fmemopen implementation

[Andreas Jaeger <aj at suse dot de>]

Hi glibc folks,

As part of a bug report, Hanno send us a implementation of fmemopen.
Could somebody check this, please?

Thanks,
Andreas

/*
 * fmemopen() - "my" version of a string stream
 * Hanno Mueller, kontakt@hanno.de
 *
 *
 * I needed fmemopen() for an application that I currently work on,
 * but couldn't find it in libio. The following snippet of code is an
 * attempt to implement what glibc's documentation describes.
 *
 * No, it isn't really tested yet. :-)
 *
 *
 *
 * I already see some potential problems:
 *
 * - I never used the "original" fmemopen(). I am sure that "my"
 *   fmemopen() behaves differently than the original version.
 *
 * - The documentation doesn't say wether a string stream allows
 *   seeks. I checked the old fmemopen implementation in glibc's stdio
 *   directory, wasn't quite able to see what is going on in that
 *   source, but as far as I understand there was no seek there. For
 *   my application, I needed fseek() and ftell(), so it's here.
 *
 * - "append" mode and fseek(p, SEEK_END) have two different ideas
 *   about the "end" of the stream.
 *
 *   As described in the documentation, when opening the file in
 *   "append" mode, the position pointer will be set to the first null
 *   character of the string buffer (yet the buffer may already
 *   contain more data). For fseek(), the last byte of the buffer is
 *   used as the end of the stream.
 *
 * - It is unclear to me what the documentation tries to say when it
 *   explains what happens when you use fmemopen with a NULL
 *   buffer.
 *
 *   Quote: "fmemopen [then] allocates an array SIZE bytes long. This
 *   is really only useful if you are going to write things to the
 *   buffer and then read them back in again."
 *
 *   What does that mean if the original fmemopen() did not allow
 *   seeking? How do you read what you just wrote without seeking back
 *   to the beginning of the stream?
 *
 * - I think there should be a second version of fmemopen() that does
 *   not add null characters for each write. (At least in my
 *   application, I am not actually using strings but binary data and
 *   so I don't need the stream to add null characters on its own.)
 */


#define _GNU_SOURCE

#include <string.h>
#include <sys/types.h>
#include <libio.h>
#include <stdio.h>


typedef struct fmemopen_cookie_struct fmemopen_cookie_t;
struct fmemopen_cookie_struct {
  char *buffer;
  int mybuffer;
  size_t size;
  size_t pos;
  size_t maxpos;
};


ssize_t fmemopen_read(void *cookie, char *b, size_t s)
{
  fmemopen_cookie_t *c;
  c = (fmemopen_cookie_t*) cookie;

  if ((c->pos + s) > c->size) {
    if (c->pos == c->size) return -1;
    s = c->size - c->pos;
  }

  memcpy(b, &(c->buffer[c->pos]), s);
  
  c->pos += s;
  if (c->pos > c->maxpos) c->maxpos = c->pos;

  return s;
}


ssize_t fmemopen_write (void *cookie, const char *b, size_t s)
{
  fmemopen_cookie_t *c;
  int addnullc;

  c = (fmemopen_cookie_t*) cookie;

  addnullc = ( (s == 0) || (b[s - 1] != '\0') ) ? 1 : 0;

  if ((c->pos + s + addnullc) > c->size) {
    if ((c->pos + addnullc) == c->size) return -1;
    s = c->size - c->pos - addnullc;
  }
  
  memcpy(&(c->buffer[c->pos]), b, s);

  c->pos += s;
  if (c->pos > c->maxpos) {
    c->maxpos = c->pos;
    if (addnullc) c->buffer[c->maxpos] = '\0';
  }

  return s;
}


int fmemopen_seek (void *cookie, fpos_t p, int w)
{
  fpos_t np;
  fmemopen_cookie_t *c;

  c = (fmemopen_cookie_t*) cookie;

  switch(w) {

  case SEEK_SET: 
    np = p; 
    break;

  case SEEK_CUR: 
    np = c->pos + p; 
    break;

  case SEEK_END: 
    np = c->size - p; 
    break;

  }

  if ( (np < 0) || (np > c->size) ) return -1;

  c->pos = np;

  return np;
}


int fmemopen_close (void *cookie)
{
  fmemopen_cookie_t *c;
  c = (fmemopen_cookie_t*) cookie;

  if (c->mybuffer) free(c->buffer);
  free(c);

  return 0;
}


FILE *fmemopen (void *BUF, size_t SIZE, const char *OPENTYPE)
{
  cookie_io_functions_t iof;
  fmemopen_cookie_t *c;

  c = (fmemopen_cookie_t*) malloc(sizeof(fmemopen_cookie_t));
  if (c == NULL) return NULL;
  
  c->mybuffer = (BUF == NULL);

  if (c->mybuffer) {
    c->buffer = (char*) malloc(SIZE);
    if (c->buffer == NULL) {
      free(c);
      return NULL;
    }
    c->buffer[0] = '\0';
  } else {
    c->buffer = BUF;
  }

  c->size = SIZE;

  if (OPENTYPE[0] == 'w') c->buffer[0] = '\0';

  c->maxpos = strlen(c->buffer);

  if (OPENTYPE[0] == 'a') {
    c->pos = c->maxpos;
  } else {
    c->pos = 0;
  }

  iof.read = fmemopen_read;
  iof.write = fmemopen_write;
  iof.seek = fmemopen_seek;
  iof.close = fmemopen_close;

  return fopencookie(c, OPENTYPE, iof);
}





static char buffer[] = "foobar";

int main (void)
{
  int ch;
  FILE *stream;
  
  stream = fmemopen (buffer, strlen (buffer), "r");

  while ((ch = fgetc (stream)) != EOF)
    printf ("Got %c\n", ch);

  fclose (stream);
  
  return 0;
}



-- 
 Andreas Jaeger
  SuSE Labs aj@suse.de
   private aj@arthur.rhein-neckar.de