Function not working on c program selection sort

There are 3 main issues in your code:

1. You are getting user input twice. One in main function:

   printf("Enter array elements:\n");
   for(i=0;i<n;i++)
     scanf("%d",&a[i]);
   

   Another in selection_sort function:

   for(i=0;i<n;i++)
     scanf("%d",&a[i]);
   

   Fix this by removing the one in selection_sort function.

2. As noted in the comments section, you are returning from main function before calling selection_sort function:

   return 0;
   

   Fix this by moving it to the end of main function.

3. You are printing the expected results from selection_sort function before calling it:

   printf("Sorted array: ");
   for(i=0;i<n;i++)
     printf(" %d",a[i]);
   ...
   selection_sort(i,j,n,a,temp,min);
   

   Fix this by calling selection_sort before printing its results.

Here is the fixed code:

#include <stdio.h>

int selection_sort (int a[], int n, int i, int j, int temp, int min)
{
   for(i=0;i<n;i++) {

        min=i;

        for(j=i+1;j<n;j++) {

           if(a[j]<a[min]) {

              min=j;
           }
        }

        temp=a[i];
        a[i]=a[min];
        a[min]=temp;
   }
}

int main()
{
    int i, j, n,a[20], temp, min;
    
    printf("How many elements: ");
    scanf("%d",&n);
    
    printf("Enter array elements: ");
    for(i=0;i<n;i++) {

        scanf("%d",&a[i]);
    }
    
    selection_sort(a, n, i, j, temp, min);

    printf("Sorted array:");
    for(i=0;i<n;i++) {

        printf(" %d", a[i]);
    }
    printf("\n");
    
    return 0;
}

Here is a test:

$ gcc main.c && ./a.out
How many elements: 3
Enter array elements: 1 4 2
Sorted array: 1 2 4

int main(){

   int i, j, n,a[20], temp,min;

   printf("How many elements:\n ");
   scanf("%d",&n);

   printf("Enter array elements:\n");
    for(i=0;i<n;i++)
    scanf("%d",&a[i]);

    selection_sort(a,n,i,temp,min);//Not need to use j,cause j isn't used in your function
    
    for(i=0;i<n;i++)
    printf("%d\t",a[i]);//You should call your function before print your data,and also before "return 0"
    
    return 0; 
}

AND,"int i,int temp,int min" these don't need to be the parameters.You can just create them in your function