Question

I have a couple of functions I created a while ago for reading and writing std::strings to a FILE* opened for reading in binary mode. They have worked fine before (and WriteString() still works) but ReadString() keeps giving me memory corruption errors at run-time. The strings are stored by writing their size as an unsigned int before the string data as char.

bool WriteString(std::string t_str, FILE* t_fp) {
// Does the file stream exist and is it valid? If not, return false.
if (t_fp == NULL) return false;
// Create char pointer from string.
char* text = const_cast<char*>(t_str.c_str());
// Find the length of the string.
unsigned int size = t_str.size();
// Write the string's size to the file.
fwrite(&size, sizeof(unsigned int), 1, t_fp);
// Followed by the string itself.
fwrite(text, 1, size, t_fp);
// Everything worked, so return true.
return true;

}



std::string ReadString(FILE* t_fp) {
// Does the file stream exist and is it valid? If not, return false.
if (t_fp == NULL) return false;
// Create new string object to store the retrieved text and to return to the calling function.
std::string str;
// Create a char pointer for temporary storage.
char* text = new char;
// UInt for storing the string's size.
unsigned int size;
// Read the size of the string from the file and store it in size.
fread(&size, sizeof(unsigned int), 1, t_fp);
// Read [size] number of characters from the string and store them in text.
fread(text, 1, size, t_fp);
// Store the contents of text in str.
str = text;
// Resize str to match the size else we get extra cruft (line endings methinks).
str.resize(size);
// Finally, return the string to the calling function.
return str;

}

Can anyone see any problems with this code or have any alternative suggestions?

Answer 1

Biggest major problem that jumped out at me:

// Create a char pointer for temporary storage.
char* text = new char;
// ...
// Read [size] number of characters from the string and store them in text.
fread(text, 1, size, t_fp);

This creates text as a pointer to a single character, and then you try to read an arbitrary number of characters (potentially many more than one) into it. In order for this to work right, you would have to create text as an array of characters after you figured out what the size was, like this:

// UInt for storing the string's size.
unsigned int size;
// Read the size of the string from the file and store it in size.
fread(&size, sizeof(unsigned int), 1, t_fp);
// Create a char pointer for temporary storage.
char* text = new char[size];
// Read [size] number of characters from the string and store them in text.
fread(text, 1, size, t_fp);

Second, you don’t free the memory that you allocated to text. You need to do that:

// Free the temporary storage
delete[] text;

Finally, is there a good reason why you are choosing to use C file I/O in C++? Using C++-style iostreams would have alleviated all of this and made your code much, much shorter and more readable.