c++ - Segmentation Fault on string conversion? -

i working on encryption project , making simple test takes file name terminal , runs encryption. have following encryption code following segmentation fault:

terminate called after throwing instance of 'std::logic_error'   what():  basic_string::_m_construct null not valid<br><br> program received signal sigabrt, aborted. 0x00007ffff74ab428 in __gi_raise (sig=sig@entry=6)     @ ../sysdeps/unix/sysv/linux/raise.c:54 54  ../sysdeps/unix/sysv/linux/raise.c: no such file or directory. 

after running trace gdb have confirmed fault triggered after following loc:

string plain(reinterpret_cast(filecontents), filelength);

my main function below calls piece of code:

#include <iostream> #include <stdlib.h> #include <stdio.h> #include <string> #include <fstream> #include <limits> #include <sstream> #include <stdlib.h> #include <sys/types.h> #include <sys/stat.h> #include <fcntl.h> #include "rc4_enc.c" #include "rc4_skey.c"  using namespace std; void foo(int);  int main(int argc, char * argv[]){     if(argc!=2){         cout << "please enter filename want encrypt or decrypt\n";         exit(2);     }     int fd;     fd = open(argv[1], o_rdonly);     cout << "the file descriptor " << fd << endl;     //close(fd);// can modify if it's closed?     foo(fd);      return 0; } 

and function follows:

void foo(int fd) {       int filelength = lseek(fd, 0, seek_end);      unsigned char* filecontents;      filecontents = (unsigned char*) calloc(filelength, sizeof(char));      pread(fd, filecontents, filelength, 0);      string plain(reinterpret_cast<char*>(filecontents), filelength); // segfault happens here. why?          rc4_key key;         int length = plain.size();         unsigned char *buf = (unsigned char*)malloc(length+1);         memset(buf, 0, length+1);          ifstream pass;         pass.open("pass.txt");         if(!pass.good()){             return;         }         else{             string password;             getline(pass,password);             rc4_set_key(&key, password.length(), (const unsigned char*)password.c_str());         }         rc4(&key, length, (const unsigned char*)plain.c_str(), buf);         string result((char*)buf, length);         free(buf);         const char *outputbuf = result.c_str();         pwrite(fd, outputbuf, result.length(), 0);         ftruncate(fd, result.length());     } 

i leave comment, not have enough reputation.

how large file? calloc failing , returning null? if it's not that, might wise check calloc's return value. or use new operator, try/catch.