return an array of structs

This is a discussion on return an array of structs within the C Programming forums, part of the General Programming Boards category; So I have been obsessing over creating a merge sort for linked lists. I can do a merge sort with ...

  1. #1
    spurious conceit MK27's Avatar
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    return an array of structs

    So I have been obsessing over creating a merge sort for linked lists. I can do a merge sort with an int array fine, but there seems to be much less flexability dealing with an array of structs which is giving me a lot of problems. How do you return an array of structs? (After trying it and googling I am not sure if it is possible, strangely). Trying to pass such a list as an argument seems even less possible.

    Ideally, I would to do this with just an array of struct pointers, or an array of any kind of pointers but my compiler will not let me cast them into struct pointers in a useful way.
    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
    Kernel hacker
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    You can not return an array from a function, no matter what.

    You can pass in an array and fill it in, or you can return a pointer, or pass in a pointer to pointer and allocate space (and either pass in a pointer to an itneger that gives you the number of items, or return the number of items if you pass a pointer to pointer).

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  3. #3
    spurious conceit MK27's Avatar
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    Quote Originally Posted by matsp View Post
    You can not return an array from a function, no matter what.

    You can pass in an array and fill it in, or you can return a pointer, or pass in a pointer to pointer and allocate space (and either pass in a pointer to an itneger that gives you the number of items, or return the number of items if you pass a pointer to pointer).

    --
    Mats
    If I pass in an array, I have to declare it's length?
    Code:
    void testfunc (struct *ray[10]) {
    If I return a pointer (eg, struct *ptr (struct *ray) {) should I allocate for the array it points to inside the function, or before I call it?
    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

  4. #4
    spurious conceit MK27's Avatar
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    Here's an example of the wierdness I'm talking about:
    Code:
    #include <stdio.h>
    #include <stdlib.h>
    #include <string.h>
    
    struct Test {
    	char *name;
    	int age;
    } ;  
    
    void testfunc (struct Test *examp) {
    	if ((examp=realloc(examp,3*sizeof(*examp)))==NULL) puts ("malloc fail");
    	puts("HERE");
    	fflush(stdout);
    	examp[0].name=malloc(5);
    	strcpy(examp[0].name,"five");
    	examp[0].age=5;
    	examp[1].name=malloc(5);
    	strcpy(examp[1].name,"four");
    	examp[1].age=4;
    	examp[2].name=malloc(5);
    	strcpy(examp[2].name,"nine");
    	examp[2].age=9;
    }
    
    int main () {
    	int i=0;
    	struct Test *examp=malloc(sizeof(*examp)); 	 
    	
    	testfunc (examp);
    	printf("sizeof *examp:%d sizeof examp:%d\n",sizeof(*examp),sizeof(examp));
    	printf("%s is %d\n", examp[0].name,examp[0].age);
    	for (i=0;i<3;i++) {puts("X");printf("%s is %d\n", examp[i].name,examp[i].age);}
    	
    }
    Everything is fine. I am surprised that *examp is 8 bytes while examp is 4 bytes, but whatever (???). However, I am a little worried the realloc doesn't really realloc inside the function, because this causes a segfault:
    Code:
    #include <stdio.h>
    #include <stdlib.h>
    #include <string.h>
    
    struct Test {
    	char *name;
    	int age;
    } ;  
    
    void testfunc (struct Test *examp) {
    	if ((examp=malloc(3*sizeof(*examp)))==NULL) puts ("malloc fail");
    	puts("HERE");
    	fflush(stdout);
    	examp[0].name=malloc(5);
    	strcpy(examp[0].name,"five");
    	examp[0].age=5;
    	examp[1].name=malloc(5);
    	strcpy(examp[1].name,"four");
    	examp[1].age=4;
    	examp[2].name=malloc(5);
    	strcpy(examp[2].name,"nine");
    	examp[2].age=9;
    }
    
    int main () {
    	int i=0;
    	struct Test *examp; 	 
    	
    	testfunc (examp);
    	printf("sizeof *examp:%d sizeof examp:%d\n",sizeof(*examp),sizeof(examp));
    	printf("%s is %d\n", examp[0].name,examp[0].age);
    	for (i=0;i<3;i++) {puts("X");printf("%s is %d\n", examp[i].name,examp[i].age);}
    	
    }
    Why can't I malloc the struct array inside another function and return it?
    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

  5. #5
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    Quote Originally Posted by MK27 View Post
    Everything is fine. I am surprised that *examp is 8 bytes while examp is 4 bytes, but whatever (???).
    examp is a pointer, which (on a 32-bit system) is 4 bytes in size. *examp is what its points to, which in this case is the size of your structure.

    Quote Originally Posted by MK27 View Post
    Why can't I malloc the struct array inside another function and return it?
    You can. The drawback to this technique is that the callers of your function will have to remember to free() the array.

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    Computer Programming: An Introduction for the Scientifically Inclined

  6. #6
    Kernel hacker
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    However, if you want to pass in a pointer, to be filled in inside the function, you should pass a pointer to a pointer - just like if you want to change an integer inside a function, you need to pass a pointer to an integer.

    --
    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.

  7. #7
    Registered User slingerland3g's Avatar
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    What you need or have is a 2d array and you need to properly set aside memory for that as well.

    Code:
    int **createMyExamp(int r, int c)
    {
       int i;
       int **examp = malloc(r * sizeof(int *));
       for (i = 0 ; i < r; i++)
          examp[i] = malloc (c * sizeof(int));
      return examp;
    }

  8. #8
    C++まいる!Cをこわせ! Elysia's Avatar
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    Code:
    int (*p)[dimension2] = malloc(dimension1 * sizeof(type) + dimension2);
    That you need a p2p to create a 2D array is a myth. All you need is the correct type.
    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.
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    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
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    Changed the function to return the structure and this works

    Code:
    #include <stdio.h>
    #include <stdlib.h>
    #include <string.h>
    
    typedef struct Test {
            char *name;
            int age;
    } ARF ;
    
    int main () {
            int i;
            ARF *examp;
    
            ARF *testfunc() ;
    
            examp = testfunc();
    
            printf("sizeof *examp:%d sizeof examp:%d\n",sizeof(*examp),sizeof(examp));
            printf("%s is %d \n", examp[0].name, examp[0].age );
    
             for (i=0;i<3;i++) printf("%s is %d\n", examp[i].name,examp[i].age);
    
            return 0 ;
    
    }
    
    ARF *testfunc()
    {
            ARF *bexamp ;
            if (  ( bexamp =  (struct Test *) malloc( 3 * sizeof(ARF) ) ) == NULL)   puts ("malloc fail");
    
            puts("HERE");
            fflush(stdout);
    
            bexamp[0].name=(char *) malloc ( (strlen("five")+1) * sizeof(char ) );
            strcpy(bexamp[0].name,"five");
            bexamp[0].age=5;
    
            bexamp[1].name=(char * ) malloc(5 * sizeof(char ) );
            strcpy(bexamp[1].name,"four");
            bexamp[1].age=4;
    
            bexamp[2].name=(char * ) malloc(5 * sizeof(char ) );
            strcpy(bexamp[2].name,"nine");
            bexamp[2].age=9;
    
            return bexamp ;
    }

  10. #10
    CSharpener vart's Avatar
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    you should not use null-pointer (after failed malloc)
    The first 90% of a project takes 90% of the time,
    the last 10% takes the other 90% of the time.

  11. #11
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    Quote Originally Posted by Elysia View Post
    Code:
    int (*p)[dimension2] = malloc(dimension1 * sizeof(type) + dimension2);
    That you need a p2p to create a 2D array is a myth. All you need is the correct type.
    Sorry to dig this up, but I've been looking into this thread and got some nice info. I have a few doubts regarding the quoted statement, can someone elaborate on it? What is p2p? (In this context, I think its not peer to peer)


    I believe this is an answer to , if yes how does it apply?

    Code:
    int **createMyExamp(int r, int c)
    {
       int i;
       int **examp = malloc(r * sizeof(int *));
       for (i = 0 ; i < r; i++)
          examp[i] = malloc (c * sizeof(int));
      return examp;
    }
    Thanks!

  12. #12
    spurious conceit MK27's Avatar
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    Quote Originally Posted by sugarfree View Post
    What is p2p?
    A "pointer to a pointer", eg:
    Code:
      int **examp = malloc(r * sizeof(int *));
    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

  13. #13
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    haha, silly me. That was easy.

  14. #14
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    Another question, what is the point of having examp[i] = malloc (c * sizeof(int)); in slingerland3g post? Is this examp[r][c]?

  15. #15
    spurious conceit MK27's Avatar
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    Quote Originally Posted by sugarfree View Post
    Is this examp[r][c]?
    Yes.

    example[r] is a pointer and needs c*sizeof(int) bytes allocated to it to store an array (row) of ints.
    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

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