glibc invalid pointer error

Hello,
after Compilation with the gnu compiler ( g++-4.2 dbtest.cpp -Wall -pedantic -Wextra -o test -g ) and running the Program, i get the following error:

Code:

*** glibc detected *** ./test: free(): invalid pointer: 0x0804b04f ***
======= Backtrace: =========
/lib/i686/cmov/libc.so.6[0xb7d998f5]
/lib/i686/cmov/libc.so.6(cfree+0x90)[0xb7d9d360]
/usr/lib/libstdc++.so.6(_ZdlPv+0x21)[0xb7f67151]
/usr/lib/libstdc++.so.6(_ZNSs4_Rep10_M_destroyERKSaIcE+0x1d)[0xb7f439ed]
/usr/lib/libstdc++.so.6(_ZNSsD1Ev+0x51)[0xb7f453c1]
./test[0x80490e6]
./test[0x804915b]
./test(__gxx_personality_v0+0x4d4)[0x804905c]
/lib/i686/cmov/libc.so.6(__libc_start_main+0xe0)[0xb7d44450]
./test(__gxx_personality_v0+0x49)[0x8048bd1]
======= Memory map: ========
08048000-0804a000 r-xp 00000000 08:03 4726528    /home/ben/dateien/arbeit/coding/c++/projects/filedb/test
0804a000-0804b000 rw-p 00001000 08:03 4726528    /home/ben/dateien/arbeit/coding/c++/projects/filedb/test
0804b000-0806c000 rw-p 0804b000 00:00 0          [heap]
b7c00000-b7c21000 rw-p b7c00000 00:00 0 
b7c21000-b7d00000 ---p b7c21000 00:00 0 
b7d2d000-b7d2e000 rw-p b7d2d000 00:00 0 
b7d2e000-b7e76000 r-xp 00000000 08:03 147285     /lib/i686/cmov/libc-2.7.so
b7e76000-b7e77000 r--p 00148000 08:03 147285     /lib/i686/cmov/libc-2.7.so
b7e77000-b7e79000 rw-p 00149000 08:03 147285     /lib/i686/cmov/libc-2.7.so
b7e79000-b7e7c000 rw-p b7e79000 00:00 0 
b7e7c000-b7e88000 r-xp 00000000 08:03 784965     /lib/libgcc_s.so.1
b7e88000-b7e89000 rw-p 0000b000 08:03 784965     /lib/libgcc_s.so.1
b7e89000-b7e8a000 rw-p b7e89000 00:00 0 
b7e8a000-b7ead000 r-xp 00000000 08:03 147290     /lib/i686/cmov/libm-2.7.so
b7ead000-b7eaf000 rw-p 00023000 08:03 147290     /lib/i686/cmov/libm-2.7.so
b7eaf000-b7f91000 r-xp 00000000 08:03 1880512    /usr/lib/libstdc++.so.6.0.10
b7f91000-b7f94000 r--p 000e2000 08:03 1880512    /usr/lib/libstdc++.so.6.0.10
b7f94000-b7f96000 rw-p 000e5000 08:03 1880512    /usr/lib/libstdc++.so.6.0.10
b7f96000-b7f9c000 rw-p b7f96000 00:00 0 
b7fb5000-b7fb8000 rw-p b7fb5000 00:00 0 
b7fb8000-b7fd2000 r-xp 00000000 08:03 4137293    /lib/ld-2.7.so
b7fd2000-b7fd4000 rw-p 00019000 08:03 4137293    /lib/ld-2.7.so
bf95f000-bf974000 rw-p bffeb000 00:00 0          [stack]
ffffe000-fffff000 r-xp 00000000 00:00 0          [vdso]

Here is an extraction of the code, but please read first my comments at the end, before analysing it.

filedb.h:

Code:

template <typename typ>
class filedb{
  typedef const typ ctyp;

  private:
    char path_db[512];
    unsigned int number_of_records;
    typ* recarray;

    //shall be positiv if rec1 > rec2, negativ if rec1 < rec2
    //and 0 if rec1 = rec2
    int (*comp)( const typ& rec1, const typ& rec2 );

  public:

    //opens file with records and reads all records into memory
    filedb( std::string path_db,
	    int (*comp)( const typ& rec1, const typ& rec2 ) );
    ~filedb();

    //inserts record into database (does not write change to file)
    void insert( const typ* record );
};

filedb.cpp:

Code:

//opens file with records and reads all records into memory
template <typename typ>
filedb<typ>::filedb( std::string p_db,
                     int (*comp)( const typ& rec1, const typ& rec2 ) ) :
                     comp(comp) 
{
  strcpy( path_db, p_db.c_str() );

  number_of_records = 0;
  recarray = new typ[number_of_records];
}

//Destructor
template <typename typ>
filedb<typ>::~filedb(){
  delete [] recarray;
}


//inserts record into database (does not write change to file)
template <typename typ>
void filedb<typ>::insert( const typ* record ){
  typ* rectemp = recarray;
  recarray = new typ[number_of_records + 1];
  memcpy( recarray, rectemp, number_of_records*sizeof(typ) );
  delete [] rectemp;

  ++number_of_records;

  int i = number_of_records-1;
  for( ; i > 0 && comp(recarray[i], *record); --i ){
    memcpy( &recarray[i], &recarray[i-1], sizeof(typ) );
  }

  //insert record to the right place
  memcpy( &recarray[i], record, sizeof(typ) );                      //<------- the important line
}

dbtest.cpp

Code:

int main(){
  filedb<record> db( "./hello", comperator );

  record rec1;
  record rec2;
  record rec3;
  record rec4;
  record rec5;
  
  rec1.id = 5;
  rec1.str = "ich bins";

  rec2.id = 2;
  rec2.str = "du bists";

  rec3.id = 4;
  rec3.str = "wir sinds";

  db.insert( &rec1 );
  db.insert( &rec2 );
  db.insert( &rec3 );
}

Running gdb i discoverd, that the error occeurs when the programm frees the memory of recarray in the destructor. Furthermore, i discoverd, that after uncommenting "the important line" from filedb.cpp, the error ceased to exist. The memcpy function seems to be the evil element here, but i cannot figure out, why it leads to this error.
Any help appreciated
Ben

Code:

//inserts record into database (does not write change to file)
template <typename typ>
void filedb<typ>::insert( const typ* record ){
  typ* rectemp = recarray;
  recarray = new typ[number_of_records + 1];
  memcpy( recarray, rectemp, number_of_records*sizeof(typ) );
  delete [] rectemp;

  ++number_of_records;

  int i = number_of_records-1;
  for( ; i > 0 && comp(recarray[i], *record); --i ){
    memcpy( &recarray[i], &recarray[i-1], sizeof(typ) );
  }

  //insert record to the right place
  memcpy( &recarray[i], record, sizeof(typ) );                      //<------- the important line
}

Isn't 'i' equal to -1 when you end the loop?

Edit: or at least, i = -1 if there are NO records in the list

--
Mats

Hi, thanks for the answer, but i don't think i will ever be -1, cause the decremention of i only is applied, after every loop, so it wont even be applied for i being 0 at the beginning.

edit: it definitly wont get to -1, testet it with following program:

Code:

#include <iostream>
int main(){
  int i = 0;
  for( ; i > 0; --i ) ;
  std::cout << i << std::endl;
}

Output: 0

You should not be using memcpy() unless the type is restricted to a POD type.

Thank you a lot,
that was my mistake and i'm sorry, that i left out the definition of record, which was a struct including a string.
Ben