Pwan Pwan - 3 months ago 23
C Question

Char* function Never Return value

I'm a newbie of C. Here I write a function to parse http post from browser. Here is my code:

char* HTTP_POST_GET_BODY(char* HttpPost){

char* HttpPostBody = strstr(HttpPost,"\r\n\r\n");
HttpPostBody = HttpPostBody + 4;
if(strcmp(HttpPostBody,"\r\n\r\n") != 0 && strcmp(HttpPostBody,"") != 0){
return HttpPostBody;
}
else{
char* HttpPostBody_IE;
HttpPostBody = strstr(HttpPost,"::");
char* HttpPostBodyEnd = strstr(HttpPost,"HTTP/1.1");
int body_length = HttpPostBodyEnd - HttpPostBody;
strncpy(HttpPostBody_IE,HttpPostBody+2,body_length-2);
return HttpPostBody_IE;
}

}


So basically, if the procedure goes in the "else" it should return a char pointer to caller. I check the debugger. HttpPostBody_IE has a value but when it return it is a null string.

char* http_body = HTTP_POST_GET_BODY(recieve_buffer);


Anyone has an idea about it?

Answer

You declare the pointer-variable HttpPostBody_IE but never allocate memory for it.
The call to strncpy(....) should create a core dump. Try this:

int body_length = HttpPostBodyEnd - HttpPostBody;
HttpPostBody_IE = (char*)malloc(body_length+1);
strncpy(HttpPostBody_IE,HttpPostBody+2,body_length-2);
return HttpPostBody_IE;

Of course, make sure the caller of this functions releases the allocated memory afterwards.
You have a problem in case the function returns from within the if statement. This because no memory is allocated in that case.

You might resolve it this way:

static char HttpPostBody_IE[BIG_ENOUGH_FOR_ANY_SOURCE];
if (....)
{
  ...
}
else
{
  ...
  strncpy(HttpPostBody_IE, ...);
  return (HttpPostBody_IE);
}

Please notice that in this way the variable needs to be static.