Почему может быть undefined reference на функцию, котороая присутствует в коде?

Выдает ошибку undefined reference to 'sortirovka', но никак не могу понять, что не так.

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <malloc.h>
#define MAX 50
  
int n; // Кол-во фильмов

struct Buyer // Шаблон структуры
{
    char surname[30];
    char name[30];
    char middlename[30];
    char city[30];
    char street[30];
    int homenumber;
    int flatnumber;
    int creditcard;
    int purchasecode;
};

struct Buyer shopbuyer[MAX];

void menu()
{
    void add(); // Объяление функции add()
    void search(); // Объяление функции search()
    void sortirovka(); // Объяление функции sortirovka()
    void display(); // Объяление функции display()
    int choice;
    printf("\tMENU:\n");
    printf("1.Add\n");
    printf("2.Search\n");
    printf("3.Sort\n");
    printf("4.Show\n");
    printf("5.Exit\n");
    printf("Enter choosen position:");
    scanf("%d",&choice);
    switch (choice)
    {
        case 1: add(); break;
        case 2: search(); break;
        case 3: sortirovka(); break;
        case 4: display(); break;
        case 5: exit(0);
        default: printf("Error. Try again\n"); menu(); 
    };
};

void add()
{
    system("chcp 1251");
    system("cls"); // Windows -     system("cls");
    for (int i = n; i < MAX; i++)
        {
        printf("Add new buyer\n");
        printf("Surname: ");
        fgets(shopbuyer[i].surname, 30, stdin);
        printf("Name: ");
        fgets(shopbuyer[i].name, 30, stdin);
        printf("Middle Name: ");
        fgets(shopbuyer[i].middlename, 30, stdin);
        printf("City: ");
        fgets(shopbuyer[i].city, 30, stdin);
        printf("Street: ");
        fgets(shopbuyer[i].street, 30, stdin);
        printf("Home Number: ");
        scanf("%d",&shopbuyer[i].homenumber);
        printf("Flat Number: ");
        scanf("%d",&shopbuyer[i].flatnumber);
        printf("Credit Card: ");
        scanf("%d",&shopbuyer[i].creditcard);
        printf("Purchase Code: ");
        scanf("%d",&shopbuyer[i].purchasecode);
        n++;
        break; 
        };
    system("chcp 1251");
    system("cls");
    menu();
};

void display()
{
    system("chcp 1251");
    system("cls"); // Windows -     system("cls");
    printf("List of Buyers:\n");
    for (int i = 0; i < n; i++)
        {
        printf("Surname: %s\n", shopbuyer[i].surname);
        printf("Name: %s\n", shopbuyer[i].name);
        printf("Middle Name: %s\n", shopbuyer[i].middlename);
        printf("City: %s\n", shopbuyer[i].city);
        printf("Street: %s\n", shopbuyer[i].street);
        printf("Home Number: %d\n", shopbuyer[i].homenumber);
        printf("Flat Number: %d\n", shopbuyer[i].flatnumber);
        printf("Credit Card: %d\n", shopbuyer[i].creditcard);
        printf("Purchase Code: %d\n", shopbuyer[i].purchasecode);
        printf("\n");
        };
    menu();
};
// Функция поиска
void search()
{
    system("clear");
    printf("Enter keyword: ");
    char keyword[20];
    fgets(keyword, 20, stdin);

    /////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////
    if ((strcmp(keyword, "Surname") == 0) || (strcmp(keyword, "surname") == 0)) // Если ключевое слово равно title или Title
        {
            char surname2[30];
            printf("Enter Surname: ");
            fgets(surname2, 30, stdin);
            int c = 0;
            for (int i = 0; i < n; i++)
        { 
            if (strcmp (surname2, shopbuyer[i].surname) == 0) // Проверка на совпадение (0 - совпадение, иначе - 1)
            {
                printf("Surname: %s", shopbuyer[i].surname);
                c++;
            };
        
        };

            if (c == 0) {
                printf("Not Found");
            }  // Полная проверка списка title, если совпадений не найдено, вывод - Not found 
        };

    /////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////

    if ((strcmp(keyword, "Name") == 0) || (strcmp(keyword, "name") == 0)) // 
        {
            char name2[30];
            printf("Enter Name: ");
            fgets(name2, 30, stdin);
            int c = 0;
            for (int i = 0; i < n; i++)
        { 
            if (strcmp (name2, shopbuyer[i].name) == 0) 
            {
                printf("Name: %s", shopbuyer[i].name);
                c++;
            };
        
        };

            if (c == 0) {
                printf("Not Found");
            }  
        };

    /////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////

    if ((strcmp(keyword, "Middle Name") == 0) || (strcmp(keyword, "middle name") == 0) || (strcmp(keyword, "Middle name") == 0)) // 
    {
        char middlename2[30];
        printf("Enter Middle Name: ");
        fgets(middlename2, 30, stdin);
        int c = 0;
        for (int i = 0; i < n; i++)
    { 
        if (strcmp (middlename2, shopbuyer[i].middlename) == 0) 
        {
            printf("Middle Name: %s", shopbuyer[i].middlename);
            c++;
        };
      
    };

        if (c == 0) {
            printf("Not Found");
        }  
    };

    /////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////

    if ((strcmp(keyword, "city") == 0) || (strcmp(keyword, "City") == 0)) // 
        {
            char city2[30];
            printf("Enter City: ");
            fgets(city2, 30, stdin);
            int c = 0;
            for (int i = 0; i < n; i++)
        { 
            if (strcmp (city2, shopbuyer[i].city) == 0) 
            {
                printf("City: %s", shopbuyer[i].city);
                c++;
            };
        
        };

            if (c == 0) {
                printf("Not Found");
            }  
        };

    /////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////
    
    if ((strcmp(keyword, "Street") == 0) || (strcmp(keyword, "street") == 0)) // 
        {
            char street2[30];
            printf("Enter Street: ");
            fgets(street2, 30, stdin);
            int c = 0;
            for (int i = 0; i < n; i++)
        { 
            if (strcmp (street2, shopbuyer[i].street) == 0) 
            {
                printf("Street: %s", shopbuyer[i].street);
                c++;
            };
        
        };

            if (c == 0) {
                printf("Not Found");
            }  
        };

    /////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////
    
    if ((strcmp(keyword, "Home Number") == 0) || (strcmp(keyword, "Home number") == 0) || (strcmp(keyword, "home number") == 0))
    {
        int homenumber2;
        printf("Enter Home Number: ");
        scanf("%d",&homenumber2);
        int c = 0;
        for (int i = 0; i < n; i++)
    { 
        if (homenumber2 == shopbuyer[i].homenumber)
        {
            printf("Home Number: %d", shopbuyer[i].homenumber);
            c++;
        };
      
    };

        if (c == 0) {
            printf("Not Found");
        }  
    };

    /////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////

    if ((strcmp(keyword, "Flat Number") == 0) || ((strcmp(keyword, "Flat number") == 0)) || ((strcmp(keyword, "flat number") == 0))) // 
    {
        int flatnumber2;
        printf("Enter Flat Number: ");
        scanf("%d",&flatnumber2);
        int c = 0;
        for (int i = 0; i < n; i++)
    { 
        if (flatnumber2 == shopbuyer[i].flatnumber) 
        {
            printf("Flat Number: %d", shopbuyer[i].flatnumber);
            c++;
        };
      
    };

        if (c == 0) {
            printf("Not Found");
        }  
    };

    /////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////

    if ((strcmp(keyword, "Credit Card") == 0) || ((strcmp(keyword, "Credit card") == 0)) || ((strcmp(keyword, "credit card") == 0))) // 
    {
        int creditcard2;
        printf("Enter Credit Card: ");
        scanf("%d",&creditcard2);
        int c = 0;
        for (int i = 0; i < n; i++)
    { 
        if (creditcard2 == shopbuyer[i].creditcard) 
        {
            printf("Credit Card: %d", shopbuyer[i].creditcard);
            c++;
        };
      
    };

        if (c == 0) {
            printf("Not Found");
        }  
    };

    /////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////

    if ((strcmp(keyword, "Purchase Code") == 0) || ((strcmp(keyword, "Purchase code") == 0)) || ((strcmp(keyword, "purchase Code") == 0))) // 
    {
        int purchasecode2;
        printf("Enter Purchase Code: ");
        scanf("%d",&purchasecode2);
        int c = 0;
        for (int i = 0; i < n; i++)
    { 
        if (purchasecode2 == shopbuyer[i].purchasecode) 
        {
            printf("Purchase Code: %d", shopbuyer[i].purchasecode);
            c++;
        };
      
    };

        if (c == 0) {
            printf("Not Found");
        }  
    };

    /////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////
// Функция сортировки пузырьком
void sortirovka()
{
    system("chcp 1251");
    system("cls");
    char keyword[20];
    printf("Enter keyword to sort (choose one of them) \n");
    printf("Home Number, Flat Number, Credit Card, Purchase Code: ");
    fgets(keyword, 20, stdin);

  //Сортировка по Home_Number
    if ((strcmp(keyword, "Home Number") == 0) || (strcmp(keyword, "Home number") == 0) || (strcmp(keyword, "home number") == 0))
    {
        for (int i = 0; i < n; i++)
        {
            for (int j = n - 1; j >= i; j--)
            {
                if (shopbuyer[j-1].homenumber > shopbuyer[j].homenumber)
                {
                    int temp = 0;
                    temp = shopbuyer[j-1].homenumber;
                    shopbuyer[j-1].homenumber = shopbuyer[j].homenumber;
                    shopbuyer[j].homenumber = temp;
                };
            };
        };
    };
    
    if ((strcmp(keyword, "Flat Number") == 0) || (strcmp(keyword, "Flat number") == 0) || (strcmp(keyword, "flat number") == 0))
    {
        for (int i = 0; i < n; i++)
        {
            for (int j = n - 1; j >= i; j--)
            {
                if (shopbuyer[j-1].flatnumber > shopbuyer[j].flatnumber)
                {
                    int temp = 0;
                    temp = shopbuyer[j-1].flatnumber;
                    shopbuyer[j-1].flatnumber = shopbuyer[j].flatnumber;
                    shopbuyer[j].flatnumber = temp;
                };
            };
        };
    };

    if ((strcmp(keyword, "Credit Card") == 0) || (strcmp(keyword, "Credit card") == 0) || (strcmp(keyword, "credit card") == 0))
    {
        for (int i = 0; i < n; i++)
        {
            for (int j = n - 1; j >= i; j--)
            {
                if (shopbuyer[j-1].creditcard > shopbuyer[j].creditcard)
                {
                    int temp = 0;
                    temp = shopbuyer[j-1].creditcard;
                    shopbuyer[j-1].creditcard = shopbuyer[j].creditcard;
                    shopbuyer[j].creditcard = temp;
                };
            };
        };
    };

    if ((strcmp(keyword, "Purchase Code") == 0) || (strcmp(keyword, "Purchase code") == 0) || (strcmp(keyword, "purchase code") == 0))
    {
        for (int i = 0; i < n; i++)
        {
            for (int j = n - 1; j >= i; j--)
            {
                if (shopbuyer[j-1].purchasecode > shopbuyer[j].purchasecode)
                {
                    int temp = 0;
                    temp = shopbuyer[j-1].purchasecode;
                    shopbuyer[j-1].purchasecode = shopbuyer[j].purchasecode;
                    shopbuyer[j].purchasecode = temp;
                };
            };
        };
    };
    printf("Sort complete\n");
    menu();
};

int main()
{
    printf("Enter count of buyers: ");
    scanf("%d",&n);
    system("chcp 1251");
    system("cls");
    for (int i = 0; i < n; i++)
        {
        printf("Surname №%d: ", i+1);
        fgets(shopbuyer[i].surname, 30, stdin);
        printf("Name №%d: ", i+1);
        fgets(shopbuyer[i].name, 30, stdin);
        printf("Middle Name №%d: ", i+1);
        fgets(shopbuyer[i].middlename, 30, stdin);
        printf("City №%d: ", i+1);
        fgets(shopbuyer[i].city, 30, stdin);
        printf("Street №%d: ", i+1);
        fgets(shopbuyer[i].street, 30, stdin);
        printf("Home Number №%d", i+1);
        scanf("%d", &shopbuyer[i].homenumber);
        printf("Flat Number №%d", i+1);
        scanf("%d", &shopbuyer[i].flatnumber);
        printf("Credit Card №%d", i+1);
        scanf("%d", &shopbuyer[i].creditcard);
        printf("Purchase Code №%d", i+1);
        scanf("%d", &shopbuyer[i].purchasecode);
        };  
    menu();
    return 0;
}}

Ошибки такие:

/usr/bin/x86_64-linux-gnu-ld: /usr/lib/debug/usr/lib/x86_64-linux-gnu/crt1.o(.debug_info): relocation 0 has invalid symbol index 11
/usr/bin/x86_64-linux-gnu-ld: /usr/lib/debug/usr/lib/x86_64-linux-gnu/crt1.o(.debug_info): relocation 1 has invalid symbol index 12
/usr/bin/x86_64-linux-gnu-ld: /usr/lib/debug/usr/lib/x86_64-linux-gnu/crt1.o(.debug_info): relocation 2 has invalid symbol index 2
/usr/bin/x86_64-linux-gnu-ld: /usr/lib/debug/usr/lib/x86_64-linux-gnu/crt1.o(.debug_info): relocation 3 has invalid symbol index 2
/usr/bin/x86_64-linux-gnu-ld: /usr/lib/debug/usr/lib/x86_64-linux-gnu/crt1.o(.debug_info): relocation 4 has invalid symbol index 11
/usr/bin/x86_64-linux-gnu-ld: /usr/lib/debug/usr/lib/x86_64-linux-gnu/crt1.o(.debug_info): relocation 5 has invalid symbol index 13
/usr/bin/x86_64-linux-gnu-ld: /usr/lib/debug/usr/lib/x86_64-linux-gnu/crt1.o(.debug_info): relocation 6 has invalid symbol index 13
/usr/bin/x86_64-linux-gnu-ld: /usr/lib/debug/usr/lib/x86_64-linux-gnu/crt1.o(.debug_info): relocation 7 has invalid symbol index 13
/usr/bin/x86_64-linux-gnu-ld: /usr/lib/debug/usr/lib/x86_64-linux-gnu/crt1.o(.debug_info): relocation 8 has invalid symbol index 12
/usr/bin/x86_64-linux-gnu-ld: /usr/lib/debug/usr/lib/x86_64-linux-gnu/crt1.o(.debug_info): relocation 9 has invalid symbol index 13
/usr/bin/x86_64-linux-gnu-ld: /usr/lib/debug/usr/lib/x86_64-linux-gnu/crt1.o(.debug_info): relocation 10 has invalid symbol index 13
/usr/bin/x86_64-linux-gnu-ld: /usr/lib/debug/usr/lib/x86_64-linux-gnu/crt1.o(.debug_info): relocation 11 has invalid symbol index 13
/usr/bin/x86_64-linux-gnu-ld: /usr/lib/debug/usr/lib/x86_64-linux-gnu/crt1.o(.debug_info): relocation 12 has invalid symbol index 13
/usr/bin/x86_64-linux-gnu-ld: /usr/lib/debug/usr/lib/x86_64-linux-gnu/crt1.o(.debug_info): relocation 13 has invalid symbol index 13
/usr/bin/x86_64-linux-gnu-ld: /usr/lib/debug/usr/lib/x86_64-linux-gnu/crt1.o(.debug_info): relocation 14 has invalid symbol index 13
/usr/bin/x86_64-linux-gnu-ld: /usr/lib/debug/usr/lib/x86_64-linux-gnu/crt1.o(.debug_info): relocation 15 has invalid symbol index 13
/usr/bin/x86_64-linux-gnu-ld: /usr/lib/debug/usr/lib/x86_64-linux-gnu/crt1.o(.debug_info): relocation 16 has invalid symbol index 13
/usr/bin/x86_64-linux-gnu-ld: /usr/lib/debug/usr/lib/x86_64-linux-gnu/crt1.o(.debug_info): relocation 17 has invalid symbol index 13
/usr/bin/x86_64-linux-gnu-ld: /usr/lib/debug/usr/lib/x86_64-linux-gnu/crt1.o(.debug_info): relocation 18 has invalid symbol index 13
/usr/bin/x86_64-linux-gnu-ld: /usr/lib/debug/usr/lib/x86_64-linux-gnu/crt1.o(.debug_info): relocation 19 has invalid symbol index 21
/usr/bin/x86_64-linux-gnu-ld: /usr/lib/debug/usr/lib/x86_64-linux-gnu/crt1.o(.debug_line): relocation 0 has invalid symbol index 2
/usr/lib/gcc/x86_64-linux-gnu/6/../../../x86_64-linux-gnu/crt1.o: In function `_start':
(.text+0x20): undefined reference to `main'
/tmp/ccr8pCVg.o: In function `menu':
main.c:(.text+0x9b): undefined reference to `sortirovka'
collect2: error: ld returned 1 exit status