Thread: return an array of structs

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

Originally Posted by matsp

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?

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?

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