CODEWITHSUNDEEP
cc++11c-stringsstrcpystrncpy
AlexG
AlexG

Reputation: 5919

Value of char array changing, upon calling strcpy on a different char array

When experimenting ways to copy strings to a C style array, due to some weird limitations I stumbled upon this weird behavior, firstly trying to copy the string with std::string::copy() seems to work fine until I'm trying to copy the same string from another variable into another empty C style array with std::strcpy(), then the contents from the first array get some remaining content from the string.

#include <iostream>
#include <string>
#include <algorithm>
#include "string.h"

int main()
{
    std::string str = "Lorem ipsum dolor sit amet, consetetur sadipscing elitr, sed diam nonumy eirmod tempor invidunt ut l";
    std::string str2 = "Lorem ipsum dolor sit amet, consetetur sadipscing elitr, sed diam nonumy eirmod tempor invidunt ut l";

    char cstr[64] = {0};
    char cstr2[64] = {0};

    str.copy(cstr, std::min(str.size(), sizeof(cstr) / sizeof(cstr[0]) - 1));

    // cstr:  "Lorem ipsum dolor sit amet, consetetur sadipscing elitr, sed di"
    std::cout << "cstr:  \"" << cstr << "\"" << std::endl;

    strcpy(cstr2, str2.c_str());
    cstr2[sizeof(cstr2) / sizeof(cstr2[0]) - 1] = '\0';

    // cstr:  "m nonumy eirmod tempor invidunt ut l"
    std::cout << "cstr:  \"" << cstr << "\"" << std::endl;

    // cstr2: "Lorem ipsum dolor sit amet, consetetur sadipscing elitr, sed di"
    std::cout << "cstr2: \"" << cstr2 << "\"" << std::endl;    
    return 0;
}

Surely I'm just missing some stupid cruicial point here.

Upvotes: 2

Views: 132

Answers (2)

Vlad from Moscow
Vlad from Moscow

Reputation: 310950

In the first case

str.copy(cstr, std::min(str.size(), sizeof(cstr) / sizeof(cstr[0]) - 1));

you limited the number of copied characters by the value of sizeof( cstr ) - 1.

So the array cstr contains a string because it was zero-initialized.

In the second case you did not limit the number of copied characters.

strcpy(cstr2, str2.c_str());

So the memory beyond the array cstr2 was overwritten because the string pointed to by str2.c_str() contains more characters than the size of the array cstr2.

Instead you could write

strncpy( cstr2, str2.c_str(), sizeof( cstr2 ) - 1 );

using the function strncpy.

Upvotes: 1

chux
chux

Reputation: 153358

strcpy(cstr2, str2.c_str()); is undefined behavior as that attempted to copy more than 64 bytes.

Rest of code is irrelevant.

Lorem ipsum dolor sit amet, consetetur sadipscing elitr, sed diam nonumy eirmod tempor invidunt ut l
1234567890123456789012345678901234567890123456789012345678901234

Upvotes: 0

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