

This is not a bug or problem, rather, an expected behavior. Please read on.

fgets() reads and stores the trailing newline ( \\n ). you need to remove (strip) that before storing the input.

That said, please note:

1. Please don't allow boundless increment of i when you have defined a fixed size buffer. May overflow.

2. make sure your buff[i] is big enough to hold the concatenated string. Otherwise, it will invoke undefined behaviour .

Answer 2

Below code will work for you. You need to add Null Character before doing any String operations. I commented the code wherever I modified.

#define _CRT_SECURE_NO_WARNINGS
#include <cstdlib>
#include <iostream>
#include <string.h>
#include <stdio.h>

int main() {
    FILE *f = fopen("Amol.txt", "r");
    char line[20];
    char buff[15][20];
    int i = 0;
    while (fgets(line, 18, f)) {
        line[strlen(line) -1] = '\0';  // Here I added NULL character 
        strcpy(buff[i], line);
        i++;
    }
    FILE *h = fopen("Out.txt", "w+");
    for (int j = 0; j < i; ++j) {       
        char ct[5] = "smt\n";       // As \n will be at the end,so changed this array
        strcat(buff[j], ct);        
        fputs(buff[j], h);
    }
    return 0;
}