beginner question: returning a string from a function

This is a discussion on beginner question: returning a string from a function within the C Programming forums, part of the General Programming Boards category; I've been looking at how to achieve this, and I see that it's not possible using the approach that I ...

  1. #1
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    beginner question: returning a string from a function

    I've been looking at how to achieve this, and I see that it's not possible using the approach that I first assumed would work (return "the string"

    For example, I need the first line here:
    Code:
    char *getString(void);
    main(){
        char *test = getString();
        printf("%s\n\n", test);
    }
    
    char *getString(){
        char *test = (char *)malloc(20*sizeof(char));
        test = "this is a string";
    
        return test;
    }
    I've tried looking for an explanation on why this is needed, what it does etc but I still cant understand what's happening.

    Could someone explain what the above code is doing?
    thanks for reading!

  2. #2
    C++まいる!Cをこわせ! Elysia's Avatar
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    This code "works", but not because it's correct, just because it "happens to".
    Multiple issues here. First, string literals, such as "hello world", are really pointers. const char* to be exact. So first you malloc memory, then you overwrite that pointer with something else. Memory leak.
    But since string literals are stored in a part of your executable, you can return them as you wish. They work just fine.

    Otherwise, you need to send in a buffer to your function, including the size of your buffer, and copy data into it. Make sure you don't copy more data than you have room. That's why you need your size parameter.
    And finally, main returns int and your getString prototype is mismatching.
    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.

  3. #3
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    In other words, instead of
    Code:
    test = "this is a string";
    you should have
    Code:
    strcpy(test, "this is a string");

  4. #4
    C++まいる!Cをこわせ! Elysia's Avatar
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    Don't forget that test must an array with sufficient storage, such as:
    Code:
    char test[100];
    strcpy(test, "This is a string");
    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.

  5. #5
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    Well, he's allocating 20 bytes which is sufficient for his example.

  6. #6
    C++ Witch laserlight's Avatar
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    nonoob's suggestion is a correct one and Elysia's assertion is certainly right, but if you integrate Elysia's example into your code, you would then be returning a pointer to a local variable, which is wrong. Rather, one might write:
    Code:
    #include <stdio.h>
    #include <stdlib.h>
    #include <string.h>
    
    char *getString(void);
    
    int main(void) {
        char *test = getString();
        printf("%s\n", test);
        free(test);
        return 0;
    }
    
    char *getString(void) {
        char *test = malloc(20 * sizeof(*test));
        strcpy(test, "this is a string");
        return test;
    }
    Observe that the correct headers were included, and that I removed the cast of the return value of malloc. Furthermore, sizeof(char) is always 1, so you could leave it out, but I chose to change it to the more general sizeof(*test).

    EDIT:
    Additionally, one should check that malloc did not return a null pointer before accessing the memory presumed to be allocated.
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  7. #7
    C++まいる!Cをこわせ! Elysia's Avatar
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    This amounts to having to free the malloc later which is certainly easy to forget and error prone. Furthermore, it is inflexible, because you cannot choose whether to use dynamic memory or not. And it is slower because dynamic memory is slower than stack based allocation.
    Which amounts to passing in a buffer to the function as I mentioned in the first place.
    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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