use fcntl to lock file interactively

file_lock.cpp

#include <iostream>
#include <dirent.h>
#include <unistd.h>
#include <string.h>
#include <sys/types.h>
#include <sys/stat.h>
#include <sys/file.h>
#include <errno.h>
#include "util.h"
#include <vector>

using namespace std;

int GetDirFiles(const std::string &dir, std::vector<std::string> *result) {
    int res = 0;
    result->clear();
    DIR *d = opendir(dir.c_str());
    if (d == NULL) {
        return errno;
    }
    struct dirent *entry;
    while ((entry = readdir(d)) != NULL) {
        if (strcmp(entry->d_name, "..") == 0 || strcmp(entry->d_name, ".") == 0) {
            continue;
        }
        result->push_back(entry->d_name);
    }
    closedir(d);
    return res;
}


int GetFileLength(const std::string &file) {
    struct stat stat_buf;
    int rc = stat(file.c_str(), &stat_buf);
    return rc == 0 ? stat_buf.st_size : -1;
}


int FileAppend(int fd, const std::string &value) {
    if (fd < 0) {
        return -1;
    }
    size_t value_len = value.size();
    const char *pos = value.data();
    while (value_len > 0) {
        ssize_t r = write(fd, pos, value_len);
        if (r < 0) {
            if (errno == EINTR) {
                continue;  // Retry
            }
            return -1;
        }
        pos += r;
        value_len -= r;
    }
    return 0;
}

bool FileExists(const std::string &path) {
    return access(path.c_str(), F_OK) == 0;
}


void test() {
    const string file_name = "/Users/unity/git/engine/Makefile";
    int return_code = 0;

    vector<string> result;
    GetDirFiles("/", &result);
    for (auto x:result) {
        cout << x << endl;
    }


    int l = GetFileLength(file_name);
    cout << l << endl;


    return_code = access(file_name.c_str(), F_OK | R_OK | W_OK);
    cout << return_code << endl;
}

void prompt(string msg) {
    cout << msg << endl;
    getchar();
}

void checkrc(int rc, string err_msg) {
    if (rc < 0) {
        cerr << "code:" << rc << endl;
        cerr << "when: " << err_msg << endl;
        perror("error");
        exit(-1);
    }
}

int main() {
    int fd = open("test5.txt", O_CREAT|O_RDWR,0666);
    int rc;

    checkrc(fd, "cannot open file");


    prompt("press enter to set lock");
    struct flock f;
    memset(&f, 0, sizeof(f));
    f.l_type = F_WRLCK;
    f.l_whence = SEEK_SET;
    rc = fcntl(fd, F_SETLK, &f);
    checkrc(rc, "set lock");

    prompt("press enter to unlock");
    f.l_type = F_UNLCK;
    rc = fcntl(fd, F_UNLCK, &f);

    prompt("press enter to exit");

    close(fd);
    return 0;
}