Wrong string concatenation using strcat

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?

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

Natasha Dutta · Accepted Answer · 2015-06-05T13:44:15.320

5

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:

Please don't allow boundless increment of i when you have defined a fixed size buffer. May overflow.
make sure your buff[i] is big enough to hold the concatenated string. Otherwise, it will invoke undefined behaviour.

edited Jun 05 '15 at 13:44

answered Jun 05 '15 at 13:36

Natasha Dutta

3,242
21
23

Ok, so could you explain how to remove that trailing newline? – k1ber Jun 05 '15 at 13:38
But I don't hold concatenated string in line, I hold it in buff[i] – k1ber Jun 05 '15 at 13:43
@KiberPrestupnik many ways are there, I prefer a one-liner, `line[strchr(line, '\n') -1] = 0;` or something like this along the line. – Natasha Dutta Jun 05 '15 at 13:43
1

@KiberPrestupnik You can do it the following way size_t n = strlen( line ); if ( n && line[n-1] == '\n' ) line[n-1] = '\0'; – Vlad from Moscow Jun 05 '15 at 13:44
@KiberPrestupnik yeah, that was a mistake from my side , updated. – Natasha Dutta Jun 05 '15 at 13:45
@VladfromMoscow hey man, how're you? Don't you like the above one liner I posted? – Natasha Dutta Jun 05 '15 at 13:45
@VladfromMoscow So kind of you. Thanks. :-) – Natasha Dutta Jun 05 '15 at 13:47
1

Be warned that the suggested `line[strchr(line, '\n') -1] = 0;` will make the program crash if you enter exactly 17 characters or more. Use a safer method, such as in http://stackoverflow.com/questions/29875285/code-not-reaching-statements-using-fgets/29875290#29875290 – Jongware Jun 05 '15 at 14:43
@Jongware Thank you for the correction. Hope OP is making a note of this. – Natasha Dutta Jun 05 '15 at 14:46
@Natasha Dutta The code `line[strchr(line, '\n') - 1] = 0;` doesn't seem to work in MVS2015. The text of error I get: expression must have integral or unscoped enum type. – k1ber Jun 05 '15 at 22:50

score 1 · Answer 2 · answered Jun 05 '15 at 14:28

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;
}

Wrong string concatenation using strcat

2 Answers2