C to python conversion

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  • Michele Petrazzo
    #1

    C to python conversion

    Hi all,
    I'm trying to translate a simple C code into a python + ctypes (where
    need), but I have some problems on char conversion. The code have
    to work on Linux and talk with the serial port. I think that the problem
    is that I don't translate correctly the strings.

    C code:
    #define START 0x33
    #define RETURN_START 0x22
    #define ADDR 0x01
    #define WRITE_CMD 0x03
    #define ALL_CMD 0xFF
    ....
    char buf[10];
    char buf_ret[10];

    buf[0]=0;
    buf[0]=START;
    buf[1]=ADDR;
    buf[2]=WRITE_CMD;

    write(_fd, buf, 6);
    read(_fd,buf_re t,6);

    It works

    python:
    START = 0x33
    RETURN_START = 0x22
    ADDR = 0x01
    WRITE_CMD = 0x03
    ALL_CMD = 0xFF

    lib = C.CDLL('libc.so .6')

    items = [START, ADDR, WRITE_CMD]
    buf = C.c_char * 10
    buffer_rec = buf()
    buffer_send = buf(*items)

    (Here I receive: TypeError: one character string expected)
    If I do:
    chr(int()) of every value, it work, but:

    lib.write(fd, buffer_send, 6)
    lib.read(fd, buffer_rec, 6)

    I stay there and block the program execution, until a CTRL+C

    What can I do?

    Thanks,
    Michele
    Tags: None
    • Paul McGuire
      #2
      Re: C to python conversion

      On Apr 12, 5:58 am, Michele Petrazzo <michele.petra. ..@TOGLIunipex. it>
      wrote:
      Hi all,
      I'm trying to translate a simple C code into a python + ctypes (where
      need), but I have some problems on char conversion. The code have
      to work on Linux and talk with the serial port. I think that the problem
      is that I don't translate correctly the strings.
      >
      C code:
      #define START 0x33
      #define RETURN_START 0x22
      #define ADDR 0x01
      #define WRITE_CMD 0x03
      #define ALL_CMD 0xFF
      ...
      char buf[10];
      char buf_ret[10];
      >
      buf[0]=0;
      buf[0]=START;
      buf[1]=ADDR;
      buf[2]=WRITE_CMD;
      >
      write(_fd, buf, 6);
      read(_fd,buf_re t,6);
      >
      It works
      >
      python:
      START = 0x33
      RETURN_START = 0x22
      ADDR = 0x01
      WRITE_CMD = 0x03
      ALL_CMD = 0xFF
      >
      lib = C.CDLL('libc.so .6')
      >
      items = [START, ADDR, WRITE_CMD]
      buf = C.c_char * 10
      buffer_rec = buf()
      buffer_send = buf(*items)
      >
      (Here I receive: TypeError: one character string expected)
      If I do:
      chr(int()) of every value, it work, but:
      >
      lib.write(fd, buffer_send, 6)
      lib.read(fd, buffer_rec, 6)
      >
      I stay there and block the program execution, until a CTRL+C
      >
      What can I do?
      >
      Thanks,
      Michele
      Here are some differences between the C and Python programs:
      - the C program uses a separate buffer for reading and writing - the
      Python program uses buf for both
      - the C program *may* have null-filled the output buffer, I don't know
      what the Python buffer contains after the first 3 characters
      - in the Python program, items has only 3 characters to write; the C
      program has a buffer of 10 characters

      Here are some things to try in the Python program:

      items = [START, ADDR, WRITE_CMD] + [0]*7

      inbuf = C.c_char * 10
      outbuf = C.c_char * 10
      buffer_rec = inbuf()
      buffer_send = outbuf(*items)

      -- Paul

        Comment

        • Gabriel Genellina
          #3
          Re: C to python conversion

          En Sat, 12 Apr 2008 07:58:47 -0300, Michele Petrazzo
          <michele.petraz zo@TOGLIunipex. itescribió:
          Hi all,
          I'm trying to translate a simple C code into a python + ctypes (where
          need), but I have some problems on char conversion. The code have
          to work on Linux and talk with the serial port. I think that the problem
          is that I don't translate correctly the strings.
          >
          C code:
          #define START 0x33
          #define RETURN_START 0x22
          #define ADDR 0x01
          #define WRITE_CMD 0x03
          #define ALL_CMD 0xFF
          ...
          char buf[10];
          char buf_ret[10];
          >
          buf[0]=0;
          buf[0]=START;
          buf[1]=ADDR;
          buf[2]=WRITE_CMD;
          >
          write(_fd, buf, 6);
          read(_fd,buf_re t,6);
          You don't even need ctypes. In C, `char` is a small integer: 'A' and the
          number 65 are interchangeable . In Python, there are no chars but strings
          of length 1, which are not the same thing as their ordinal integer.
          The easiest way is to define those constants as strings instead:

          START = chr(0x33)
          RETURN_START = chr(0x22)
          ADDR = chr(0x01)
          WRITE_CMD = chr(0x03)
          ALL_CMD = chr(0xFF)
          NUL = chr(0)

          buf = START + ADDR + WRITE_CMD + NUL + NUL + NUL
          # I assume the buffer was initialized to NULs, because only 3 bytes
          # are filled but 6 bytes are written.
          os.write(_fd, buf)
          buf_ret = os.read(_fd, 6)

          --
          Gabriel Genellina

            Comment

            • Gabriel Genellina
              #4
              Re: C to python conversion

              En Sat, 12 Apr 2008 07:58:47 -0300, Michele Petrazzo
              <michele.petraz zo@TOGLIunipex. itescribió:
              Hi all,
              I'm trying to translate a simple C code into a python + ctypes (where
              need), but I have some problems on char conversion. The code have
              to work on Linux and talk with the serial port. I think that the problem
              is that I don't translate correctly the strings.
              >
              C code:
              #define START 0x33
              #define RETURN_START 0x22
              #define ADDR 0x01
              #define WRITE_CMD 0x03
              #define ALL_CMD 0xFF
              ...
              char buf[10];
              char buf_ret[10];
              >
              buf[0]=0;
              buf[0]=START;
              buf[1]=ADDR;
              buf[2]=WRITE_CMD;
              >
              write(_fd, buf, 6);
              read(_fd,buf_re t,6);
              You don't even need ctypes. In C, `char` is a small integer: 'A' and the
              number 65 are interchangeable . In Python, there are no chars but strings
              of length 1, which are not the same thing as their ordinal integer.
              The easiest way is to define those constants as strings instead:

              START = chr(0x33)
              RETURN_START = chr(0x22)
              ADDR = chr(0x01)
              WRITE_CMD = chr(0x03)
              ALL_CMD = chr(0xFF)
              NUL = chr(0)

              buf = START + ADDR + WRITE_CMD + NUL + NUL + NUL
              # I assume the buffer was initialized to NULs, because only 3 bytes
              # are filled but 6 bytes are written.
              os.write(_fd, buf)
              buf_ret = os.read(_fd, 6)

              --
              Gabriel Genellina

                Comment

                • Ivan Illarionov
                  #5
                  Re: C to python conversion

                  On Apr 13, 7:58 am, "Gabriel Genellina" <gagsl-...@yahoo.com.a r>
                  wrote:
                  En Sat, 12 Apr 2008 07:58:47 -0300, Michele Petrazzo
                  <michele.petra. ..@TOGLIunipex. itescribió:
                  >
                  >
                  >
                  Hi all,
                  I'm trying to translate a simple C code into a python + ctypes (where
                  need), but I have some problems on char conversion. The code have
                  to work on Linux and talk with the serial port. I think that the problem
                  is that I don't translate correctly the strings.
                  >
                  C code:
                  #define START 0x33
                  #define RETURN_START 0x22
                  #define ADDR 0x01
                  #define WRITE_CMD 0x03
                  #define ALL_CMD 0xFF
                  ...
                  char buf[10];
                  char buf_ret[10];
                  >
                  buf[0]=0;
                  buf[0]=START;
                  buf[1]=ADDR;
                  buf[2]=WRITE_CMD;
                  >
                  write(_fd, buf, 6);
                  read(_fd,buf_re t,6);
                  >
                  You don't even need ctypes. In C, `char` is a small integer: 'A' and the
                  number 65 are interchangeable . In Python, there are no chars but strings
                  of length 1, which are not the same thing as their ordinal integer.
                  The easiest way is to define those constants as strings instead:
                  >
                  START = chr(0x33)
                  RETURN_START = chr(0x22)
                  ADDR = chr(0x01)
                  WRITE_CMD = chr(0x03)
                  ALL_CMD = chr(0xFF)
                  NUL = chr(0)
                  >
                  buf = START + ADDR + WRITE_CMD + NUL + NUL + NUL
                  # I assume the buffer was initialized to NULs, because only 3 bytes
                  # are filled but 6 bytes are written.
                  os.write(_fd, buf)
                  buf_ret = os.read(_fd, 6)
                  >
                  --
                  Gabriel Genellina
                  The easiest way is to use struct:

                  START = 0x33
                  RETURN_START = 0x22
                  ADDR = 0x01
                  WRITE_CMD = 0x03
                  ALL_CMD = 0xFF

                  buf = struct.pack('3b 3x', START, ADDR, WRITE_CMD)
                  os.write(_fd, buf)
                  buf_ret = os.read(_fd, 6)

                  And, definitely, no need for ctypes here.

                  --
                  Ivan Illarionov

                    Comment

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