Writing to .txt with WriteFile() C++ - c++

Question

(Using C++, Windows 10, Microsoft Visual Studio 2017)
Hello, I am new to serial ports but trying to learn how to open, close, read, and write with them
Right now, I am trying to use the CreateFile() and WriteFile() functions to write to a txt file.
I created a .txt file called "write.txt" and saved it in my Documents folder. Then I added it to my project's source files via "Add" --> "Existing Item". The file is empty.
I wrote the program below, hoping I could write to the file, but after running then checking on the file, the file is still empty. The program compiles but I am guessing I have an error somewhere. Am I going about writing to a file correctly?
#include <iostream>
#include <Windows.h>
#include <string>
using namespace std;
int main() {
HANDLE write;
write = CreateFile(TEXT("\\write.txt"), GENERIC_WRITE, 0, NULL, OPEN_ALWAYS, FILE_ATTRIBUTE_NORMAL, NULL);
if (write == INVALID_HANDLE_VALUE) {
DWORD errorWrite = GetLastError();
printf("Error in opening file\n");
printf("Error = 0x%x\n", errorWrite);
}
else {
printf("Opening file successful\n");
}
WriteFile(write, "hello", 5, 0, 0);
CloseHandle(write);
return 0;
}
The output says the file was opened successfully, but the write.txt file remains empty. Any advice?
Thank you in advance :D

Answer 1

You are not accessing the text file that is located in your Documents folder. You are accessing a text file that is located at the root of the drive where the calling process's current working directory is currently pointing at.
You need to explicitly query the OS for the path to the Documents folder (ie, via SHGetFolderPath() or SHGetKnownFolderPath()), and then append your filename to that path.
Also, you are calling WriteFile() incorrectly. In particular, the lpNumberOfBytesWritten parameter is wrong:
lpNumberOfBytesWritten
A pointer to the variable that receives the number of bytes written when using a synchronous hFile parameter. WriteFile sets this value to zero before doing any work or error checking. Use NULL for this parameter if this is an asynchronous operation to avoid potentially erroneous results.
This parameter can be NULL only when the lpOverlapped parameter is not NULL.
And, you are not doing any error handling on WriteFile(), either:
If the function succeeds, the return value is nonzero (TRUE).
If the function fails, or is completing asynchronously, the return value is zero (FALSE). To get extended error information, call the GetLastError function.
Try something more like this instead:
#include <iostream>
#include <string>
#include <Windows.h>
#include <Shlobj.h>
#include <shlwapi.h>
using namespace std;
int main() {
TCHAR path[MAX_PATH] = {};
HRESULT hRes = SHGetFolderPath(NULL, CSIDL_MYDOCUMENTS, NULL, SHGFP_TYPE_CURRENT, path);
if (FAILED(hRes)) {
printf("Can't get Documents folder path\nError = 0x%08x\n", hRes);
}
else {
PathAppend(path, TEXT("write.txt"));
HANDLE write = CreateFile(path, GENERIC_WRITE, 0, NULL, OPEN_ALWAYS, FILE_ATTRIBUTE_NORMAL, NULL);
if (write == INVALID_HANDLE_VALUE) {
DWORD errorWrite = GetLastError();
printf("Can't open file\nError = %u\n", errorWrite);
}
else {
printf("File opened successful\n");
DWORD written;
if (!WriteFile(write, "hello", 5, &written, NULL)) {
DWORD errorWrite = GetLastError();
printf("Can't write to file\nError = %u\n", errorWrite);
} else {
printf("File written successful\n");
}
CloseHandle(write);
}
}
return 0;
}