Search code examples
c++fwrite

c++ binary write or read

I have a problem when I tried to make a conversion of a text to binary format:

As described in the code below, it worked well for test3.txt or test4.txt, however, fout3.write(str1.data() ,len); doesn't work for test5.txt (WHICH REALLY puzzled me) so I used fout3.write(reinterpret_cast<char *>(& str1) ,len); instead, and it worked well.

#include <cassert>
#include <iostream>
#include <fstream>
#include <string>

using namespace std;

int main(void)
{
   ofstream fout("test3.txt", ios::out | ios::binary);
   fout << "ABC\n";
   fout.close();

   int test =  111;
   ofstream fout1("test4.txt", ios::out | ios::binary);
   fout1.write(reinterpret_cast<char *>(&test), sizeof(int));
   fout1.close();

   int test2;
   ifstream fin("test4.txt", ios::in | ios::binary);
   fin.read(reinterpret_cast<char *>(&test2), sizeof(int));
   cout << test2 << endl;

   string str1 = "ATEGWGEf";
   int len = str1.length();
   cout << str1.data() << endl;
   ofstream fout3("test6.txt", ios::out | ios::binary);
// fout3.write(str1.data() ,len);
   fout3.write(reinterpret_cast<char *>(& str1) ,len);
   fout3.close();

   string line;
   ifstream fin2("test6.txt",  ios::in | ios::binary);
   fin2.read(reinterpret_cast<char *>(&line),300);
   cout << line << endl;

   return 0;
}

But another question come to me. When I tried to read test6.txt, which was just wrote, it failed with error like this.

*** glibc detected *** ./binary_file.debug: double free or corruption (fasttop): 0x0000000001e4c260 ***
======= Backtrace: =========
/lib64/libc.so.6[0x3155275366]
/usr/lib64/libstdc++.so.6(_ZNSsD1Ev+0x39)[0x317209d4c9]
./binary_file.debug[0x4010f3]
/lib64/libc.so.6(__libc_start_main+0xfd)[0x315521ecdd]
./binary_file.debug[0x400d69]
======= Memory map: ========
......................
01e4a000-01e6b000 rw-p 00000000 00:00 0                                  [heap]
3154a00000-3154a20000 r-xp 00000000 fd:00 3031286                        /lib64/ld-2.12.so
3154c1f000-3154c20000 r--p 0001f000 fd:00 3031286                        /lib64/ld-2.12.so
3154c20000-3154c21000 rw-p 00020000 fd:00 3031286                        /lib64/ld-2.12.so
3154c21000-3154c22000 rw-p 00000000 00:00 0
3155200000-3155389000 r-xp 00000000 fd:00 3033037                        /lib64/libc-2.12.so
3155389000-3155588000 ---p 00189000 fd:00 3033037                        /lib64/libc-2.12.so
3155588000-315558c000 r--p 00188000 fd:00 3033037                        /lib64/libc-2.12.so
315558c000-315558d000 rw-p 0018c000 fd:00 3033037                        /lib64/libc-2.12.so
315558d000-3155592000 rw-p 00000000 00:00 0
3155600000-3155683000 r-xp 00000000 fd:00 3033062                        /lib64/libm-2.12.so
3155683000-3155882000 ---p 00083000 fd:00 3033062                        /lib64/libm-2.12.so
3155882000-3155883000 r--p 00082000 fd:00 3033062                        /lib64/libm-2.12.so
3155883000-3155884000 rw-p 00083000 fd:00 3033062                        /lib64/libm-2.12.so
3171c00000-3171c16000 r-xp 00000000 fd:00 3014674                        /lib64/libgcc_s-4.4.7-20120601.so.1
3171c16000-3171e15000 ---p 00016000 fd:00 3014674                        /lib64/libgcc_s-4.4.7-20120601.so.1
3171e15000-3171e16000 rw-p 00015000 fd:00 3014674                        /lib64/libgcc_s-4.4.7-20120601.so.1
3172000000-31720e8000 r-xp 00000000 fd:00 1719247                        /usr/lib64/libstdc++.so.6.0.13
31720e8000-31722e8000 ---p 000e8000 fd:00 1719247                        /usr/lib64/libstdc++.so.6.0.13
31722e8000-31722ef000 r--p 000e8000 fd:00 1719247                        /usr/lib64/libstdc++.so.6.0.13
31722ef000-31722f1000 rw-p 000ef000 fd:00 1719247                        /usr/lib64/libstdc++.so.6.0.13
31722f1000-3172306000 rw-p 00000000 00:00 0
7f1142209000-7f114220e000 rw-p 00000000 00:00 0
7f114222d000-7f1142230000 rw-p 00000000 00:00 0
7fff7ff9c000-7fff7ffb1000 rw-p 00000000 00:00 0                          [stack]
7fff7ffff000-7fff80000000 r-xp 00000000 00:00 0                          [vdso]
ffffffffff600000-ffffffffff601000 r-xp 00000000 00:00 0                  [vsyscall]
Aborted (core dumped)

Together with this error, I successfully convert binary test6.txt to texts by accident. I really want to know why and how to fix this bug.

Thx!

Solution

  • Well the problem with reading is in this line:

    fin2.read(reinterpret_cast<char *>(&line),300);

    What are you doing is casting the address of string object into a char * pointer. Then you are overwriting its internal content. This is wrong and it will never work. Some people suggest using this:

    fin2.read(reinterpret_cast<char *>(&line.c_str()),300);

    Well it can compile, but it should crash at runtime. The reason is that std::string::c_str() returns a const char * and you are casting away the const with reinterpret_cast. Also this is UB because the C++98 standard says:

    A program shall not alter any of the characters in this sequence.