C ++ ReadFile，CreateProcess，管道标准输出重定向，输出翻倍

Question

i have this piece of code :

        for (;;) {
            BOOL ReadSuccess = ReadFile(rdPipe, StdOutBuffer, 8192, &dwRead, NULL);

            if (strlen(StdOutBuffer) <= 0) {
                Sleep(100);
                send(sock, RECIEVE_BREAK, strlen(RECIEVE_BREAK), 0);
                break;
            }

            else if (!ReadSuccess || dwRead == 0) {
                Sleep(100);
                send(sock, RECIEVE_BREAK, strlen(RECIEVE_BREAK), 0);
                break;
            }

            else {
                send(sock, StdOutBuffer, strlen(StdOutBuffer), 0);
            }

            cout << StdOutBuffer << endl;
            cout << "\n\n\n\n";
        }

And i have problem with doubled output in my StdOutBuffer, for example, when i call "tasklist" i've got some parts twice.

Dont u someone know where can be problem?

Answer 1

The function ReadFile() doesn't read a null terminated c-string, but a block of (here 8192) chars without any guarantee to have a null terminator.

Therefore, calling strlen(StdOutBuffer) is not guaranteed to give any useful result. You should rather rely on dwRead instead. If you read text data that might be not null terminated (in the file or, as it seems, via the pipe), you should add it yourself:

 StdOutBuffer[dwLen] = '\0'; 

This works even if the read fails, because the first thing ReadFile() does is to set the length to 0. Note that your buffer should then have a size of at least 8192+1 char.

Why do you get the error ? Suppose you receive a full line of data, and that fortunately, it includes some null terminator. You'd then process it. Suppose now that in the next iteration you'd only receive a couple of chars (eg just a newline): the remaining of the buffer would not overwritten, and as you'd not have a null terminator at the end of the new bytes read, strlen() would cause to think there is more data to process than there is; some data would be processed a second time.