Question

出于某种原因，Valgrind似乎在调用ifstream.read的行中（在searchInFile函数中）进行无效写入。 gdb似乎在同一行上有段错误，但我无法弄清楚问题。如果正在调用读取函数，那么如何进行写入操作？

#include <iostream>
#include <fstream>
#include <cstring>
#include <cstdlib>

#define FILENAME "./test.dat"

using namespace std;

struct record
{
    long long int phoneNumber;
    char name[10];
    char address[75];
};

int searchInFile(long long int phoneNo)
{
    /*
    Search if a record with the given phone no.
    exists in the file. Returns the number of 
    records before the required record. Returns
    -1 if no such record exists.
    */

    int ctr = 0;
    record * buffer = NULL;
    ifstream file(FILENAME, ios::binary);

    while(file.read( reinterpret_cast<char*>(buffer), sizeof(record)))
    {
        if(buffer->phoneNumber == phoneNo)
        {
            file.close();
            return(ctr);
        }

        ctr++;
    }

    file.close();

    if(buffer->phoneNumber == phoneNo)
        return(ctr);

    return(-1);
}

void displayRecord(long long int phoneNo)
{
    /*
    Displays the record with given phone no.
    */

    int location = searchInFile(phoneNo);
    record * buffer = NULL;


    if(location == -1)
        cout << "Invalid Number! No such record exists!" << endl;

    else
    {
        ifstream file(FILENAME, ios::binary);

        cout << "Reading Error Code: " << file.read(reinterpret_cast<char*>(buffer), sizeof(record));

        cout << "Phone Number: " << buffer->phoneNumber << endl;
        cout << "Name: " << buffer->name << endl;
        cout << "Address: " << buffer->address << endl;

        file.close();
    }
}

void saveRecord(record toSave)
{
    /*
    Saves a new record to file
    */
    ofstream ofile(FILENAME, ios::app | ios::binary);
    //ofile.seekp(0, ios::end);
    ofile.write( reinterpret_cast<char*>(&toSave), sizeof(record));
    ofile.close();
}


int main()
{
    record test = {(long long int)9988776655, "Debug", "Address of Debugger"};
    saveRecord(test);
    displayRecord((long long int) 9988776655);
    return(0);
}

这是Valgrind和gdb的输出：

    Syscall param write(buf) points to uninitialised byte(s)
   at 0x543B870: __write_nocancel (syscall-template.S:81)
   by 0x4EB18D5: ??? (in /usr/lib/x86_64-linux-gnu/libstdc++.so.6.0.19)
   by 0x4EE9A77: std::basic_filebuf<char, std::char_traits<char> >::_M_convert_to_external(char*, long) (in /usr/lib/x86_64-linux-gnu/libstdc++.so.6.0.19)
   by 0x4EEA4D2: std::basic_filebuf<char, std::char_traits<char> >::overflow(int) (in /usr/lib/x86_64-linux-gnu/libstdc++.so.6.0.19)
   by 0x4EE9CA2: std::basic_filebuf<char, std::char_traits<char> >::_M_terminate_output() (in /usr/lib/x86_64-linux-gnu/libstdc++.so.6.0.19)
   by 0x4EE9D4A: std::basic_filebuf<char, std::char_traits<char> >::close() (in /usr/lib/x86_64-linux-gnu/libstdc++.so.6.0.19)
   by 0x4EEBB4C: std::basic_ofstream<char, std::char_traits<char> >::close() (in /usr/lib/x86_64-linux-gnu/libstdc++.so.6.0.19)
   by 0x4011A5: saveRecord(record) (test.cpp:84)
   by 0x4012F1: main (test.cpp:91)
 Address 0x5a1b31d is 93 bytes inside a block of size 8,192 alloc'd
   at 0x4C2B8A8: operator new[](unsigned long) (vg_replace_malloc.c:423)
   by 0x4EE98AB: std::basic_filebuf<char, std::char_traits<char> >::_M_allocate_internal_buffer() (in /usr/lib/x86_64-linux-gnu/libstdc++.so.6.0.19)
   by 0x4EE9EC1: std::basic_filebuf<char, std::char_traits<char> >::open(char const*, std::_Ios_Openmode) (in /usr/lib/x86_64-linux-gnu/libstdc++.so.6.0.19)
   by 0x4EEB747: std::basic_ofstream<char, std::char_traits<char> >::basic_ofstream(char const*, std::_Ios_Openmode) (in /usr/lib/x86_64-linux-gnu/libstdc++.so.6.0.19)
   by 0x40117E: saveRecord(record) (test.cpp:81)
   by 0x4012F1: main (test.cpp:91)

Invalid write of size 1
   at 0x4ED338E: std::basic_streambuf<char, std::char_traits<char> >::xsgetn(char*, long) (in /usr/lib/x86_64-linux-gnu/libstdc++.so.6.0.19)
   by 0x4EE8D0D: std::basic_filebuf<char, std::char_traits<char> >::xsgetn(char*, long) (in /usr/lib/x86_64-linux-gnu/libstdc++.so.6.0.19)
   by 0x4EB4CAA: std::istream::read(char*, long) (in /usr/lib/x86_64-linux-gnu/libstdc++.so.6.0.19)
   by 0x400EDD: searchInFile(long long) (test.cpp:30)
   by 0x400FAA: displayRecord(long long) (test.cpp:55)
   by 0x401300: main (test.cpp:92)
 Address 0x0 is not stack'd, malloc'd or (recently) free'd


Process terminating with default action of signal 11 (SIGSEGV)
 Access not within mapped region at address 0x0
   at 0x4ED338E: std::basic_streambuf<char, std::char_traits<char> >::xsgetn(char*, long) (in /usr/lib/x86_64-linux-gnu/libstdc++.so.6.0.19)
   by 0x4EE8D0D: std::basic_filebuf<char, std::char_traits<char> >::xsgetn(char*, long) (in /usr/lib/x86_64-linux-gnu/libstdc++.so.6.0.19)
   by 0x4EB4CAA: std::istream::read(char*, long) (in /usr/lib/x86_64-linux-gnu/libstdc++.so.6.0.19)
   by 0x400EDD: searchInFile(long long) (test.cpp:30)
   by 0x400FAA: displayRecord(long long) (test.cpp:55)
   by 0x401300: main (test.cpp:92)
 If you believe this happened as a result of a stack
 overflow in your program's main thread (unlikely but
 possible), you can try to increase the size of the
 main thread stack using the --main-stacksize= flag.
 The main thread stack size used in this run was 8388608.

HEAP SUMMARY:
    in use at exit: 8,760 bytes in 2 blocks
  total heap usage: 4 allocs, 2 frees, 17,520 bytes allocated

LEAK SUMMARY:
   definitely lost: 0 bytes in 0 blocks
   indirectly lost: 0 bytes in 0 blocks
     possibly lost: 0 bytes in 0 blocks
   still reachable: 8,760 bytes in 2 blocks
        suppressed: 0 bytes in 0 blocks

ERROR SUMMARY: 2 errors from 2 contexts (suppressed: 0 from 0)
Segmentation fault (core dumped)

请注意，每次运行前都删除了test.dat文件。

    Program received signal SIGSEGV, Segmentation fault.
0x00007ffff7b7338e in std::basic_streambuf<char, std::char_traits<char> >::xsgetn(char*, long) () from /usr/lib/x86_64-linux-gnu/libstdc++.so.6
(gdb) bt
#0  0x00007ffff7b7338e in std::basic_streambuf<char, std::char_traits<char> >::xsgetn(char*, long) () from /usr/lib/x86_64-linux-gnu/libstdc++.so.6
#1  0x00007ffff7b88d0e in std::basic_filebuf<char, std::char_traits<char> >::xsgetn(char*, long) () from /usr/lib/x86_64-linux-gnu/libstdc++.so.6
#2  0x00007ffff7b54cab in std::istream::read(char*, long) () from /usr/lib/x86_64-linux-gnu/libstdc++.so.6
#3  0x0000000000400ede in searchInFile (phoneNo=9988776655) at test.cpp:30
#4  0x0000000000400fab in displayRecord (phoneNo=9988776655) at test.cpp:55
#5  0x0000000000401301 in main () at test.cpp:92

我唯一能想到的是buffer正在写进入这里，但这并不能让我在任何地方。

Answer 1

read（）方法将读取的信息写入缓冲区，而且我怀疑valgrind认为这封写无效，所以开始看看你的缓冲区是如何分配的。

在这种情况下，valgrind显然是正确的。您需要分配内存来缓冲它以将文件内容放入。

例如

record * buffer = new record;

Answer 2

buffer为空。您没有将任何存储分配到read，传递错误的大小（它应该是0，而不是sizeof(record)）并在以后取消引用它会导致未定义的行为。 valgrind中的write和read指的是内存访问，而不是文件读取或写入。

Valgrind中的读操作无效写入

2 个答案: