comparing char buffer to string not working

Question

When trying to compare a char buffer to a std string in an if statement it's not working as intended Here is the code

if (ConnectNamedPipe(hPipe, NULL) != FALSE)   // wait for someone to connect to the pipe
    {
        while (ReadFile(hPipe, buffer, sizeof(buffer) - 1, &dwRead, NULL) != FALSE)
        {
            /* add terminating zero */
            buffer[dwRead] = 0x00;

            /* do something with data in buffer */
            printf("%s\n", buffer);
            
            string cmd = bufferToString(buffer, sizeof(buffer));

            printf("%s", cmd.c_str());

            if (cmd.c_str() == "help") //HERE is the issue
            {
                printf("hello");
            }

        }
    }

When comparing it doesn't work I've tried using different types of conversions of char buffer[1024] to a string but not getting anywhere

EDIT: I've tried so far

    cmd.resize(dwRead);
if (cmd == string("help"))

and

if (0 == ::std::strcmp(buffer, "help"))

none of them work

Does this answer your question? What is the proper function for comparing two C-style strings? See also: C++ Compare char array with string — scohe001
– scohe001, Commented Mar 30, 2021 at 16:07

Ted Lyngmo · Accepted Answer · 2021-03-30 16:19:22Z

1

Instead of reading the data into a char[] and then copying that data into a std::string, you could use a std::string directly.

Building blocks:

std::string cmd;

cmd.resize(wanted_buffer_size); // Set the proper buffer size

ReadFile(hPipe, cmd.data(), cmd.size(), &dwRead, nullptr); // Read directly into cmd

cmd.resize(dwRead); // Shrink down to dwRead afterwards. Automatically null terminated.

answered Mar 30, 2021 at 16:19

Ted Lyngmo

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2 Comments

DavidTheTech Over a year ago

Thank you very much I'm new to strings and chars in c++ and this has helped a ton

Ted Lyngmo Over a year ago

@DavidTheTech Glad to hear it! You're welcome!

user7860670 · Accepted Answer · 2021-03-30 16:07:29Z

0

You are creating string copying sizeof(buffer) characters while it only contains dwRead of initialized characters.

Then you compare two pointers instead of string content (which still won't work because cmd has wrong length). You should actually compare using strcmp without creating temporary std::string object.

if (0 == ::std::strcmp(buffer, "help"))
{
    printf("hello");
}

answered Mar 30, 2021 at 16:07

user7860670

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Comments

Devolus · Accepted Answer · 2021-03-30 16:15:28Z

0

You should either do

cmd.resize(dwRead);

This will set the string to the length of the actual read data, instead of the whole buffer. A c++ std::string can contain any data , including 0-bytes.

Or you must call

string cmd = bufferToString(buffer, dwRead);

Which should have the same effect (without seeing the implementation of bufferToString).

Also your comparison is wrong. In c++ you would do

if (cmd == "help")

edited Mar 30, 2021 at 16:15

answered Mar 30, 2021 at 16:10

Devolus

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Comments

Kola73 · Accepted Answer · 2021-03-30 16:10:18Z

-1

This should work:

if (cmd == string("help")) // should work
{
    printf("hello");
}

The code:

if (cmd.c_str() == "help")

will compare pointers (#1 returned from cmd.c_str() , #2 - constant pointer to "help"), that is not quite correct

answered Mar 30, 2021 at 16:10

Kola73

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