c - Creating a FILE * stream that results in a string

Question

I'm looking for a way to pass in a FILE * to some function so that the function can write to it with fprintf. This is easy if I want the output to turn up in an actual file on disk, say. But what I'd like instead is to get all the output as a string (char *). The kind of API I'd like is:

/** Create a FILE object that will direct writes into an in-memory buffer. */
FILE *open_string_buffer(void);

/** Get the combined string contents of a FILE created with open_string_buffer
    (result will be allocated using malloc). */
char *get_string_buffer(FILE *buf);

/* Sample usage. */
FILE *buf;
buf = open_string_buffer();
do_some_stuff(buf);   /* do_some_stuff will use fprintf to write to buf */
char *str = get_string_buffer(buf);
fclose(buf);
free(str);

The glibc headers seem to indicate that a FILE can be set up with hook functions to perform the actual reading and writing. In my case I think I want the write hook to append a copy of the string to a linked list, and for there to be a get_string_buffer function that figures out the total length of the list, allocates memory for it, and then copies each item into it in the correct place.

I'm aiming for something that can be passed to a function such as do_some_stuff without that function needing to know anything other than that it's got a FILE * it can write to.

Is there an existing implementation of something like this? It seems like a useful and C-friendly thing to do -- assuming I'm right about the FILE extensibility.

Answer 1

If portability is not important for you, you can take a look on fmemopen and open_memstream. They are GNU extensions, hence only available on glibc systems. Although it looks like they are part of POSIX.1-2008 (fmemopen and open_memstream).