0

I am developing a limited version of shell for my self. Here is a C++ program 

#include <iostream>
#include <cstdio>
#include <cstring>
#include <cstdlib>
#include <unistd.h>
using namespace std;
const int MAX = 256;
const int CMD_MAX = 10;
char *valid_cmds = " ls ps df ";
int main()
{
        char line_input[MAX], the_cmd[CMD_MAX];
        char *new_args[CMD_MAX], *cp;
        int i;
        while (1) {
                cout << "cmd> ";
                if (cin.getline(line_input, MAX, '\n') != NULL) {
                        cp = line_input;
                        i = 0;
                        if ((new_args[i] = strtok(cp, " ")) != NULL) {
                                strcpy(the_cmd, new_args[i]);
                                strcat(the_cmd, " ");
                                if ((strstr(valid_cmds, the_cmd) -valid_cmds)% 4 == 1) {
                                        do {
                                                cp = NULL;
                                                new_args[++i] = strtok(cp, " ");
                                        } while (i < CMD_MAX
                                                 && new_args[i] != NULL);
                                        new_args[i] = NULL;
                                        switch (fork()) {
                                        case 0:
                                                execvp(new_args[0], new_args);
                                                perror("exec failure");
                                                exit(1);
                                        case -1:
                                                perror("fork failure");
                                                exit(2);
                                        default:
                                                // In the parent we should be waiting for
                                                // the child to finis h
                                                ;
                                        }
                                } else
                                        cout << "huh?" << endl;
                        }
                }
        }
}

What I am not able to understand is when I compile this program I am getting error in line 

 (cin.getline(line_input, MAX, '\n') != NULL)

and the error is following I do a $ g++ p3.7.cpp 

p3.7.cpp:10:20: warning: ISO C++ forbids converting a string constant to ‘char*’ [-Wwrite-strings]
 char *valid_cmds = " ls ps df ";
                    ^~~~~~~~~~~~
p3.7.cpp: In function ‘int main()’:
p3.7.cpp:18:42: error: no match for ‘operator!=’ (operand types are ‘std::basic_istream<char>’ and ‘long int’)
   if (cin.getline(line_input, MAX, '\n') != NULL) {
                                          ^
p3.7.cpp:18:42: note: candidate: operator!=(int, long int) <built-in>
p3.7.cpp:18:42: note:   no known conversion for argument 1 from ‘std::basic_istream<char>’ to ‘int’
In file included from /usr/lib/gcc/x86_64-pc-cygwin/7.3.0/include/c++/iosfwd:40:0,
                 from /usr/lib/gcc/x86_64-pc-cygwin/7.3.0/include/c++/ios:38,
                 from /usr/lib/gcc/x86_64-pc-cygwin/7.3.0/include/c++/ostream:38,
                 from /usr/lib/gcc/x86_64-pc-cygwin/7.3.0/include/c++/iostream:39,
                 from p3.7.cpp:2:
/usr/lib/gcc/x86_64-pc-cygwin/7.3.0/include/c++/bits/postypes.h:221:5: note: candidate: template<class _StateT> bool std::operator!=(const std::fpos<_StateT>&, const std::fpos<_StateT>&)
     operator!=(const fpos<_StateT>& __lhs, const fpos<_StateT>& __rhs)
     ^~~~~~~~
/usr/lib/gcc/x86_64-pc-cygwin/7.3.0/include/c++/bits/postypes.h:221:5: note:   template argument deduction/substitution failed:
In file included from /usr/include/sys/unistd.h:14:0,
                 from /usr/include/unistd.h:4,
                 from p3.7.cpp:6:
p3.7.cpp:18:45: note:   ‘std::basic_istream<char>’ is not derived from ‘const std::fpos<_StateT>’
   if (cin.getline(line_input, MAX, '\n') != NULL) {
                                             ^
In file included from /usr/lib/gcc/x86_64-pc-cygwin/7.3.0/include/c++/bits/stl_algobase.h:64:0,
                 from /usr/lib/gcc/x86_64-pc-cygwin/7.3.0/include/c++/bits/char_traits.h:39,
                 from /usr/lib/gcc/x86_64-pc-cygwin/7.3.0/include/c++/ios:40,
                 from /usr/lib/gcc/x86_64-pc-cygwin/7.3.0/include/c++/ostream:38,
                 from /usr/lib/gcc/x86_64-pc-cygwin/7.3.0/include/c++/iostream:39,
                 from p3.7.cpp:2:
/usr/lib/gcc/x86_64-pc-cygwin/7.3.0/include/c++/bits/stl_pair.h:456:5: note: candidate: template<class _T1, class _T2> constexpr bool std::operator!=(const std::pair<_T1, _T2>&, const std::pair<_T1, _T2>&)
     operator!=(const pair<_T1, _T2>& __x, const pair<_T1, _T2>& __y)
     ^~~~~~~~
/usr/lib/gcc/x86_64-pc-cygwin/7.3.0/include/c++/bits/stl_pair.h:456:5: note:   template argument deduction/substitution failed:
In file included from /usr/include/sys/unistd.h:14:0,
                 from /usr/include/unistd.h:4,
                 from p3.7.cpp:6:
p3.7.cpp:18:45: note:   ‘std::basic_istream<char>’ is not derived from ‘const std::pair<_T1, _T2>’
   if (cin.getline(line_input, MAX, '\n') != NULL) {
                                             ^
In file included from /usr/lib/gcc/x86_64-pc-cygwin/7.3.0/include/c++/bits/stl_algobase.h:67:0,
                 from /usr/lib/gcc/x86_64-pc-cygwin/7.3.0/include/c++/bits/char_traits.h:39,
                 from /usr/lib/gcc/x86_64-pc-cygwin/7.3.0/include/c++/ios:40,
                 from /usr/lib/gcc/x86_64-pc-cygwin/7.3.0/include/c++/ostream:38,
                 from /usr/lib/gcc/x86_64-pc-cygwin/7.3.0/include/c++/iostream:39,
                 from p3.7.cpp:2:
/usr/lib/gcc/x86_64-pc-cygwin/7.3.0/include/c++/bits/stl_iterator.h:311:5: note: candidate: template<class _Iterator> bool std::operator!=(const std::reverse_iterator<_Iterator>&, const std::reverse_iterator<_Iterator>&)
     operator!=(const reverse_iterator<_Iterator>& __x,
     ^~~~~~~~
/usr/lib/gcc/x86_64-pc-cygwin/7.3.0/include/c++/bits/stl_iterator.h:311:5: note:   template argument deduction/substitution failed:
In file included from /usr/include/sys/unistd.h:14:0,
                 from /usr/include/unistd.h:4,
                 from p3.7.cpp:6:
p3.7.cpp:18:45: note:   ‘std::basic_istream<char>’ is not derived from ‘const std::reverse_iterator<_Iterator>’
   if (cin.getline(line_input, MAX, '\n') != NULL) {
                                             ^
In file included from /usr/lib/gcc/x86_64-pc-cygwin/7.3.0/include/c++/bits/stl_algobase.h:67:0,
                 from /usr/lib/gcc/x86_64-pc-cygwin/7.3.0/include/c++/bits/char_traits.h:39,
                 from /usr/lib/gcc/x86_64-pc-cygwin/7.3.0/include/c++/ios:40,
                 from /usr/lib/gcc/x86_64-pc-cygwin/7.3.0/include/c++/ostream:38,
                 from /usr/lib/gcc/x86_64-pc-cygwin/7.3.0/include/c++/iostream:39,
                 from p3.7.cpp:2:
/usr/lib/gcc/x86_64-pc-cygwin/7.3.0/include/c++/bits/stl_iterator.h:349:5: note: candidate: template<class _IteratorL, class _IteratorR> bool std::operator!=(const std::reverse_iterator<_Iterator>&, const std::reverse_iterator<_IteratorR>&)
     operator!=(const reverse_iterator<_IteratorL>& __x,
     ^~~~~~~~
/usr/lib/gcc/x86_64-pc-cygwin/7.3.0/include/c++/bits/stl_iterator.h:349:5: note:   template argument deduction/substitution failed:
In file included from /usr/include/sys/unistd.h:14:0,
                 from /usr/include/unistd.h:4,
                 from p3.7.cpp:6:
p3.7.cpp:18:45: note:   ‘std::basic_istream<char>’ is not derived from ‘const std::reverse_iterator<_Iterator>’
   if (cin.getline(line_input, MAX, '\n') != NULL) {
                                             ^
In file included from /usr/lib/gcc/x86_64-pc-cygwin/7.3.0/include/c++/bits/stl_algobase.h:67:0,
                 from /usr/lib/gcc/x86_64-pc-cygwin/7.3.0/include/c++/bits/char_traits.h:39,
                 from /usr/lib/gcc/x86_64-pc-cygwin/7.3.0/include/c++/ios:40,
                 from /usr/lib/gcc/x86_64-pc-cygwin/7.3.0/include/c++/ostream:38,
                 from /usr/lib/gcc/x86_64-pc-cygwin/7.3.0/include/c++/iostream:39,
                 from p3.7.cpp:2:
/usr/lib/gcc/x86_64-pc-cygwin/7.3.0/include/c++/bits/stl_iterator.h:1130:5: note: candidate: template<class _IteratorL, class _IteratorR> bool std::operator!=(const std::move_iterator<_IteratorL>&, const std::move_iterator<_IteratorR>&)
     operator!=(const move_iterator<_IteratorL>& __x,
     ^~~~~~~~
/usr/lib/gcc/x86_64-pc-cygwin/7.3.0/include/c++/bits/stl_iterator.h:1130:5: note:   template argument deduction/substitution failed:
In file included from /usr/include/sys/unistd.h:14:0,
                 from /usr/include/unistd.h:4,
                 from p3.7.cpp:6:
p3.7.cpp:18:45: note:   ‘std::basic_istream<char>’ is not derived from ‘const std::move_iterator<_IteratorL>’
   if (cin.getline(line_input, MAX, '\n') != NULL) {
                                             ^
In file included from /usr/lib/gcc/x86_64-pc-cygwin/7.3.0/include/c++/bits/stl_algobase.h:67:0,
                 from /usr/lib/gcc/x86_64-pc-cygwin/7.3.0/include/c++/bits/char_traits.h:39,
                 from /usr/lib/gcc/x86_64-pc-cygwin/7.3.0/include/c++/ios:40,
                 from /usr/lib/gcc/x86_64-pc-cygwin/7.3.0/include/c++/ostream:38,
                 from /usr/lib/gcc/x86_64-pc-cygwin/7.3.0/include/c++/iostream:39,
                 from p3.7.cpp:2:
/usr/lib/gcc/x86_64-pc-cygwin/7.3.0/include/c++/bits/stl_iterator.h:1136:5: note: candidate: template<class _Iterator> bool std::operator!=(const std::move_iterator<_IteratorL>&, const std::move_iterator<_IteratorL>&)
     operator!=(const move_iterator<_Iterator>& __x,
     ^~~~~~~~
/usr/lib/gcc/x86_64-pc-cygwin/7.3.0/include/c++/bits/stl_iterator.h:1136:5: note:   template argument deduction/substitution failed:
In file included from /usr/include/sys/unistd.h:14:0,
                 from /usr/include/unistd.h:4,
                 from p3.7.cpp:6:
p3.7.cpp:18:45: note:   ‘std::basic_istream<char>’ is not derived from ‘const std::move_iterator<_IteratorL>’
   if (cin.getline(line_input, MAX, '\n') != NULL) {
                                             ^
In file included from /usr/lib/gcc/x86_64-pc-cygwin/7.3.0/include/c++/string:41:0,
                 from /usr/lib/gcc/x86_64-pc-cygwin/7.3.0/include/c++/bits/locale_classes.h:40,
                 from /usr/lib/gcc/x86_64-pc-cygwin/7.3.0/include/c++/bits/ios_base.h:41,
                 from /usr/lib/gcc/x86_64-pc-cygwin/7.3.0/include/c++/ios:42,
                 from /usr/lib/gcc/x86_64-pc-cygwin/7.3.0/include/c++/ostream:38,
                 from /usr/lib/gcc/x86_64-pc-cygwin/7.3.0/include/c++/iostream:39,
                 from p3.7.cpp:2:
/usr/lib/gcc/x86_64-pc-cygwin/7.3.0/include/c++/bits/allocator.h:158:5: note: candidate: template<class _T1, class _T2> bool std::operator!=(const std::allocator<_CharT>&, const std::allocator<_T2>&)
     operator!=(const allocator<_T1>&, const allocator<_T2>&)
     ^~~~~~~~
/usr/lib/gcc/x86_64-pc-cygwin/7.3.0/include/c++/bits/allocator.h:158:5: note:   template argument deduction/substitution failed:
In file included from /usr/include/sys/unistd.h:14:0,
                 from /usr/include/unistd.h:4,
                 from p3.7.cpp:6:
p3.7.cpp:18:45: note:   ‘std::basic_istream<char>’ is not derived from ‘const std::allocator<_CharT>’
   if (cin.getline(line_input, MAX, '\n') != NULL) {
                                             ^
In file included from /usr/lib/gcc/x86_64-pc-cygwin/7.3.0/include/c++/string:41:0,
                 from /usr/lib/gcc/x86_64-pc-cygwin/7.3.0/include/c++/bits/locale_classes.h:40,
                 from /usr/lib/gcc/x86_64-pc-cygwin/7.3.0/include/c++/bits/ios_base.h:41,
                 from /usr/lib/gcc/x86_64-pc-cygwin/7.3.0/include/c++/ios:42,
                 from /usr/lib/gcc/x86_64-pc-cygwin/7.3.0/include/c++/ostream:38,
                 from /usr/lib/gcc/x86_64-pc-cygwin/7.3.0/include/c++/iostream:39,
                 from p3.7.cpp:2:
/usr/lib/gcc/x86_64-pc-cygwin/7.3.0/include/c++/bits/allocator.h:164:5: note: candidate: template<class _Tp> bool std::operator!=(const std::allocator<_CharT>&, const std::allocator<_CharT>&)
     operator!=(const allocator<_Tp>&, const allocator<_Tp>&)
     ^~~~~~~~
/usr/lib/gcc/x86_64-pc-cygwin/7.3.0/include/c++/bits/allocator.h:164:5: note:   template argument deduction/substitution failed:
In file included from /usr/include/sys/unistd.h:14:0,
                 from /usr/include/unistd.h:4,
                 from p3.7.cpp:6:
p3.7.cpp:18:45: note:   ‘std::basic_istream<char>’ is not derived from ‘const std::allocator<_CharT>’
   if (cin.getline(line_input, MAX, '\n') != NULL) {
                                             ^
In file included from /usr/lib/gcc/x86_64-pc-cygwin/7.3.0/include/c++/string:52:0,
                 from /usr/lib/gcc/x86_64-pc-cygwin/7.3.0/include/c++/bits/locale_classes.h:40,
                 from /usr/lib/gcc/x86_64-pc-cygwin/7.3.0/include/c++/bits/ios_base.h:41,
                 from /usr/lib/gcc/x86_64-pc-cygwin/7.3.0/include/c++/ios:42,
                 from /usr/lib/gcc/x86_64-pc-cygwin/7.3.0/include/c++/ostream:38,
                 from /usr/lib/gcc/x86_64-pc-cygwin/7.3.0/include/c++/iostream:39,
                 from p3.7.cpp:2:
/usr/lib/gcc/x86_64-pc-cygwin/7.3.0/include/c++/bits/basic_string.h:6044:5: note: candidate: template<class _CharT, class _Traits, class _Alloc> bool std::operator!=(const std::basic_string<_CharT, _Traits, _Alloc>&, const std::basic_string<_CharT, _Traits, _Alloc>&)
     operator!=(const basic_string<_CharT, _Traits, _Alloc>& __lhs,
     ^~~~~~~~
/usr/lib/gcc/x86_64-pc-cygwin/7.3.0/include/c++/bits/basic_string.h:6044:5: note:   template argument deduction/substitution failed:
In file included from /usr/include/sys/unistd.h:14:0,
                 from /usr/include/unistd.h:4,
                 from p3.7.cpp:6:
p3.7.cpp:18:45: note:   ‘std::basic_istream<char>’ is not derived from ‘const std::basic_string<_CharT, _Traits, _Alloc>’
   if (cin.getline(line_input, MAX, '\n') != NULL) {
                                             ^
In file included from /usr/lib/gcc/x86_64-pc-cygwin/7.3.0/include/c++/string:52:0,
                 from /usr/lib/gcc/x86_64-pc-cygwin/7.3.0/include/c++/bits/locale_classes.h:40,
                 from /usr/lib/gcc/x86_64-pc-cygwin/7.3.0/include/c++/bits/ios_base.h:41,
                 from /usr/lib/gcc/x86_64-pc-cygwin/7.3.0/include/c++/ios:42,
                 from /usr/lib/gcc/x86_64-pc-cygwin/7.3.0/include/c++/ostream:38,
                 from /usr/lib/gcc/x86_64-pc-cygwin/7.3.0/include/c++/iostream:39,
                 from p3.7.cpp:2:
/usr/lib/gcc/x86_64-pc-cygwin/7.3.0/include/c++/bits/basic_string.h:6057:5: note: candidate: template<class _CharT, class _Traits, class _Alloc> bool std::operator!=(const _CharT*, const std::basic_string<_CharT, _Traits, _Alloc>&)
     operator!=(const _CharT* __lhs,
     ^~~~~~~~
/usr/lib/gcc/x86_64-pc-cygwin/7.3.0/include/c++/bits/basic_string.h:6057:5: note:   template argument deduction/substitution failed:
In file included from /usr/include/sys/unistd.h:14:0,
                 from /usr/include/unistd.h:4,
                 from p3.7.cpp:6:
p3.7.cpp:18:45: note:   mismatched types ‘const _CharT*’ and ‘std::basic_istream<char>’
   if (cin.getline(line_input, MAX, '\n') != NULL) {
                                             ^
In file included from /usr/lib/gcc/x86_64-pc-cygwin/7.3.0/include/c++/string:52:0,
                 from /usr/lib/gcc/x86_64-pc-cygwin/7.3.0/include/c++/bits/locale_classes.h:40,
                 from /usr/lib/gcc/x86_64-pc-cygwin/7.3.0/include/c++/bits/ios_base.h:41,
                 from /usr/lib/gcc/x86_64-pc-cygwin/7.3.0/include/c++/ios:42,
                 from /usr/lib/gcc/x86_64-pc-cygwin/7.3.0/include/c++/ostream:38,
                 from /usr/lib/gcc/x86_64-pc-cygwin/7.3.0/include/c++/iostream:39,
                 from p3.7.cpp:2:
/usr/lib/gcc/x86_64-pc-cygwin/7.3.0/include/c++/bits/basic_string.h:6069:5: note: candidate: template<class _CharT, class _Traits, class _Alloc> bool std::operator!=(const std::basic_string<_CharT, _Traits, _Alloc>&, const _CharT*)
     operator!=(const basic_string<_CharT, _Traits, _Alloc>& __lhs,
     ^~~~~~~~
/usr/lib/gcc/x86_64-pc-cygwin/7.3.0/include/c++/bits/basic_string.h:6069:5: note:   template argument deduction/substitution failed:
In file included from /usr/include/sys/unistd.h:14:0,
                 from /usr/include/unistd.h:4,
                 from p3.7.cpp:6:
p3.7.cpp:18:45: note:   ‘std::basic_istream<char>’ is not derived from ‘const std::basic_string<_CharT, _Traits, _Alloc>’
   if (cin.getline(line_input, MAX, '\n') != NULL) {
                                             ^
In file included from /usr/lib/gcc/x86_64-pc-cygwin/7.3.0/include/c++/bits/ios_base.h:46:0,
                 from /usr/lib/gcc/x86_64-pc-cygwin/7.3.0/include/c++/ios:42,
                 from /usr/lib/gcc/x86_64-pc-cygwin/7.3.0/include/c++/ostream:38,
                 from /usr/lib/gcc/x86_64-pc-cygwin/7.3.0/include/c++/iostream:39,
                 from p3.7.cpp:2:
/usr/lib/gcc/x86_64-pc-cygwin/7.3.0/include/c++/system_error:319:3: note: candidate: bool std::operator!=(const std::error_code&, const std::error_code&)
   operator!=(const error_code& __lhs, const error_code& __rhs) noexcept
   ^~~~~~~~
/usr/lib/gcc/x86_64-pc-cygwin/7.3.0/include/c++/system_error:319:3: note:   no known conversion for argument 1 from ‘std::basic_istream<char>’ to ‘const std::error_code&’
/usr/lib/gcc/x86_64-pc-cygwin/7.3.0/include/c++/system_error:323:3: note: candidate: bool std::operator!=(const std::error_code&, const std::error_condition&)
   operator!=(const error_code& __lhs, const error_condition& __rhs) noexcept
   ^~~~~~~~
/usr/lib/gcc/x86_64-pc-cygwin/7.3.0/include/c++/system_error:323:3: note:   no known conversion for argument 1 from ‘std::basic_istream<char>’ to ‘const std::error_code&’
/usr/lib/gcc/x86_64-pc-cygwin/7.3.0/include/c++/system_error:327:3: note: candidate: bool std::operator!=(const std::error_condition&, const std::error_code&)
   operator!=(const error_condition& __lhs, const error_code& __rhs) noexcept
   ^~~~~~~~
/usr/lib/gcc/x86_64-pc-cygwin/7.3.0/include/c++/system_error:327:3: note:   no known conversion for argument 1 from ‘std::basic_istream<char>’ to ‘const std::error_condition&’
/usr/lib/gcc/x86_64-pc-cygwin/7.3.0/include/c++/system_error:331:3: note: candidate: bool std::operator!=(const std::error_condition&, const std::error_condition&)
   operator!=(const error_condition& __lhs,
   ^~~~~~~~
/usr/lib/gcc/x86_64-pc-cygwin/7.3.0/include/c++/system_error:331:3: note:   no known conversion for argument 1 from ‘std::basic_istream<char>’ to ‘const std::error_condition&’
In file included from /usr/lib/gcc/x86_64-pc-cygwin/7.3.0/include/c++/bits/locale_facets.h:48:0,
                 from /usr/lib/gcc/x86_64-pc-cygwin/7.3.0/include/c++/bits/basic_ios.h:37,
                 from /usr/lib/gcc/x86_64-pc-cygwin/7.3.0/include/c++/ios:44,
                 from /usr/lib/gcc/x86_64-pc-cygwin/7.3.0/include/c++/ostream:38,
                 from /usr/lib/gcc/x86_64-pc-cygwin/7.3.0/include/c++/iostream:39,
                 from p3.7.cpp:2:
/usr/lib/gcc/x86_64-pc-cygwin/7.3.0/include/c++/bits/streambuf_iterator.h:210:5: note: candidate: template<class _CharT, class _Traits> bool std::operator!=(const std::istreambuf_iterator<_CharT, _Traits>&, const std::istreambuf_iterator<_CharT, _Traits>&)
     operator!=(const istreambuf_iterator<_CharT, _Traits>& __a,
     ^~~~~~~~
/usr/lib/gcc/x86_64-pc-cygwin/7.3.0/include/c++/bits/streambuf_iterator.h:210:5: note:   template argument deduction/substitution failed:
In file included from /usr/include/sys/unistd.h:14:0,
                 from /usr/include/unistd.h:4,
                 from p3.7.cpp:6:
p3.7.cpp:18:45: note:   ‘std::basic_istream<char>’ is not derived from ‘const std::istreambuf_iterator<_CharT, _Traits>’
   if (cin.getline(line_input, MAX, '\n') != NULL) {

What is the mistake above I am doing. Do I need to include some header file which I have forgotten.

ss321c
  • 689
  • 2
  • 11
  • 23
  • You have quite a few problems besides that, most of which should be easily solvable if you [read a couple of good books](https://stackoverflow.com/questions/388242/the-definitive-c-book-guide-and-list/388282#388282). And [a good reference](https://en.cppreference.com/w/cpp/io/basic_istream/getline) should solve your current question (besides reading the error messages and attempting to decipher what they're saying). – Some programmer dude Aug 26 '18 at 05:17
  • 1
    The first is pretty self-explanatory. The rest are caused by your assuming that `getline` returns something that it doesn't. – molbdnilo Aug 26 '18 at 06:12

1 Answers1

0

More of a style comment than anything. I'd be inclined to break up your main function into separate functions, each dealing with a different concern of the problem.

In this example I've used c++ data constructs rather than c-style ones.

I've also used boost::optional and boost::tokenizer.

The boost library suite is invaluable to any c++ developer as it provides many of the essential application programming tools that the c++ standard library fails to provide.

#include <boost/optional.hpp>
#include <boost/tokenizer.hpp>
#include <boost/algorithm/string.hpp>

#include <iostream>

#include <unistd.h>
#include <sys/wait.h>

using boost::optional;
using std::string;
using std::vector;

void prompt(std::istream& is)
{
    if (std::addressof(is) == std::addressof(std::cin))
    {
        std::cout << "cmd> ";
    }
}

optional<string> next_line(std::istream& is)
{
    auto        result = optional<string>();
    std::string line;

    while (1)
    {
        prompt(is);
        if (!getline(is, line)) break;
        if (!line.empty())
        {
            result = line;
            break;
        }
    }
    return result;
}

vector<string> tokenise(std::string const& source)
{
    using char_function = boost::char_separator<char>;

    auto tokens = boost::tokenizer<char_function>(source, char_function(" "));

    return vector<string>(tokens.begin(), tokens.end());
}

auto to_char_data_vector(vector<string>::iterator first, vector<string>::iterator last) -> vector<char *>
{
    auto to_char_data = [](std::string& str) { return str.data(); };
    auto result = vector<char *>();
    std::transform(first, last,
                   back_inserter(result),
                   to_char_data);
    return result;
};

auto append_null(vector<char*> v) -> vector<char *>
{
    v.push_back(nullptr);
    return v;
}

int sub_command(string const& cmd, vector<string>& args) // note - not const.
{
    auto do_error = [](const char *message)
    {
        auto retcode = errno;
        std::cerr << message << std::endl;
        return retcode;
    };

    auto pid = fork();
    if (pid == -1)
    {
        return do_error("fork failure");
    }
    else if (pid == 0)
    {
        auto result = execv(cmd.data(),
                            append_null(to_char_data_vector(begin(args),
                                                            std::end(args)))
                                .data());
        std::exit(do_error("execv failure"));
    }
    else
    {
        int  stat       = 0;
        auto waitresult = waitpid(pid, &stat, 0);
        if (waitresult == -1)
            return do_error("failed to wait");
        return WEXITSTATUS(stat);
    }
}

/// @pre tokens.length() >= 1
int interpret(vector<string> const& tokens)
{
    auto args = vector<string>(next(begin(tokens)), std::end(tokens));
    auto& cmd = tokens[0];

    if (boost::iequals(cmd, "exit"))
    {
        std::exit(args.empty() ? 0 : std::stoi(args[0]));
    }
    else
    {
        return sub_command(cmd, args);
    }

}

int main()
{
    while (auto opt_line = next_line(std::cin))
    {
        auto tokens = tokenise(*opt_line);
        if (tokens.empty()) continue;

        auto return_code = interpret(tokens);
        std::cout << "command returned: " << return_code << std::endl;
    }
}

example run:

cmd> /bin/ls .
CMakeCache.txt      TryThings.cbp       trythings
CMakeFiles      _3rdParty
Makefile        cmake_install.cmake
command returned: 0
cmd> exit 20

Process finished with exit code 20

Note that I had to specify the exact path of the ls command. evecv knows nothing about search paths.

Richard Hodges
  • 68,278
  • 7
  • 90
  • 142