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).

Answer 2

I m not sure if it s possible to non-portably extend FILE objects, but if you are looking for something a little bit more POSIX friendly, you can use pipe and fdopen.

It s not exactly the same as having a FILE* that returns bytes from a buffer, but it certainly is a FILE* with programmatically determined contents.

int fd[2];
FILE *in_pipe;

if (pipe(fd))
{
   /* TODO: handle error */
}

in_pipe = fdopen(fd[0], "r");
if (!in_pipe)
{
   /* TODO: handle error */
}

From there you will want to write your buffer into fd[1] using write(). Careful with this step, though, because write() may block if the pipe s buffer is full (i.e. someone needs to read the other end), and you might get EINTR if your process gets a signal while writing. Also watch out for SIGPIPE, which happens when the other end closes the pipe. Maybe for your use you might want to do the write of the buffer in a separate thread to avoid blocking and make sure you handle SIGPIPE.

Of course, this won t create a seekable FILE*...

Answer 3

I m not sure I understand why you want to mess up with FILE *. Couldn t you simply write to a file and then load it in string?

 char *get_file_in_buf(char *filename) {
   char *buffer;
   ... get file size with fseek or fstat ...
   ... allocate buffer ...
   ... read buffer from file ...
   return buffer;
 }

If you only want to "write" formatted text into a string, another option could be to handle an extensible buffer using snprintf() (see the answers to this SO question for a suggestion on how to handle this: Resuming [vf]?nprintf after reaching the limit).

If, instead, you want to create a type that can be passed transparently to any function taking a FILE * to make them act on string buffers, it s a much more complex matter ...

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