#include <stdio.h>
#include <shlobj.h>

int main(int argc, char *argv[])
{
    const char *szFileName = "text.txt";
    const char *szContent = "test string";

    char szPath[_MAX_PATH];

    SHGetSpecialFolderPath(NULL, szPath, CSIDL_DESKTOPDIRECTORY, FALSE);

    strcat(szPath, "\\");
    strcat(szPath, szFileName);

    FILE *pFile = fopen(szPath, "a+");

    if(pFile != NULL)
    {
        fputs(szContent, pFile);
        fclose(pFile);
    }
    return 0;
}