从函数返回的错误数组值

Question

I am making this program in which my main function calls a function which returns an array after the calculation. I checked already that calculation is right inside the local function. But when I return that array to 'main' function then I only can print the correct value one time and it prints wrong value all other times.

#include <math.h>
#include <stdio.h>
#include <string.h>
#include <stdlib.h>
#include <assert.h>
#include <limits.h>
#include <stdbool.h>
int* getJoinedPipes(int input1_size, int* input1,int* output_size){
  int i,j,temp;
  int op1[input1_size-1];
  *output_size = input1_size - 1;
  for(i=0; i<input1_size; i++){
    for(j=i+1; j<input1_size; j++){
      if(input1[i] > input1[j]){
        temp     = input1[i];
        input1[i] = input1[j];
        input1[j] = temp;
      }
    }
  }

  op1[0]=input1[0] + input1[1];
  for(i=1;i<input1_size-1;i++){
    op1[i] = op1[i-1]+input1[i+1];
  }
  //printf("%d\n",op1[2]);

  return op1; 
}

int main() {
  int output_size;
  int* output;

  int ip1_size = 0;
  int ip1_i;
  scanf("%d\n", &ip1_size);
  int ip1[ip1_size];
  for(ip1_i = 0; ip1_i < ip1_size; ip1_i++) {
    int ip1_item;
    scanf("%d", &ip1_item);

    ip1[ip1_i] = ip1_item;
  }
  output = getJoinedPipes(ip1_size,ip1,&output_size);
  printf("a==%d\n",output[0]);
  printf("a==%d\n",output[0]);
  printf("a==%d\n",output[0]);
  int output_i;
  for(output_i=0; output_i < output_size; output_i++) {
    printf("%d\n", output[output_i]);
  }

  return 0;
}

Output should be

5
9
15

But in the console, it's showing the following (after dry run).

a==5    
a==1943372821    
a==1943372821    
1943372821    
17    
6356632

You can see first time its giving correct value (5) and later for same print its giving garbage values.

Answer 1

op1 is an automatic array. You cannot return it and use it outside its scope.

op1 exists only within getJoinedPipes , if you return it, the result is Undefined Behaviour .

To fix it you can either:

• pass op1 as a parameter to getJoinedPiped
• allocate op1 on the heap dynamically. You you do that, you can safely return op1 but you have to remember to free it when you don't need it.

Answer 2

You are returning pointer to a local variable object whose lifetime has ended yet when you are trying to get its values -> horrible mistake and undefined behaviour.

If you want to return an array from function, allocate it dynamically via malloc , and dont forget to free it after you are done with that array. You should also check return value of malloc if you got memory you asked for.

Correct

#include <math.h>
#include <stdio.h>
#include <string.h>
#include <stdlib.h>
#include <assert.h>
#include <limits.h>
#include <stdbool.h>
int* getJoinedPipes(int input1_size, int* input1,int* output_size){
int i,j,temp;
int * op1 = (int *)malloc(sizeof(int) * (input1_size-1));
*output_size = input1_size - 1;
for(i=0; i<input1_size; i++){
    for(j=i+1; j<input1_size; j++){
        if(input1[i] > input1[j]){
            temp     = input1[i];
            input1[i] = input1[j];
            input1[j] = temp;
        }
    }
}

op1[0]=input1[0] + input1[1];
for(i=1;i<input1_size-1;i++){
    op1[i] = op1[i-1]+input1[i+1];
}
//printf("%d\n",op1[2]);

return op1;
}

int main() {
int output_size;
int* output;

int ip1_size = 0;
int ip1_i;
scanf("%d\n", &ip1_size);
int ip1[ip1_size];
for(ip1_i = 0; ip1_i < ip1_size; ip1_i++) {
    int ip1_item;
    scanf("%d", &ip1_item);

    ip1[ip1_i] = ip1_item;
}
output = getJoinedPipes(ip1_size,ip1,&output_size);
printf("a==%d\n",output[0]);
printf("a==%d\n",output[0]);
printf("a==%d\n",output[0]);
int output_i;
for(output_i=0; output_i < output_size; output_i++) {

    printf("%d\n", output[output_i]);

}

free(output);
return 0;
}

Answer 3

You should check your indentation... apart from that, I guess the problem could arise because the array you output in your function is created on the function stack. So what you get as an output array is a reference on the stack of your joinedPipes-function. Try to pass your array as an argument to the function and not create it as a return value.

Hope that does the trick...