A basic question

This is a discussion on A basic question within the C Programming forums, part of the General Programming Boards category; consider the following code Code: #include <stdio.h> int main(void) { const char *a = "abc"; const char *b = "abc"; ...

  1. #1
    C/C++Newbie Antigloss's Avatar
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    A basic question

    consider the following code
    Code:
    #include <stdio.h>
    
    int main(void)
    {
        const char *a = "abc";
        const char *b = "abc";
        
        if ( a == b )
            printf("Yes, we are the same!");
            
        getchar();
        return 0;
    }
    Is it guaranteed that the two pointers hold the same address? That's, is the address of the two "abc" the same?
    Last edited by Antigloss; 11-13-2005 at 02:48 AM.

  2. #2
    Devil's Advocate SlyMaelstrom's Avatar
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    No they don't hold the same address. If you're talking about addresses, try this.

    Code:
    #include <stdio.h>
    
    int main(void)
    {
        const char *a = "abc";
        const char *b = "abc";
        
        if ( &a == &b )
            printf("Yes, we are the same!");
    
        printf("%p \n%p", &a, &b);
            
        getchar();
        return 0;
    }
    Last edited by SlyMaelstrom; 11-13-2005 at 03:23 AM.
    Sent from my iPadŽ

  3. #3
    C/C++Newbie Antigloss's Avatar
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    No, I didn't mean the addresses of a and b, I meant the address of the two "abc".
    I meant the value that a and b hold.

  4. #4
    Devil's Advocate SlyMaelstrom's Avatar
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    Oh, Try this then.

    Code:
    #include <stdio.h>
    
    int main(void)
    {
        const char *a = "abc";  // Reserves memory for 3 characters
        const char *b = "abc";  // Reserves memory for 3 more characters
        const char *x = &a[0];
        const char *y = &b[0];
       
        printf("%p\n%p\n", &x, &y);
            
        getchar();
        return 0;
    }
    Hmm... Let me think this over a bit. I'm all screwed up here.
    Last edited by SlyMaelstrom; 11-13-2005 at 03:54 AM.
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  5. #5
    Super Moderator Harbinger's Avatar
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    From the gcc manual

    `-fmerge-constants'
    Attempt to merge identical constants (string constants and
    floating point constants) across compilation units.

    This option is the default for optimized compilation if the
    assembler and linker support it. Use `-fno-merge-constants' to
    inhibit this behavior.

    Enabled at levels `-O', `-O2', `-O3', `-Os'.
    Because they're the same string, the compiler may choose (or may be told to) merge identical string constants, so writing "abc" twice in the code results in only one "abc" in the executable.
    In this case, comparing the pointers will produce a match, but not for the obvious reason (or intent)

  6. #6
    C/C++Newbie Antigloss's Avatar
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    So it's up to the compiler to do this, but not guaranteed by the standard. Right?

  7. #7
    ATH0 quzah's Avatar
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    Your first example was correct. Disregard everything Sly said. You are comparing the two values contained in the pointer, which are addresses. The comparison made in your example would be valid.

    Back to the question: The standard doesn't guarintee that two string literals of the same contents be in the same spot in memory. Purely a compiler issue.


    Quzah.
    Hope is the first step on the road to disappointment.

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