printf turns pointer into type int when %s expects type char

This is a discussion on printf turns pointer into type int when %s expects type char within the C Programming forums, part of the General Programming Boards category; I think the code and error is self-explanatory. Why is this problem arising? Intuitively speaking, I think it has something ...

  1. #1
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    printf turns pointer into type int when %s expects type char

    I think the code and error is self-explanatory. Why is this problem arising? Intuitively speaking, I think it has something to do with using pointer arrays instead of regular pointers. Other than that I'm lost!

    Code and compiler output below

    Code:
    root[~]# cc -Wall -pedantic 8.11.c
    8.11.c: In function 'main':
    8.11.c:32: warning: format '%s' expects type 'char *', but argument 2 has type 'int'
    8.11.c:32: warning: format '%s' expects type 'char *', but argument 3 has type 'int'
    8.11.c:32: warning: format '%s' expects type 'char *', but argument 4 has type 'int'
    8.11.c:32: warning: format '%s' expects type 'char *', but argument 5 has type 'int'
    8.11.c:32: warning: format '%s' expects type 'char *', but argument 6 has type 'int'
    8.11.c:32: warning: format '%s' expects type 'char *', but argument 7 has type 'int'
    
    root[~]# ./a.out
    Segmentation fault


    Code:
    #include <stdio.h>
    #include <stdlib.h>
    #include <string.h>
    #include <time.h>
    
    int main()
    {
      char *article[ 5 ] = { "the", "a", "one", "some", "any" };
      char *noun[ 5 ] = { "boy", "girl", "dog", "town", "car" };
      char *verb[ 5 ] = { "drove", "jumped", "ran", "walked", "skipped" };
      char *preposition[ 5 ] = { "to", "from", "over", "under", "on" };
    
      srand( time( NULL ) );
      printf( "%s %s %s %s %s %s\n", 
              *article[ rand() % 5 + 1 ],
              *noun[ rand() % 5 + 1 ],
              *verb[ rand() % 5 + 1 ],
              *preposition[ rand() % 5 + 1 ],
              *article[ rand() % 5 + 1 ],
              *noun[ rand() % 5 + 1 ] );
    
      return 0;
    }

  2. #2
    Dr Dipshi++ mike_g's Avatar
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    Try removing the stars:
    Code:
      printf( "%s %s %s %s %s %s\n", 
              article[ rand() % 5 + 1 ],
              noun[ rand() % 5 + 1 ],
              verb[ rand() % 5 + 1 ],
              preposition[ rand() % 5 + 1 ],
              article[ rand() % 5 + 1 ],
              noun[ rand() % 5 + 1 ] );

  3. #3
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    Thanks that did the trick.

    I have some questions.
    1) Don't pointers need to be dereferenced to work? I'm guessing that using array syntax already implies it's a dereferenced pointer but it's confusing nonetheless.

    2) Why did the compiler choose to turn the type char values into type int in this instance?

  4. #4
    C++ Witch laserlight's Avatar
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    1) Don't pointers need to be dereferenced to work? I'm guessing that using array syntax already implies it's a dereferenced pointer but it's confusing nonetheless.
    If a is an array or a pointer to the first element of an array, then a[n] is equivalent to *(a + n). (a[n] is also equivalent to n[a], but I find that potentially misleading.)
    C + C++ Compiler: MinGW port of GCC
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  5. #5
    Registered User slingerland3g's Avatar
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    A good read to help explain this. And this link is scattered all over this forum:

    http://www.c-faq.com/

  6. #6
    C++まいる!Cをこわせ! Elysia's Avatar
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    Quote Originally Posted by yougene View Post
    1) Don't pointers need to be dereferenced to work? I'm guessing that using array syntax already implies it's a dereferenced pointer but it's confusing nonetheless.
    Yes, they do, but you misunderstand.
    Printf wants a pointer to the string because it does not want one character, it wants it all. That's why it takes a pointer.
    char* [] is also the same as char** when passed to a function.
    And when using [] on an array, you automatically dereference it.
    Code:
    char mystr[] = "My string";
    mystr[5]; /* == *(mystr + 5) */
    In other words, you are dereferencing it!
    Quote Originally Posted by Adak View Post
    io.h certainly IS included in some modern compilers. It is no longer part of the standard for C, but it is nevertheless, included in the very latest Pelles C versions.
    Quote Originally Posted by Salem View Post
    You mean it's included as a crutch to help ancient programmers limp along without them having to relearn too much.

    Outside of your DOS world, your header file is meaningless.

  7. #7
    Registered User slingerland3g's Avatar
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    Quote Originally Posted by laserlight View Post
    If a is an array or a pointer to the first element of an array, then a[n] is equivalent to *(a + n). (a[n] is also equivalent to n[a], but I find that potentially misleading.)
    Funny I recently was reading on how you could use n[a] or even n + a. Though anyone coding in such obfuscated style would not find much work.

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    Quote Originally Posted by Elysia View Post
    Yes, they do, but you misunderstand.
    Printf wants a pointer to the string because it does not want one character, it wants it all. That's why it takes a pointer.
    So %s is actually looking for a referenced pointer address? It dereferences itself?

    char* [] is also the same as char** when passed to a function.
    And when using [] on an array, you automatically dereference it.
    That's what is confusing me. I thought the [ ] dereference one of the stars, so I thought I need to add another * to dereference the second pointer( I don't know if that's the correct way of saying it )

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    Quote Originally Posted by laserlight View Post
    If a is an array or a pointer to the first element of an array, then a[n] is equivalent to *(a + n). (a[n] is also equivalent to n[a], but I find that potentially misleading.)
    Would n[ a ] be considered good programming? I can see some situations where this might be useful.


    Quote Originally Posted by slingerland3g View Post
    A good read to help explain this. And this link is scattered all over this forum:

    http://www.c-faq.com/
    Thanks for the link. It's going in my browser toolbar.

  10. #10
    C++まいる!Cをこわせ! Elysia's Avatar
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    Quote Originally Posted by yougene View Post
    So %s is actually looking for a referenced pointer address? It dereferences itself?
    No, it simply expects a pointer. An array can only be passed via a pointer and it expects an array.

    That's what is confusing me. I thought the [ ] dereference one of the stars, so I thought I need to add another * to dereference the second pointer( I don't know if that's the correct way of saying it )
    It dereferences one level, not two.
    But you thought wrong in that you needed to dereference twice. It wants an array, a pointer, not a char.
    Quote Originally Posted by Adak View Post
    io.h certainly IS included in some modern compilers. It is no longer part of the standard for C, but it is nevertheless, included in the very latest Pelles C versions.
    Quote Originally Posted by Salem View Post
    You mean it's included as a crutch to help ancient programmers limp along without them having to relearn too much.

    Outside of your DOS world, your header file is meaningless.

  11. #11
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    >>Elysia
    Crystal clear, thank you

    It's like if dealing with a regulary array &#37;s would expect string not string[ someNumber ].

  12. #12
    C++まいる!Cをこわせ! Elysia's Avatar
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    Yep.
    Quote Originally Posted by Adak View Post
    io.h certainly IS included in some modern compilers. It is no longer part of the standard for C, but it is nevertheless, included in the very latest Pelles C versions.
    Quote Originally Posted by Salem View Post
    You mean it's included as a crutch to help ancient programmers limp along without them having to relearn too much.

    Outside of your DOS world, your header file is meaningless.

  13. #13
    uint64_t...think positive xuftugulus's Avatar
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    Quote Originally Posted by yougene View Post
    Would n[ a ] be considered good programming? I can see some situations where this might be useful.
    As far as the compiler is concerned and n is not an expression, it is the same thing as a[ n ]. I would really like to see the situations where you think it might be useful, as it generally is an obfuscating C idiom.
    Code:
    ...
        goto johny_walker_red_label;
    johny_walker_blue_label: exit(-149$);
    johny_walker_red_label : exit( -22$);
    A typical example of ...cheap programming practices.

  14. #14
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    Quote Originally Posted by xuftugulus View Post
    As far as the compiler is concerned and n is not an expression, it is the same thing as a[ n ]. I would really like to see the situations where you think it might be useful, as it generally is an obfuscating C idiom.
    I was thinking it might be useful in a situation where you want to simply increment the name which in this case would be the number n. But on second though it's not very useful since there is still no easy way to increment a. Without a double pointer or something.

  15. #15
    Code Goddess Prelude's Avatar
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    >I was thinking it might be useful in a situation [...]
    I have yet to see or hear about a situation outside of the IOCCC where it's viable, much less useful. The scary part is that most descriptions of subscripting will talk about this trick like it's some sort of amazing feature of C. I suppose when people run out of interesting things to say, they fall back on trivia.

    However, a similar trick actually is useful, and that's using a string literal as the object for subscripting:
    Code:
    #include <stdio.h>
    
    int main ( void )
    {
      int i;
    
      for ( i = 0; i < 10; i++ )
        putchar ( "0123456789"[i] );
    
      return 0;
    }
    >As far as the compiler is concerned and n is not an expression, it is the same thing as a[ n ].
    It's the same with an expression too, but you have to take care to parenthesize it:
    Code:
    #include <stdio.h>
    
    int main ( void )
    {
      const char *p = "This is a test";
      
      puts ( &( 2 * 2 + 3 / 2 )[p] );
      puts ( &p[2 * 2 + 3 / 2] );
    
      return 0;
    }
    My best code is written with the delete key.

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