Segfault after fread() call - c++

Question

I have the following code:
char*
Sender::PrepareData(char* filename, unsigned long long int bytesToTransfer)
{
FILE* dataFile = fopen(filename, "rb");
if (dataFile==NULL) {fputs ("File error",stderr); exit (1);}
cout << "File Open: " << filename << endl;
char* theData;
size_t bytesRead = fread(&theData, 1, bytesToTransfer, dataFile);
if (bytesRead != bytesToTransfer) {fputs ("Reading error",stderr); exit (3);}
cout << "Data Read -- Num Bytes: " << bytesRead << endl;
cout << "Data to Send: " << *theData << endl;
return theData;
}
When this method gets hit, my output is:
File Open: t.bin
Data Read -- Num Bytes: 10
Segmentation fault (core dumped)
My t.bin file contains the following:
This is a test.
98172398172837129837
alsjdf89u32ijofiou2
TEST TEST...
!!## TESTING TEST!! ###(DLKAJ)
When I run through gdb, the segfault output is:
File Open: t.bin Data Read -- Num Bytes: 10
Program received signal SIGSEGV, Segmentation fault. 0x00000000004015e2 in Sender::PrepareData (this=0x603010, filename=0x7fffffffe363 "t.bin", bytesToTransfer=10)
at sender.cpp:98 98 cout << "Data to Send: " << *theData << endl;
Can someone tell me what I'm doing wrong?

Answer 1

You need a buffer, theData is just a pointer.
Something like
char theData[1000];
size_t bytesRead = fread(theData, 1, bytesToTransfer, dataFile);
might work depending on the maximum value of bytesToTransfer
If you are sure you are reading a string you might also need to terminate theData before writing to cout,
theData[bytesRead] = '\0';
You'll need to allocate your buffer on the heap if you want to return it.
It'd be a lot easier to do something like
std::vector<char>
PrepareData(char* filename, unsigned long long int bytesToTransfer)
{
std::ifstream file(filename, file.binary);
if (!file) {
std::cerr << "File error";
exit(1);
}
std::cout << "File Open: " << filename << '\n';
std::vector<char> data(bytesToTransfer + 1);
file.read(data.data(), bytesToTransfer);
data.back() = '\0';
if (!file) {
std::cerr << "Reading error";
exit(3);
}
std::cout << "Data Read -- Num Bytes: " << bytesToTransfer << '\n';
std::cout << "Data to Send: " << data.data() << '\n';
return data;
}
Then again if all you are doing is reading chars from a file you should probably consider using strings.