**`bsearch_s`** is a built-in function defined in the `<stdlib.h>` header. It is available for use from the *C11* version of C. 

To use the function, we must define `__STDC_LIB_EXT1__` and set `__STDC_WANT_LIB_EXT1__` to `1`.

Below is the function prototype:

```C
void* bsearch_s (const void *key, const void *arr, rsize_t count, rsize_t size, int (*comp)(const void *, const void *, void *), void *context);
```

# Parameters

Let's take a deeper look into the input parameters of the function.

- **key:** Pointer to the element being found.
- **arr:** Pointer to the array being searched.
- **count:** Total number of elements in the array.
- **size:** Size, in bytes, of each element in the array.
- **comp:** Pointer to the comparison function.
- **context:** Additional information to serve as context for *comp*.

# Return

>The `bsearch_s` function returns a pointer to the *key* element.

If the *key* element is not found or dealt with a runtime error, a *null* pointer is returned.

# The comparison function

Below the prototype for the **comp** function:

```
int cmp(const void *key, const void *element, void* context);
```

The first argument is the *key* that we find, while the second argument may be any *element* from the array.

The *comp* function returns:

- `-1` if the key is *less* than the element

- `0` if the key is *equal to* the element

- `1` if the key is *greater* than the element


# Code


#define  __STDC_WANT_LIB_EXT1__ 1

#include <stdlib.h>
#include <stdio.h>

int comp(void const *lhs, void const *rhs, void* context)
{
    const int* l = (const int*) lhs;
    const int* r = (const int*)rhs;

    if (*l < *r) return -1;
    else if (*l > *r) return 1;
    else return 0;
}

int main()
{
   int arr[] = {0, 2, 4, 6, 8, 10};

   int key = 4;
   int* result = bsearch_s(&key, arr, sizeof(arr) / sizeof(arr[0]), sizeof(arr[0]), comp, NULL);
   
   if(result)
      printf("\n%d found\n", *result);
   else
      printf( "\nNot found!\n" );
}


Here we're dealing with a simple `integer` array; hence, there is no need for context and we can pass `NULL` in its place.

>Note that the array we are searching is sorted. This is a pre-requisite for `binary search`.

cpp.tar.gz

What is bsearch_s() in C?

Available from C11, `bsearch_s` in `<stdlib.h>` performs binary search on a sorted array with context-sensitive comparison.