feof() from FAQ

This is a discussion on feof() from FAQ within the C Programming forums, part of the General Programming Boards category; Hi guys, I've just read the FAQ about NOT to use feof(fp) to check if we reached the end of ...

  1. #1
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    feof() from FAQ

    Hi guys,

    I've just read the FAQ about NOT to use feof(fp) to check if we reached the end of the file.
    It all make sense but I don't understand 1 thing...

    Code:
    while (!feof(fp))
      {
        fgets(buf, sizeof(buf), fp);
        printf ("Line %4d: %s", i, buf);
        i++;
      }
    How come that when we reach the end of the file and the fgets return NULL, buf keep its last value... Why fgets doesn't assign buf as NULL and return NULL, it seems that it only return NULL without modifying buf.... Is that how it should be??

    Many thanks !!!

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    Quote Originally Posted by salvadoravi View Post
    Why fgets doesn't assign buf as NULL...
    Because "buf" is converted to a char* when passed to fgets(). For a function to be able to set it to NULL, a char** would be required. Try this yourself. Write a function that receives a char* as a parameter. No matter how much you change the pointer inside the function, you won't be able to affect the original pointer, outside the function.
    Last edited by Mr_Miguel; 01-25-2008 at 11:48 AM. Reason: Clarification
    Name: Miguel Martins
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  3. #3
    C++ Witch laserlight's Avatar
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    I've just read the FAQ about NOT to use feof(fp) to check if we reached the end of the file.
    I believe the FAQ does not quite say that. Rather, it advises not to use feof() to control a loop that reads input. You can use it to check if the end of file has been reached, but only after a read that might have set the end of file indicator has actually been performed.
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    Thanks for your replies guys,

    I will rephrase the question.
    Why buf[0] not set to NULL when fgets read the end of the file?

    Thanks again

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    Perhaps you mean "buf[0] set to 0"; talking about NULL doesn't make much sense there, since "buf[0]" is of type "char".

    So your idea is that fgets() should set "buf" to an empty string, am I right? Well, so far, I haven't seen any fgets() implementation that does that. All the implementations I know leave "buf" unaltered when EOF is reached.
    Name: Miguel Martins
    Date of birth: 14th August 1987

    "He who hesitates is lost."

  6. #6
    Code Goddess Prelude's Avatar
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    >Why buf[0] not set to NULL when fgets read the end of the file?
    Please don't confuse NULL and '\0'. But to answer your question, that's how fgets is defined:
    The fgets function returns s if successful. If end-of- file is encountered and no characters have been read into the array, the contents of the array remain unchanged and a null pointer is returned. If a read error occurs during the operation, the array contents are indeterminate and a null pointer is returned.
    A more interesting question is why do you think fgets should set buf[0] to '\0' when the return value of fgets clearly says that the call failed in some way?
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    Got IT now!!!
    Thank you pros

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