Segmentation Fault on string conversion?

I am working on an encryption project and I am making a simple test that takes a file name from terminal and runs my encryption. I have the following encryption code but I get the following segmentation fault:

terminate called after throwing an 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)
    at ../sysdeps/unix/sysv/linux/raise.c:54
54  ../sysdeps/unix/sysv/linux/raise.c: No such file or directory.

After running a trace with gdb I have confirmed that this fault is triggered after the following loc:

string plain(reinterpret_cast(fileContents), fileLength);

My main function below calls this 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 the filename that you want to encrypt or decrypt\n";
        exit(2);
    }
    int fd;
    fd = open(argv[1], O_RDONLY);
    cout << "The file descriptor is " << fd << endl;
    //close(fd);// Can I modify it if it's closed?
    foo(fd);

    return 0;
}

And the function is as 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. But 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 would leave this as a comment, but do not have enough reputation.

How large is the file? Could calloc be failing and returning NULL? Even if it's not that, might be wise to check calloc's return value. Or use the new operator, with try/catch.