Place least significant bits of int into char in C++ -

-

i find maximally efficient way compute char contains least significant bits of int in c++11. solution must work possible standards-compliant compiler. (i'm using n3290 c++ draft spec, c++11.)

the reason i'm writing fuzz tester, , want check libraries require std::string input. need generate random characters strings. pseudo-random generator i'm using provides ints low bits pretty uniformly random, i'm not sure of exact range. (basically exact range depends on "size of test case" runtime parameter.)

if didn't care working on compiler, simple as:

inline char int2char(int i) { return i; }

before dismiss trivial question, consider that:

  • you don't know whether char signed or unsigned type.

  • if char signed, conversion unrepresentable int char "implementation-defined" (§4.7/3). far better undefined, solution i'd need see evidence standard prohibits things converting ints not between char_min , char_max '\0'.

  • reinterpret_cast not permitted between signed , unsigned char (§5.2.10). static_cast performs same conversion in previous point.

  • char c = & 0xff;--though silences compiler warnings--is not correct implementation-defined conversions. in particular, i & 0xff positive number, in case c signed quite plausibly not convert negative values of i negative values of c.

here solutions work, in of these cases i'm worried won't efficient simple conversion. these seem complicated simple:

  • using reinterpret_cast on pointer or reference, since can convert unsigned char * or unsigned char & char * or char & (but @ possible cost of runtime overhead).

  • using union of char , unsigned char, first assign int unsigned char, extract char (which again slower).

  • shifting left , right sign-extend int. e.g., if i int, running c = ((i << 8 * (sizeof(i) - sizeof(c)) >> 8 * (sizeof(i) - sizeof(c)) (but that's inelegant, , if compiler doesn't optimize away shifts, quite slow).

here's minimal working example. goal argue assertions can never fail on compiler, or define alternate int2char in assertions can never fail.

#include <algorithm> #include <cassert> #include <cstdio> #include <cstdlib>  using namespace std;  constexpr char int2char(int i) { return i; }  int main(int argc, char **argv) {   (int n = 1; n < min(argc, 127); n++) {     char c = -n;     int = (atoi(argv[n]) << 8) ^ -n;     assert(c == int2char(i));   }   return 0; }

i've phrased question in terms of c++ because standards easier find on web, equally interested in solution in c. here's mwe in c:

#include <assert.h> #include <stdlib.h>  static char int2char(int i) { return i; }  int main(int argc, char **argv) {   (int n = 1; n < argc && n < 127; n++) {     char c = -n;     int = (atoi(argv[n]) << 8) ^ -n;     assert(c == int2char(i));   }   return 0; }

a far better way have array of chars , generate random number pick char array. way 'well behaved' characters; or @ least characters defined badness. if want 256 chars (note 8 bit assumption) create array 256 entries in ('a','b',....'\t','n'.....)

this portable too


Comments

Post a Comment

Popular posts from this blog

python - jinja2: TemplateSyntaxError: expected token ',', got 'string' -

-
i new jinja2 , having issue using python regular expression (re). in following code bold lines have error string in them. {% block content %} <div class="container"> {% l in lines %} {% if re.search(r"error", l) %} {# <<< throws error #} <b> {{ l }} </b> {% else %} {{ l }} <hr> {% endif %} {% endfor %} </div> {% endblock %} the re.search above throws following error: jinja2.exceptions.templatesyntaxerror templatesyntaxerror: expected token ',', got 'string' raw python code not supported in jinja2 template syntax. {% if re.search(r"error", l) %} replace line with {% if "error" in l %} can fix problem. if logical condition more complicated, should consider defining own custom filters (which can call python code) or complicated things in view layer. go check global namespace .
Read more

Qt4: how to send QString inside a struct via QSharedMemory -

-
i have struct struct control_data{ int column_number; qstring cell; }; i need send thread of qsharememory. read can't because qstring contains pointers inside. other ways? you have serialize struct byte array. can convert qstring const char* this: mystring.tostdstring().c_str(); but serializing qstring should work. the first step serialize struct qdatastream using qt, example here . then once struct can read , written can pass shared memory. a complete example of using qsharedmemory can found here . here relevant code: // first, test whether shared memory segment attached process. // if so, detach if (sharedmem.isattached()) { sharedmem.detach(); } ... qbuffer buffer; buffer.open( qbuffer::readwrite ); qdatastream out( &buffer ); out << youstruct; int size = buffer.size(); // size of int + size of qstring in bytes if ( !sharedmem.create( size ) ) { return; } // write shared memory sharedmem.lock(); char *to = (char*...
Read more

node.js - NodeJS remote terminal to Dropbear OpenWRT-Server -

-
i wandering whether there valid solution have remote terminal openwrt-box out of nodejs-app? connecting terminal works: ssh -i ~/.mykeys/id_rsa root@192.168.178.39 busybox v1.23.2 (2015-04-22 23:25:48 utc) built-in shell (ash) root@openwrt:~# the only interactive ssh solution nodejs doesn't interactive part described in readme.md following: var client = require('ssh2').client; var conn = new client(); conn.on('ready', function() { console.log('client :: ready'); conn.shell(function(err, stream) { if (err) throw err; stream.on('close', function() { console.log('stream :: close'); conn.end(); }).on('data', function(data) { console.log('stdout: ' + data); }).stderr.on('data', function(data) { console.log('stderr: ' + data); }); stream.end('ls -l\nexit\n'); }); }).connect({ host: '192.168.100.100', port: 22, username...
Read more