How to unit test c functions involving IO?

Question

I am facing problems in writing unit tests to C functions which involve IO operation. For example, below is the code I wrote to get an input string from the user from console. I do not know as to how to automate testing user input using getchar() function.

char * GetStringFromConsole()
{

    char *strToReturn = NULL;
    int len = 128;

    strToReturn = (char*)malloc(len);
    if (strToReturn) 
    {
        int ch;
        char *ptr  = strToReturn;
        int counter = 0;
        for (; ;) 
        {
            ch = getchar();
            counter++;

            if (counter == len)
            {
                strToReturn = realloc(strToReturn, len*=2 );
                ptr = strToReturn + counter-1;
            }

            if ((ch != EOF) && (ch != '\n') && (counter < len))
            {
                *ptr++ = ch;
            }
            else 
            {
                break;
            }

        }
        *ptr = '\0';
    }
    return strToReturn;
}   

Answer 1

Mock getchar:

1. Utilizing preprocessor, e.g. in your test file.

   #define getchar mock_getchar
   #include "GetStringFromConsole.h"
   ...
   const char* mock_getchar_data_ptr;
   char mock_getchar()
   {
       return *mock_getchar_data_ptr++;
   }
   ...
   // in test function
   mock_getchar_data_ptr = "hello!\n";
   YourAssertEquals("hello", GetStringFromConsole());
   

2. Substitute symbol for the linker(harder, in my opinion), e.g. define your own getchar somewhere in your source .c files instead of linking to a stdlib(e.g. msvcrt on windows)

3. Modify function under test to accept a function returning char, best choice(IMHO) - no conflicts with stdlib. And setup a test by passing a thingy like mock_getchar from point 1 in test code.

   typedef char (*getchartype)();
   char * GetStringFromConsole(getchartype mygetchar)
   {
       ...
       c  = mygetchar()
   

For points 1 and 2 I'd propose to use your own function instead of getchar (e.g. mygetchar) - this way you could mock/substitute it without facing conflicts with std includes/libs.