"Unable to open file", when the program tries to open file in /proc

Question

Hi, I try to read file /proc/'pid'/status, using c program. The code is as follows, and even I use sudo to run it, the prompt still keeps throwing "Unable to open file". Please let me know if you have any ideas on how to fix this. thanks

Richard

...

int main (int argc, char* argv[]) {  
  string line;  
  char* fileLoc;  
  if(argc != 2)  
  {  
     cout << "a.out file_path" << endl;  
     fileLoc = "/proc/net/dev";  
  } else {  
     sprintf(fileLoc, "/proc/%d/status", atoi(argv[1]));  
  }  
  cout<< fileLoc << endl;  

  ifstream myfile (fileLoc);  
  if (myfile.is_open())  
  {  
    while (! myfile.eof() )  
    {  
      getline (myfile,line);  
      cout << line << endl;  
    }  
    myfile.close();  
  }  

  else cout << "Unable to open file";  

  return 0;  
}  

Answer 1

You have not allocated memory for the char array pointed by fileLoc

char* fileLoc; // just a char pointer...pointing to some random location.
.
.
sprintf(fileLoc, "/proc/%d/status", atoi(argv[1])); 

Allocate the array dynamically and free it later or you can use a static array with suitable size or even better use the C++ string.

Answer 2

Avoid using C strings in C++. You forgot to allocate this one. A stringstream will allocate for you and has sprintf functionality.

int main (int argc, char* argv[]) {  
  string line;
  ostringstream fileLoc;
  if(argc != 2)  
  {  
     cout << "a.out file_path" << endl;  
     fileLoc << "/proc/net/dev";  
  } else {  
     fileLoc << "/proc/" << argv[1] << "/status";
  }  
  cout<< fileLoc.str() << endl;  

  ifstream myfile (fileLoc.str().c_str());