This example should add an ‘inotify_add_watch’ functionality for a newly created subsequent directory within a given path (for example: /tmp), but does not add that watch function for the newly created directory (for example: /tmp/subdir1/subdir2).

Only for the root path directory (/tmp) level, given by argv[1], it is functional (for example: /tmp/subdir1). The returned watch descriptor (variable ‘watch_fd’) is increased.
For a directory created in a more distant tree position (/tmp/subdir1/subdir2) from root directory (/tmp), there’s that ‘no such file or directory’ error. The returned watch descriptor ‘watch_fd’ then is -1.

#include <stdio.h>
#include <stdlib.h>
#include <dirent.h>
#include <errno.h>
#include <sys/stat.h>
#include <sys/inotify.h>
#include <unistd.h>
#include <limits.h>
#include <string.h>

#define EVENT_SIZE (sizeof(struct inotify_event))
#define EVENT_BUF_LEN (1024 * (EVENT_SIZE + 16))

// Function to add inotify watch recursively up to a specified depth
void add_watch_recursive(int inotify_fd, const char *base_path, int depth, int current_depth) {
    if (current_depth > depth) {
        return;
    }

    printf("base_path %s t", base_path);
    int watch_fd = inotify_add_watch(inotify_fd, base_path, IN_CREATE | IN_DELETE);
    printf("watch_fd %d n", watch_fd);
    if (watch_fd < 0) {
        perror("inotify_add_watch");
        printf("ttt 'inotify_add_watch: No such file or directory' watch_fd %dn", watch_fd);
        return;
    }


    DIR *dir = opendir(base_path);
    if (!dir) {
        perror("opendir");
        return;
    }

    struct dirent *entry;
    while ((entry = readdir(dir)) != NULL) {
        if (strcmp(entry->d_name, ".") == 0 || strcmp(entry->d_name, "..") == 0) {
            continue;
        }

        char path[PATH_MAX];
        snprintf(path, PATH_MAX, "%s/%s", base_path, entry->d_name);

        struct stat statbuf;
        if (stat(path, &statbuf) == 0 && S_ISDIR(statbuf.st_mode)) {
            add_watch_recursive(inotify_fd, path, depth, current_depth + 1);
        }
    }

    closedir(dir);
}

int main(int argc, char *argv[]) {
    if (argc < 3) {
        fprintf(stderr, "Usage: %s <path> <depth>n", argv[0]);
        return EXIT_FAILURE;
    }

    const char *path = argv[1];
    int depth = atoi(argv[2]);

    if (depth < 0) {
        fprintf(stderr, "Error: Depth must be a non-negative integern");
        return EXIT_FAILURE;
    }

    // Print the path to verify it
    printf("Attempting to open directory: %sn", path);

    // Check if the directory exists
    struct stat statbuf;
    if (stat(path, &statbuf) != 0) {
        perror("stat");
        return EXIT_FAILURE;
    }

    // Check if it is a directory
    if (!S_ISDIR(statbuf.st_mode)) {
        fprintf(stderr, "Error: %s is not a directoryn", path);
        return EXIT_FAILURE;
    }

    // Initialize inotify
    int inotify_fd = inotify_init();
    if (inotify_fd < 0) {
        perror("inotify_init");
        return EXIT_FAILURE;
    }

    // Add watch on the specified directory and its subdirectories up to the specified depth
    add_watch_recursive(inotify_fd, path, depth, 0);

    printf("Watching directory: %s and its subdirectories up to depth: %dn", path, depth);

    // Buffer to store inotify events
    char buffer[EVENT_BUF_LEN];

    // Event loop
    while (1) {
        int length = read(inotify_fd, buffer, EVENT_BUF_LEN);
        if (length < 0) {
            perror("read");
            break;
        }

        int i = 0;
        while (i < length) {
            struct inotify_event *event = (struct inotify_event *) &buffer[i];
            if (event->len) {
                char event_path[PATH_MAX];
                snprintf(event_path, PATH_MAX, "%s/%s", path, event->name);

                if (event->mask & IN_CREATE) {
                    if (event->mask & IN_ISDIR) {
                        printf("Directory created: %sn", event_path);
                        // Add watch to the new directory if within the specified depth
                        add_watch_recursive(inotify_fd, event_path, depth, 1);
                    } else {
                        printf("File created: %sn", event_path);
                    }
                } else if (event->mask & IN_DELETE) {
                    if (event->mask & IN_ISDIR) {
                        printf("Directory deleted: %sn", event_path);
                    } else {
                        printf("File deleted: %sn", event_path);
                    }
                }
            }
            i += EVENT_SIZE + event->len;
        }
    }

    // Clean up
    close(inotify_fd);

    return EXIT_SUCCESS;
}

This example code for a C program for Linux system is compiled with
gcc inotifyaddwatch_subdirs.c -o inotifyaddwatch_subdirs and
started with
./inotifyaddwatch_subdirs /tmp 5 .

Only with restarting the program, a subsequent directory is added from startup, but not within the running program.

If it is a pointer error for a sub directory’s singular path reference within the line
int watch_fd = inotify_add_watch(inotify_fd, base_path, IN_CREATE | IN_DELETE);
for the variable ‘base_path’, then it would need an additional exception handling.

The man pages for ‘inotify_add_watch’ explains for this error return value
“ENOENT A directory component in pathname does not exist or is a dangling symbolic link.”,
but i did not succeed in adding that sub directory path (proved to exist and being recognized within that program) to that pathname pointer array.