Thread: Printing string bigger than the maximum size

Good evening. Why was possible to print a string with 14 bytes when I just allocated 6 bytes for it ?

Code:

 
#include <iostream> 
#include <cstring> 

using namespace std; 

const int MAXSIZE = 20; 

int main(void) 
{ 
    char name[MAXSIZE]; 
    char lastName[MAXSIZE]; 
    
    
    cout << "Enter your first name: "; 
    cin.get(name, MAXSIZE).get();
    cout << "Enter your last name: "; 
    cin.get(lastName, MAXSIZE).get();
    
    // lastName, name\0
    char *fullName = new char[strlen(name)]; 
    
    strcpy(fullName, lastName); 
    strcat(fullName, ", ");
    strcat(fullName, name); 
    
    cout << "Here's the information in a single string: " << fullName << endl; 
    
    delete fullName; 
    return 0;     
}

Code:

 
thames@semaht ~/C++/Strings $ g++ -g -Wall personinfo2.cpp -o personinfo2
thames@semaht ~/C++/Strings $ gdb -q personinfo2
Lendo símbolos de /home/thames/C++/Strings/personinfo2...concluído.
(gdb) break 26
Ponto de parada 1 at 0x400cd2: file personinfo2.cpp, line 26.
(gdb) r
Starting program: /home/thames/C++/Strings/personinfo2 
Enter your first name: thames
Enter your last name: thames

Breakpoint 1, main () at personinfo2.cpp:26
26        cout << "Here's the information in a single string: " << fullName << endl; 
(gdb) print fullName
$1 = 0x603010 "thames, thames"
(gdb) print strlen(fullName)
$2 = 14
(gdb) print strlen(name)
$3 = 6
(gdb) c
Continuando.
Here's the information in a single string: thames, thames
[Inferior 1 (process 2738) exited normally]
(gdb)

It's "possible", because it's all down to pure dumb luck.

Using a different tool to test your program, and you see there is much to complain about.

Code:

$ valgrind ./a.out
==3136== Memcheck, a memory error detector
==3136== Copyright (C) 2002-2010, and GNU GPL'd, by Julian Seward et al.
==3136== Using Valgrind-3.6.1-Debian and LibVEX; rerun with -h for copyright info
==3136== Command: ./a.out
==3136== 
Enter your first name: Commander
Enter your last name: Z
==3136== Invalid write of size 1
==3136==    at 0x4C29484: strcat (mc_replace_strmem.c:176)
==3136==    by 0x400B56: main (in /home/sc/Documents/a.out)
==3136==  Address 0x5973049 is 0 bytes after a block of size 9 alloc'd
==3136==    at 0x4C2864B: operator new[](unsigned long) (vg_replace_malloc.c:305)
==3136==    by 0x400AF8: main (in /home/sc/Documents/a.out)
==3136== 
==3136== Invalid write of size 1
==3136==    at 0x4C29493: strcat (mc_replace_strmem.c:176)
==3136==    by 0x400B56: main (in /home/sc/Documents/a.out)
==3136==  Address 0x597304a is 1 bytes after a block of size 9 alloc'd
==3136==    at 0x4C2864B: operator new[](unsigned long) (vg_replace_malloc.c:305)
==3136==    by 0x400AF8: main (in /home/sc/Documents/a.out)
==3136== 
==3136== Invalid write of size 1
==3136==    at 0x4C29495: strcat (mc_replace_strmem.c:176)
==3136==    by 0x400B56: main (in /home/sc/Documents/a.out)
==3136==  Address 0x597304c is 3 bytes after a block of size 9 alloc'd
==3136==    at 0x4C2864B: operator new[](unsigned long) (vg_replace_malloc.c:305)
==3136==    by 0x400AF8: main (in /home/sc/Documents/a.out)
==3136== 
==3136== Invalid read of size 1
==3136==    at 0x4C29727: strlen (mc_replace_strmem.c:282)
==3136==    by 0x4EC7800: std::basic_ostream<char, std::char_traits<char> >& std::operator<< <std::char_traits<char> >(std::basic_ostream<char, std::char_traits<char> >&, char const*) (in /usr/lib/x86_64-linux-gnu/libstdc++.so.6.0.16)
==3136==    by 0x400B74: main (in /home/sc/Documents/a.out)
==3136==  Address 0x5973049 is 0 bytes after a block of size 9 alloc'd
==3136==    at 0x4C2864B: operator new[](unsigned long) (vg_replace_malloc.c:305)
==3136==    by 0x400AF8: main (in /home/sc/Documents/a.out)
==3136== 
==3136== Invalid read of size 1
==3136==    at 0x51AC2B4: _IO_file_xsputn@@GLIBC_2.2.5 (fileops.c:1317)
==3136==    by 0x51A1F9C: fwrite (iofwrite.c:45)
==3136==    by 0x4EC7574: std::basic_ostream<char, std::char_traits<char> >& std::__ostream_insert<char, std::char_traits<char> >(std::basic_ostream<char, std::char_traits<char> >&, char const*, long) (in /usr/lib/x86_64-linux-gnu/libstdc++.so.6.0.16)
==3136==    by 0x4EC780E: std::basic_ostream<char, std::char_traits<char> >& std::operator<< <std::char_traits<char> >(std::basic_ostream<char, std::char_traits<char> >&, char const*) (in /usr/lib/x86_64-linux-gnu/libstdc++.so.6.0.16)
==3136==    by 0x400B74: main (in /home/sc/Documents/a.out)
==3136==  Address 0x597304b is 2 bytes after a block of size 9 alloc'd
==3136==    at 0x4C2864B: operator new[](unsigned long) (vg_replace_malloc.c:305)
==3136==    by 0x400AF8: main (in /home/sc/Documents/a.out)
==3136== 
==3136== Invalid read of size 1
==3136==    at 0x51AC2C9: _IO_file_xsputn@@GLIBC_2.2.5 (fileops.c:1317)
==3136==    by 0x51A1F9C: fwrite (iofwrite.c:45)
==3136==    by 0x4EC7574: std::basic_ostream<char, std::char_traits<char> >& std::__ostream_insert<char, std::char_traits<char> >(std::basic_ostream<char, std::char_traits<char> >&, char const*, long) (in /usr/lib/x86_64-linux-gnu/libstdc++.so.6.0.16)
==3136==    by 0x4EC780E: std::basic_ostream<char, std::char_traits<char> >& std::operator<< <std::char_traits<char> >(std::basic_ostream<char, std::char_traits<char> >&, char const*) (in /usr/lib/x86_64-linux-gnu/libstdc++.so.6.0.16)
==3136==    by 0x400B74: main (in /home/sc/Documents/a.out)
==3136==  Address 0x597304a is 1 bytes after a block of size 9 alloc'd
==3136==    at 0x4C2864B: operator new[](unsigned long) (vg_replace_malloc.c:305)
==3136==    by 0x400AF8: main (in /home/sc/Documents/a.out)
==3136== 
==3136== Invalid read of size 1
==3136==    at 0x51AC230: _IO_file_xsputn@@GLIBC_2.2.5 (fileops.c:1349)
==3136==    by 0x51A1F9C: fwrite (iofwrite.c:45)
==3136==    by 0x4EC7574: std::basic_ostream<char, std::char_traits<char> >& std::__ostream_insert<char, std::char_traits<char> >(std::basic_ostream<char, std::char_traits<char> >&, char const*, long) (in /usr/lib/x86_64-linux-gnu/libstdc++.so.6.0.16)
==3136==    by 0x4EC780E: std::basic_ostream<char, std::char_traits<char> >& std::operator<< <std::char_traits<char> >(std::basic_ostream<char, std::char_traits<char> >&, char const*) (in /usr/lib/x86_64-linux-gnu/libstdc++.so.6.0.16)
==3136==    by 0x400B74: main (in /home/sc/Documents/a.out)
==3136==  Address 0x5973049 is 0 bytes after a block of size 9 alloc'd
==3136==    at 0x4C2864B: operator new[](unsigned long) (vg_replace_malloc.c:305)
==3136==    by 0x400AF8: main (in /home/sc/Documents/a.out)
==3136== 
==3136== Invalid read of size 1
==3136==    at 0x51AC23E: _IO_file_xsputn@@GLIBC_2.2.5 (fileops.c:1348)
==3136==    by 0x51A1F9C: fwrite (iofwrite.c:45)
==3136==    by 0x4EC7574: std::basic_ostream<char, std::char_traits<char> >& std::__ostream_insert<char, std::char_traits<char> >(std::basic_ostream<char, std::char_traits<char> >&, char const*, long) (in /usr/lib/x86_64-linux-gnu/libstdc++.so.6.0.16)
==3136==    by 0x4EC780E: std::basic_ostream<char, std::char_traits<char> >& std::operator<< <std::char_traits<char> >(std::basic_ostream<char, std::char_traits<char> >&, char const*) (in /usr/lib/x86_64-linux-gnu/libstdc++.so.6.0.16)
==3136==    by 0x400B74: main (in /home/sc/Documents/a.out)
==3136==  Address 0x597304a is 1 bytes after a block of size 9 alloc'd
==3136==    at 0x4C2864B: operator new[](unsigned long) (vg_replace_malloc.c:305)
==3136==    by 0x400AF8: main (in /home/sc/Documents/a.out)
==3136== 
Here's the information in a single string: Z, Commander
==3136== Mismatched free() / delete / delete []
==3136==    at 0x4C27FF2: operator delete(void*) (vg_replace_malloc.c:387)
==3136==    by 0x400B8D: main (in /home/sc/Documents/a.out)
==3136==  Address 0x5973040 is 0 bytes inside a block of size 9 alloc'd
==3136==    at 0x4C2864B: operator new[](unsigned long) (vg_replace_malloc.c:305)
==3136==    by 0x400AF8: main (in /home/sc/Documents/a.out)
==3136== 
==3136== 
==3136== HEAP SUMMARY:
==3136==     in use at exit: 0 bytes in 0 blocks
==3136==   total heap usage: 1 allocs, 1 frees, 9 bytes allocated
==3136== 
==3136== All heap blocks were freed -- no leaks are possible
==3136== 
==3136== For counts of detected and suppressed errors, rerun with: -v
==3136== ERROR SUMMARY: 15 errors from 9 contexts (suppressed: 4 from 4)

edit:

ah, nevermind.

Code:

 delete [] fullname;

many thanks. Valgrind is a damn freaking cool tool.

I forgot to use Valgrind

edit:

Code:

 
#include <iostream> 
#include <cstring> 

using namespace std; 

const int MAXSIZE = 20; 

int main(void) 
{ 
    char name[MAXSIZE]; 
    char lastName[MAXSIZE]; 
    
    cout << "Enter your first name: "; 
    cin.get(name, MAXSIZE).get();
    cout << "Enter your last name: "; 
    cin.get(lastName, MAXSIZE).get();
    
    // lastName, name\0
    char *fullName = new char[strlen(name) + strlen(lastName) + 3]; 
    
    strcpy(fullName, lastName); 
    strcat(fullName, ", ");
    strcat(fullName, name); 
    
    cout << "Here's the information in a single string: " << fullName << endl; 
    
    delete fullName; 
    return 0;     
}

I thought the code above was correct.

Code:

thames@semaht ~/C++/Strings $ valgrind ./personinfo2
==3215== Memcheck, a memory error detector
==3215== Copyright (C) 2002-2011, and GNU GPL'd, by Julian Seward et al.
==3215== Using Valgrind-3.7.0 and LibVEX; rerun with -h for copyright info
==3215== Command: ./personinfo2
==3215== 
Enter your first name: thames
Enter your last name: thames
Here's the information in a single string: thames, thames
==3215== Mismatched free() / delete / delete []
==3215==    at 0x4C2A44B: operator delete(void*) (in /usr/lib/valgrind/vgpreload_memcheck-amd64-linux.so)
==3215==    by 0x400D1F: main (personinfo2.cpp:27)
==3215==  Address 0x5a06040 is 0 bytes inside a block of size 15 alloc'd
==3215==    at 0x4C2AAA4: operator new[](unsigned long) (in /usr/lib/valgrind/vgpreload_memcheck-amd64-linux.so)
==3215==    by 0x400C87: main (personinfo2.cpp:19)
==3215== 
==3215== 
==3215== HEAP SUMMARY:
==3215==     in use at exit: 0 bytes in 0 blocks
==3215==   total heap usage: 1 allocs, 1 frees, 15 bytes allocated
==3215== 
==3215== All heap blocks were freed -- no leaks are possible
==3215== 
==3215== For counts of detected and suppressed errors, rerun with: -v
==3215== ERROR SUMMARY: 1 errors from 1 contexts (suppressed: 2 from 2)

Why do you not use std::string?