using character arrays and realloc

This is a discussion on using character arrays and realloc within the C Programming forums, part of the General Programming Boards category; It was my understanding that the name of an array is a pointer to it's first element. So why does ...

  1. #1
    spurious conceit MK27's Avatar
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    using character arrays and realloc

    It was my understanding that the name of an array is a pointer to it's first element. So why does this:
    Code:
    char string[]="some words";
    string=realloc(string, 20);
    cause error: incompatible types in assignment, meaning I have to do this instead:
    Code:
    char *string;
    string=malloc(11);
    strcpy(string, "some words");
    string=realloc(string, 20);
    Can I ask why? And is there a way out? I tried using a seperate pointer (char *ptr; ptr=string) but could not get that to work either...
    C programming resources:
    GNU C Function and Macro Index -- glibc reference manual
    The C Book -- nice online learner guide
    Current ISO draft standard
    CCAN -- new CPAN like open source library repository
    3 (different) GNU debugger tutorials: #1 -- #2 -- #3
    cpwiki -- our wiki on sourceforge

  2. #2
    and the hat of sweating
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    A local array, like all other local variables is created on the stack. Since you didn't allocate it with malloc(), you cannot change its size with realloc() or delete it with free().
    Stack variables are placed right next to each other and when the function returns, all the stack variables disappear since the stack pointer just moves back to where it was before the function was called.

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    C++ Witch laserlight's Avatar
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    It was my understanding that the name of an array is a pointer to it's first element.
    The name of an array is not a pointer to the first element of the array. Rather, an array is converted to a pointer to its first element when it is passed as an argument (or used in various other contexts).
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  4. #4
    Kernel hacker
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    cause error: incompatible types in assignment
    You are trying to assign a pointer into something that is an array[11]. The compiler should not allow you to do that.

    And as pointed out, you can not realloc an object that wasn't created by malloc [or realloc or calloc] in the first place.

    --
    Mats
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  5. #5
    spurious conceit MK27's Avatar
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    hmmm...thanks (cpjust in particular), that clarifies a few things for me further.
    C programming resources:
    GNU C Function and Macro Index -- glibc reference manual
    The C Book -- nice online learner guide
    Current ISO draft standard
    CCAN -- new CPAN like open source library repository
    3 (different) GNU debugger tutorials: #1 -- #2 -- #3
    cpwiki -- our wiki on sourceforge

  6. #6
    C++まいる!Cをこわせ! Elysia's Avatar
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    Quote Originally Posted by matsp View Post
    You are trying to assign a pointer into something that is an array[11]. The compiler should not allow you to do that.
    Well, it's typical C. It does work in C. Go figure.
    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.

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    Kernel hacker
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    Quote Originally Posted by Elysia View Post
    Well, it's typical C. It does work in C. Go figure.
    Neither does it in C++.

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    Mats
    Compilers can produce warnings - make the compiler programmers happy: Use them!
    Please don't PM me for help - and no, I don't do help over instant messengers.

  8. #8
    C++まいる!Cをこわせ! Elysia's Avatar
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    Uhh yes? You mentioned it shouldn't work and I agree. It certainly doesn't in C++, but it does work in C (as in, it compiles without errors).
    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.

  9. #9
    and the Hat of Guessing tabstop's Avatar
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    Quote Originally Posted by Elysia View Post
    Uhh yes? You mentioned it shouldn't work and I agree. It certainly doesn't in C++, but it does work in C (as in, it compiles without errors).
    Which part of "error: incompatible types in assignment" are you having trouble with? (Yes, that's the C error.)

  10. #10
    C++まいる!Cをこわせ! Elysia's Avatar
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    Everything apparently...
    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.

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