Functions, stack

This is a discussion on Functions, stack within the C Programming forums, part of the General Programming Boards category; Hi, I have the following code: Code: #include <stdio.h> char * returnString () { char *string; string = "Blah"; return ...

  1. #1
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    Functions, stack

    Hi,

    I have the following code:

    Code:
    #include <stdio.h>
    
    char * returnString () {
    	
    	char *string;
    	
    	string = "Blah";
    	
    	return (string);
    		
    }
    
    int main() {
    	
    	printf("%s\n", returnString());	
    	
    	return (0);	
    }
    First question: Is the string "Blah" stored on stack? If so, how come it can be printed in main()? Shouldn't it be removed off the stack right upon exiting function returnString()?

    Second question: After compiling this code, my compiler gives me the following warning:

    Call to function 'returnString' with no prototype in function main
    Could you tell me why?


    Thanks a lot, guys.

  2. #2
    B26354 Deckard's Avatar
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    Quote Originally Posted by Ariod
    First question: Is the string "Blah" stored on stack? If so, how come it can be printed in main()? Shouldn't it be removed off the stack right upon exiting function returnString()?
    "Blah" is in the text segment, and the variable string is on the stack. For what it's worth, data on the stack is not erased when the stack pointer is moved to reclaim the space allocated during the call.
    Jason Deckard

  3. #3
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    Regarding the "no prototype" warning message, are you compiling your C code as C++? Which compiler are you using?
    If you understand what you're doing, you're not learning anything.

  4. #4
    Frequently Quite Prolix dwks's Avatar
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    Your error
    Code:
     Call to function 'returnString' with no prototype in function main
    means that you need a prototype before the function's first use.
    Code:
        char *string;
    	
        string = "Blah";
    You have no way of knowing what string points to, and you assign "Blah" to what ever that is! Not good. malloc() the string first (or declare it as [number]).
    dwk

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  5. #5
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    Actually, you don't need a prototype in C for a function that's defined before it's called. The OP shouldn't be getting that warning.

    And the assignment of "Blah" to string is perfectly legal. If he was trying to do something like strcpy(string, "Blah") without allocating memory for string then it would be bad. But all the OP is doing in this case is saying string points to "Blah" which is valid. Just don't try changing the contents of "Blah" (i.e. string[0] = 'r') because string literals are commonly stored in read-only memory.
    Last edited by itsme86; 07-27-2005 at 11:12 AM.
    If you understand what you're doing, you're not learning anything.

  6. #6
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    Quote Originally Posted by Deckard
    "Blah" is in the text segment, and the variable string is on the stack. For what it's worth, data on the stack is not erased when the stack pointer is moved to reclaim the space allocated during the call.
    So the string (the actual text) is not stored on the stack? Sorry, I'm just trying to understand this.

    Quote Originally Posted by itsme86
    Regarding the "no prototype" warning message, are you compiling your C code as C++? Which compiler are you using?
    I'm using Borland's free C/C++ compiler. The file's extension is .c if that means anything, I'm new to this compiler so I don't know if that's enough information for it to compile as C, not C++.

    I wrote a few programs where the function definition was before where it was being called, and it compiled without any warnings. This is the first time I got this warning.


    Thanks for your replies.

  7. #7
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    So the string (the actual text) is not stored on the stack? Sorry, I'm just trying to understand this.
    Correct. Only local, non-static variables are stored on the stack. Your pointer string is stored on the stack, but that only contains the address of "Blah", not "Blah" itself. If your function returned &string it would be bad (a pointer to the variable on the stack).

    I'm not familiar with Borland's compiler, but I use DEV-C++ in Windows and it seems to be a very decent free IDE.
    If you understand what you're doing, you're not learning anything.

  8. #8
    Frequently Quite Prolix dwks's Avatar
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    Actually, you don't need a prototype in C for a function that's defined before it's called. The OP shouldn't be getting that warning.
    You're right, you don't (although it's a good habit to get into).
    dwk

    Seek and ye shall find. quaere et invenies.

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  9. #9
    Yes, my avatar is stolen anonytmouse's Avatar
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    To add to itsme86's answer, it is the C standard which requires that a string literal will be valid for the lifetime of a program.
    http://dev.unicals.com/papers/c89-draft.html#3.1.4

    A character string literal has static storage duration and type ``array of char ,'' and is initialized with the given characters.
    http://dev.unicals.com/papers/c89-draft.html#3.1.2.4

    An object declared with external or internal linkage, or with the storage-class specifier static has static storage duration. For such an object, storage is reserved and its stored value is initialized only once, prior to program startup. The object exists and retains its last-stored value throughout the execution of the entire program.
    Second question: After compiling this code, my compiler gives me the following warning:
    Call to function 'returnString' with no prototype in function main
    Could you tell me why?
    Code:
    char * returnString ()
    In C, this is not a complete function definition.
    Empty parameter lists

    C distinguishes between a function declared with an empty parameter list and a function declared with a parameter list consisting of only void. The former is an unprototyped function taking an unspecified number of arguments, while the latter is a prototyped function taking no arguments.
    To define a function that takes no arguments, you must use the keyword void:
    Code:
    char * returnString (void) {
    
    int main(void) {

  10. #10
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    Quote Originally Posted by anonytmouse
    Code:
    char * returnString ()
    In C, this is not a complete function definition.

    To define a function that takes no arguments, you must use the keyword void:
    Code:
    char * returnString (void) {
    
    int main(void) {
    Thanks for the tip, I get no warnings now.

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