Determine the maximum number of requests (OS-level queue depth) for the given file path

path_queue_depth.cc

#include <sys/sysmacros.h>
#include <sys/types.h>
#include <sys/stat.h>
#include <fcntl.h>
#include <unistd.h>
#include <iostream>
#include <string>

int main(int argc, char** argv) {
    if (argc != 2) {
        std::cerr << "Usage: " << argv[0] << "<path>\n";
        return 1;
    }

    struct stat statbuf;
    if (stat(argv[1], &statbuf)) {
        perror("stat() failed");
        return 1;
    }

    std::string dqdPath = "/sys/dev/block/";
    dqdPath += std::to_string(major(statbuf.st_dev)) + ":" +
        std::to_string(minor(statbuf.st_dev)) + "/queue/nr_requests";

    auto fd = open(dqdPath.c_str(), O_RDONLY);
    if (fd == -1) {
        std::string msg{"open('" + dqdPath + "') failed"};
        perror(msg.c_str());
        return 1;
    }

    std::string buffer;
    buffer.resize(16);
    auto len = read(fd, &buffer[0], buffer.size());
    if ((len <= 0) || (len >= buffer.size())) {
        std::string msg{"read('" + dqdPath + "') failed"};
        perror(msg.c_str());
        return 1;
    }
    buffer.resize(len);

    std::cout << "queue_depth: " << buffer << "\n";
    return 0;
}