Why gethostname function in C causes the PATH env to be cleared?

This is a code for a small shell called imcsh:

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <unistd.h>
#include <sys/types.h>
#include <sys/wait.h>
#include <signal.h>

#define MAX_INPUT_SIZE 1024
#define MAX_TOKEN_SIZE 64
#define MAX_NUM_TOKENS 64

// Tokenizes the input string based on whitespace
void tokenizeInput(char *input, char **tokens, int *numTokens)
{
    char *token = strtok(input, " \t\n");

    *numTokens = 0;
    while (token != NULL && *numTokens < MAX_NUM_TOKENS)
    {
        tokens[*numTokens] = token;
        (*numTokens)++;
        token = strtok(NULL, " \t\n");
    }
}

// Executes the given command and waits for it to finish
void executeCommand(char **tokens, int numTokens, int background)
{
    pid_t pid = fork();

    if (pid == 0)
    { // Child process
        if (background)
        {
            // Ignore SIGINT signal for background processes
            signal(SIGINT, SIG_IGN);
        }

        // Execute the command
        execvp(tokens[0], tokens);

        // If execvp returns, there was an error
        printf("imcsh: command not found: %s\n", tokens[0]);
        exit(1);
    }
    else if (pid > 0)
    { // Parent process
        if (!background)
        {
            // Wait for the child process to finish
            waitpid(pid, NULL, 0);
        }
        else
        {
            // Print the PID of the background process
            printf("Background process PID: %d\n", pid);
        }
    }
    else
    { // Error occurred
        printf("Failed to fork()\n");
        exit(1);
    }
}

// Prints the version of imcsh
void showVersion()
{
    printf("IMCSH Version 1.1 created by me.\n");
}

// Redirects the output to a file
void redirectOutput(char **tokens, int numTokens)
{
    // Find the ">" symbol in the tokens
    int i;
    for (i = 0; i < numTokens; i++)
    {
        if (strcmp(tokens[i], ">") == 0)
        {
            break;
        }
    }

    // If ">" symbol found and there is a file name after it
    if (i < numTokens - 1)
    {
        char *filename = tokens[i + 1];

        // Open the file for writing (create if it doesn't exist, append if it does)
        FILE *file = fopen(filename, "a");
        if (file != NULL)
        {
            // Duplicate the file descriptor of the opened file
            int fd = fileno(file);

            // Redirect the stdout to the file descriptor
            dup2(fd, STDOUT_FILENO);

            fclose(file);
        }
        else
        {
            printf("imcsh: failed to open file: %s\n", filename);
        }
    }
    else
    {
        printf("imcsh: invalid command syntax\n");
    }
}

int main()
{
    char input[MAX_INPUT_SIZE];
    char *tokens[MAX_NUM_TOKENS];
    int numTokens;
    char *username = getlogin();
    if (username == NULL)
    {
        printf("Failed to get username.\n");
    }

    char hostname[1024];
    if (gethostname(hostname, sizeof(hostname)) != 0)
    {
        printf("Failed to get hostname.\n");
        exit(1);
    }

    while (1)
    {
        // Print the command prompt
        // printf("%s@%s> ", username, hostname);
        printf("user@host> ");

        // Read user input
        fgets(input, sizeof(input), stdin);
        // Tokenize the input
        tokenizeInput(input, tokens, &numTokens);

        if (numTokens > 0)
        {
            // Check for internal commands
            if (strcmp(tokens[0], "exec") == 0)
            {
                int background = 0;

                // Check for "&" symbol at the end (for background execution)
                if (numTokens > 1 && strcmp(tokens[numTokens - 1], "&") == 0)
                {
                    background = 1;
                    tokens[numTokens - 1] = NULL; // Remove "&" symbol
                }

                // Execute the command
                executeCommand(tokens + 1, numTokens - 1, background);
            }
            else if (strcmp(tokens[0], "globalusage") == 0)
            {
                showVersion();
            }
            else if (strcmp(tokens[0], "quit") == 0)
            {
                // Check for running processes
                int status;
                pid_t result;

                do
                {
                    result = waitpid(-1, &status, WNOHANG);
                    if (result > 0)
                    {
                        printf("Running process: %d\n", result);
                    }
                } while (result > 0);

                // Prompt for confirmation
                printf("The following processes are running, are you sure you want to quit? [Y/n]: ");

                char confirm[5];
                fgets(confirm, sizeof(confirm), stdin);

                if (confirm[0] == 'Y' || confirm[0] == 'y')
                {
                    // Terminate all running processes and exit the shell
                    kill(0, SIGTERM);
                    break;
                }
            }
            else
            {
                // External command, execute it
                executeCommand(tokens, numTokens, 0);
            }
        }
    }

    return 0;
}

if I run this and enter a command, for example, ls, it says command not found. But if I comment this section:

if (gethostname(hostname, sizeof(hostname)) != 0)
    {
        printf("Failed to get hostname.\n");
        exit(1);
    } 

ls will work. I tried to set PATH variable again using setenv("PATH", "/bin:/usr/bin:/usr/local/bin", 1);
But the problem still persists. What is the problem?

You're not constructing the token list correctly.

The second argument to the execv function is an array of strings which is expected to be terminated by a NULL pointer. You don't set this null pointer, to execv keeps reading arguments until it finds one. This will read into uninitialized array elements and possibly past the end of the array, triggering undefined behavior.

After you've populated the token list, add a NULL pointer to the end:

void tokenizeInput(char *input, char **tokens, int *numTokens)
{
    char *token = strtok(input, " \t\n");

    *numTokens = 0;
    while (token != NULL && *numTokens < MAX_NUM_TOKENS - 1)
    {
        tokens[*numTokens] = token;
        (*numTokens)++;
        token = strtok(NULL, " \t\n");
    }
    tokens[*numTokens] = NULL;
}