Reading file using ifstream

This is a discussion on Reading file using ifstream within the C++ Programming forums, part of the General Programming Boards category; I'm trying to read the contents of a file using the ifstream constructs. I've written a program that is a ...

  1. #1
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    Reading file using ifstream

    I'm trying to read the contents of a file using the ifstream constructs. I've written a program that is a carbon copy off the website, under Reading From a File:

    C++ Binary File I/O

    Code:
    1 #include <fstream>
    2 using namespace std;
    3 int main() {
    4 char buffer[100];
    5 ifstream myFile("temp", ios::in);
    6 myFile.read(buffer, 100);
    7 if(!myFile) {
    8         fprintf(stderr, "Fail\n");
    9 }
    10 }
    There is a file called temp in the directory where the program was built and the executable is handled. Everytime I execute the program, it prints "Fail." The file temp has chmod 777.

    The heck is going wrong here?

  2. #2
    and the Hat of Guessing tabstop's Avatar
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    You can check for more specifics about the error in several ways:
    1. Immediately after line 5, you can check whether the file even opened.
    2. If it did open, you can be more specific than just !myFile -- is at end of file (eof) or did it encounter a read error (bad)?
    3. You can also use perror or errno to get the error message from the operating system.

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    myFile.is_open returns 1
    myFile.good returns 1
    myFile.eof returns 0

  4. #4
    and the Hat of Guessing tabstop's Avatar
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    If myFile.good() returns true, then !myFile will be false (technically !myFile returns myFile.fail(), but if the failbit is set then good() will return false). Did you check before the read or after?

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    Those results were before the read.

    After the read, open returns 1; good returns 0; end returns 1; and bad returns 0 (??)

  6. #6
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    Since you are at end of file, the file is no longer "good" (as you see), hence your !myFile triggers. (bad is reserved for read errors; eof is not considered a read error.) Whether or not the read was successful is another story -- you'd have to call gcount to find out if anything was read (or examine the contents of buffer, I suppose, except they started out uninitialized, so who knows whether what you have was from the file or not).

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    gcount returns 0

  8. #8
    and the Hat of Guessing tabstop's Avatar
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    So that means nothing was read; presumably this means your file exists but is empty.

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