[erlang-questions] Bug in trapexit article "How to use ei ..."

Mikl Kurkov <>
Sat Nov 15 16:03:37 CET 2008


It seems that the code in the trapexit article
http://www.trapexit.org/How_to_use_ei_to_marshal_binary_terms_in_port_programs
has some bugs that I ran into.
In the next code 

[c]
int read_cmd(byte *buf, int *size)
{
  int len;

  if (read_exact(buf, 2) != 2)
    return(-1);
  len = (buf[0] << 8) | buf[1];

  if (len > *size) {
    buf = (byte *) realloc(buf, len);
    if (buf == NULL)
      return -1;
    *size = len;
  }
  return read_exact(buf, len);
}
[/c]

if the size of binary data is more than the size of the buffer then data is
reallocated, but the pointer in the main function doesn't change.
I think it should be something like this:

[c]
int read_cmd(byte **buf_ptr, int *size)
{
  int len;
  char *buf = *buf_ptr;;
  
  if (read_exact(buf, 2) != 2)
      return -1;

  len = (buf[0] << 8) | buf[1];

  if (len > *size) {
    buf = (byte *) realloc(buf, len);
    if (buf == NULL)
      return -1;
    *buf_ptr = buf;
    *size = len;
  }

  return read_exact(buf, len);
}
[/c]

The call of read_cmd in the main function should be changed into 
[c]
while (read_cmd(&buf, &size) > 0) {
[/c]

Besides the code doesn't work properly in the system with the char defined
as signed type.

Hope this information will be helpful for someone.
--
Mikl
-- 
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