Make a file pointer read/write to an in-memory location

Question

I can make a file pointer write to a file with fopen(). But can I make a file pointer that will make it so calling functions such as fputc or fprintf will write to a pointer in memory? An example of this is ByteArrayOutputStream in java. Also: could I run it in reverse, where a library needs a file pointer to read from, so I allocate memory, and make a new file pointer that will read from this memory location but return EOF when the size of the chunk runs out? (like ByteArrayInputStream in Java). Is there a way to do this in C? For example:

FILE *p = new_memory_file_pointer(); fprintf(p, "Hello World!\n"); char *data = get_written_stuff(p); printf("%s", data); //will print Hello World! 

&& / ||

char s[] = "Hello World!\n"; FILE *p = new_memory_file_pointer_read(s, sizeof(s)); char *buffer = (char *)malloc( 1024*sizeof(char) ); fread((void *)buffer, 1, sizeof(s), p); printf("%s", buffer); //prints Hello World! 

EDIT: To those reading this question years later, in addition to the accepted answer, you should look at open_memstream(3), which behaves more like these Java classes than fmemopen does.

Answer 1

If your operating system provides fmemopen, probably it will meet your purpose.

Answer 2

In C++ (and you added the C++ tag) you can write a function accepting an arbitrary input/output stream. Since std::stringstream or std::ofstream are derived classes you can pass both of them equally into this function. An example:

#include <iostream> // for std::cout #include <fstream> // for std::ofstream #include <sstream> // for std::stringstream  void write_something(std::ostream& stream) {   stream << "Hello World!" << std::endl; }  int main() {   write_something(std::cout); // write it to the screen   {     std::ofstream file("myfile.txt");     write_something(file); // write it into myfile.txt   }   {     std::stringstream stream;     write_something(stream); // write it into a string     std::cout << stream.str() << std::endl; // and how to get its content   } } 

And analogously with std::istream instead of std::ostream if you want to read the data:

void read_into_buffer(std::istream& stream, char* buffer, int length) {   stream.read(buffer, length); }  int main() {   char* buffer = new char[255];    {     std::ifstream file("myfile.txt");     read_into_buffer(file, buffer, 10); // reads 10 bytes from the file   }   {     std::string s("Some very long and useless message and ...");     std::stringstream stream(s);     read_into_buffer(stream, buffer, 10); // reads 10 bytes from the string   } }