Why do I get these disordered and unexpected result in the output file?

[CHINA_yanglei]

c++:

#include <iostream> #include <fstream> int main() {         char str[]="yanglei";         char sta[]=" welcome to you!";         static char buffer[20];         std::ofstream os("test.txt");         os.write(str,sizeof(str));         os.clear();         os.close();         std::ifstream is("test.txt");         is.seekg(0);         is.read(buffer,sizeof(buffer));         std::cout.write(buffer,sizeof(buffer));         is.close();         std::ofstream appos("test.txt",std::ios::app);         appos.seekp(0);         appos<<'Y';         appos.seekp(4);         appos<<'L;         appos.seekp(sizeof(str));         appos.write(sta,sizeof(sta));         appos.close();         system("pause");         return 0; }

I want to be a result like "YangLeiwelcome to you!",but in fact,the result is not a unexpected one.Thank you for your help very much!

Mod Edit: Remember to use syntax tags! Thanks

code:

[syntax="cpp"]Code Here[/syntax]

[Insectoid]

..can you describe the result?

[CHINA_yanglei]

The result is like "Y L welcome to you!"
Oh,I'm sorry about a negligence.
"std::ofstream appos("test.txt",std::ios::app); "
should be
"std::ofstream appos("test.txt",std::ios::ate); "