to store char* from function return value

I am trying to implement a function which reads from Serial Port ( Linux) and retuns char*. 开发者_运维技巧The function works fine but how would I store return value from function. example of function is

char  *ReadToSerialPort()
{
 char *bufptr;
 char buffer[256];  // Input buffer/ /
 //char *bufptr;      // Current char in buffer //
 int  nbytes;       // Number of bytes read //

 bufptr = buffer;

 while ((nbytes = read(fd, bufptr, buffer+sizeof(buffer)-bufptr -1 )) > 0)
 {
  bufptr += nbytes;
  //  if (bufptr[-1] == '\n' || bufptr[-1] == '\r')
  /*if ( bufptr[sizeof(buffer) -1] == '*' && bufptr[0] == '$' )
  {
   break;
  }*/

 } // while ends


 if ( nbytes ) return bufptr;
 else return 0;


 *bufptr = '\0';

} // end ReadAdrPort


//In main
int main( int argc , char *argv[])
{ 
  char *letter;
  if(strcpy(letter,  ReadToSerialPort()) >0 )
  {
   printf("Response is %s\n",letter);
  }
}

You should allocate a buffer at heap with malloc, and return it. The users of your function will be responsible for deallocating the memory (and your documentation has to clearly state this!)

A simple change would be

char* buffer = (char*)malloc(256);
// beware that now `sizeof(buffer)` will be not 256 any more, but 4, so
// you have to define your constant for it.
...
if (nbytes) return buffer;
free(buffer);
return 0;

...
int main(int argc, char *argv[])
{ 
    char *letter = ReadToSerialPort();
    if (letter)
    {
        printf("Response is %s\n", letter);
        free(letter);
        return 0;
    }
    return 1;
}

Please note that the code *bufptr = '\0'; should be before return, not after!

EDIT
Does your code look like this:

char *ReadToSerialPort()
{
    const int buffer_size = 256;
    char *buffer = (char *)malloc(buffer_size);
    char *bufptr = buffer;
    int  nbytes;

    while ((nbytes = read(fd, bufptr, buffer+buffer_size-bufptr-1)) > 0)
    {
        bufptr += nbytes;
    }

    *bufptr = '\0';

    if (bufptr != buffer)
        return bufptr;
    // else cleaning up
    free(buffer);
    return 0;
}

I am curious where does fd come from?

Fix up your main code to look like this:

//In main
int main( int argc , char *argv[])
{ 
  char *letter = ReadToSerialPort();
  if(letter != NULL)
  {
   printf("Response is %s\n",letter);
  }
}

Make sure you use the buffer declared as static within ReadToSerialPort()..... i.e.:

static char buffer[256];

You need to specify your function a bit more. You can not simply say it "returns a char *". Where are the characters it points to? In a static buffer? On the heap (allocated by new)? It looks like you are trying to return a pointer to a local buffer (allocated on the stack), which is an error. Alternatively, return a std::string.

You should change the signature of ReadToSerialPort() to also inform the caller how many bytes you are returning. So you could do either this:

int ReadToSerialPort(char** data);

or this:

void ReadToSerialPort(char** data, int* num_of_bytes);

and you stay responsible for allocating memory space inside ReadToSerialPort().

The user would do something like (not tested):

int main( int argc , char *argv[])
{ 
  char* data = NULL;
  int count = 0;
  ReadToSerialPort(data, &count);
  if (data != NULL && count > 0) // Let's suppose count returns as 5
  {
    printf("data[0]:%x data[1]:%x data[2]:%x data[3]:%x data[4]:%x\n", data[0], data[1], data[2], data[3], data[4]);
  }

  // and the user is responsible for deallocating data himself
  free(data);

  return 0;
}