passwd`¶

Problem ¶

/etc/passwd contains lines of the form

jfasch:x:1000:1000:Joerg Faschingbauer:/home/jfasch:/bin/bash

We want to parse such content conveniently in C++ [1]

Requirements ¶

Library Function (`lib/parse-passwd.cpp`)¶

Write a function

User parse_passwd_line(std::string line);

The parameter line is one line as read from /etc/passwd
On success, the function returns an object of type User (definition see below)
On failure, the function throws an exception object of type PasswdError (definition see below)

The header file lib/parse-passwd.h contains the type definitions and the function declaration, the implementation file lib/parse-passwd.cpp contains only the function skeleton which does nothing, and which has to be implemented.

Important

lib/parse-passwd.cpp is the only file that must be modified in this exercise!

lib/parse-passwd.h (download)¶

#pragma once

#include <string>


/** User

    Represents the data from one line of /etc/passwd, e.g.
    "jfasch:x:1000:1000:Joerg Faschingbauer:/home/jfasch:/bin/bash"

    See ``man -s 5 passwd``

 */
struct User
{
    User() : uid(-1), gid(-1) {}    // <--- default constructor

    std::string name;
    std::string passwd;
    int uid;
    int gid;
    std::string descr;
    std::string homedir;
    std::string shell;
};

/** PasswdError

    Exception class to be throw on error from parse_passwd_line() (see
    below)

 */
struct PasswdError
{
    enum Code {                              // <--- nested type definition (error codes)
        LINE_INVALID,
    };

    PasswdError(Code code, std::string msg)  // <--- default constructor
    : code(code), msg(msg) {}

    Code code;                               // <--- struct member
    std::string msg;                         // <--- struct member
};

/** parse_passwd_line

    Given a line, parse from it items of ``struct User``.
    Throw exceptions of type ``PasswdError`` as they are encountered.

    See the test suite for detailed specs.

 */
User parse_passwd_line(std::string line);

Tests (`tests/suite-passwd-line.cpp`)¶

The specification of the parse_passwd_line() function is in ./tests/suite-passwd-line.cpp. The other test suites in tests/ are implementation hints (see below).

Do not modify the tests!

Program (`bin/read-passwd.cpp`)¶

Finally, after all tests pass, the program built from bin/read-passwd.cpp (this file must not be modified) should give an output similar to this:

$ ./bin/read-passwd /etc/passwd
...
User.name    jfasch
    .passwd  x
    .uid     1000
    .gid     1000
    .descr   Jörg Faschingbauer
    .homedir /home/jfasch
    .shell   /bin/bash
...

Hints ¶

How To Proceed ¶

Comment out all tests
Build the project
In a top-down manner …
- Comment back in one test after the other
- Fix parse_passwd_line() until the current succeeds

During the implementation of parse_passwd_line() you will need to use

User objects that are returned on success from the function
PasswdError objects that are thrown as exceptions from the function
Miscellaneous std::string functionality to dissect a line

`User` Usage (`tests/suite-passwd-user.cpp`)¶

parse_passwd_line() returns an object of type User. See below how such objects are initialized and used.

tests/suite-passwd-user.cpp (download)¶

#include <parse-passwd.h>

#include <gtest/gtest.h>


// how to create a User object
TEST(passwd_user_suite, object_construction_and_member_access)
{
    User user;    // <--- calls default constructor

    // all default-initialized (even the numbers)
    ASSERT_EQ(user.name, "");
    ASSERT_EQ(user.passwd, "");
    ASSERT_EQ(user.uid, -1);
    ASSERT_EQ(user.gid, -1);
    ASSERT_EQ(user.descr, "");
    ASSERT_EQ(user.homedir, "");
    ASSERT_EQ(user.shell, "");
}

`PasswdError` Usage (`tests/suite-passwd-error.cpp`)¶

The exception type PasswdError has a nested enum type, which is a bit hard to use for C++ beginners. See tests/suite-passwd-error.cpp for usage examples - especially how you use the type in a throw statement.

tests/suite-passwd-error.cpp (download)¶

#include <parse-passwd.h>

#include <gtest/gtest.h>


// how to create an error
TEST(passwd_error_suite, object_construction_and_member_access)
{
    PasswdError error(/*code*/PasswdError::LINE_INVALID, /*msg*/"howdy");

    PasswdError::Code code = error.code;
    std::string msg = error.msg;

    ASSERT_EQ(code, PasswdError::LINE_INVALID);
    ASSERT_EQ(msg, "howdy");
}

// how to throw an error
static User parse_passwd_line__fake(std::string line)
{
    User u;

    // lets say we see an error during parsing the line
    if (false /*error :-)*/) {
        throw PasswdError(PasswdError::LINE_INVALID, "howdy");
    }

    return u;   // never reached. in this fake-function, we only
                // demonstrate how to cause an error.
}

// how to call code that might throw an error of type PasswdError
TEST(passwd_error_suite, throwing_and_catching)
{
    try {
        parse_passwd_line__fake("some:line:with:an:error");
    }
    catch (const PasswdError& error) {           // <--- catch-block entered if a PasswdError object is thrown
        PasswdError::Code code = error.code;
        std::string msg = error.msg;

        ASSERT_EQ(code, PasswdError::LINE_INVALID);
        ASSERT_EQ(msg, "howdy");
    }
}

`std::string` Usage ¶

Dissecting lines like

jfasch:x:1000:1000:Joerg Faschingbauer:/home/jfasch:/bin/bash

will likely require the following tools.

Searching for the field delimiter ':'. See std::string::find(). Take care of not-found conditions; s.npos is typically returned by string searches.
Given start and end positions of a fields, you need to take a substring. See std::string::substr().
Numeric fields (UID and GID) have to be converted from strings. See std::stoi(). Don’t forget to handle error conditions (see the example in the corresponding section in std::string)

Dependencies ¶

Footnotes

Exercise: Parse A Line From /etc/passwd¶

Exercise: Parse A Line From `/etc/passwd`¶