Decode a file with asn1c

Goal

Using the asn1c compiler tool, decode an UPER encoded BasicSafetyMessage file.

Procedure

1. Install asn1c

git clone https://github.com/vlm/asn1c.git
cd asn1c
test -f configure || autoreconf -iv
./configure
make
make check
make install
asn1c -v #verify it installed

2. Create a new folder to work out of and copy ASN files into there

For this example you will need J2735_201603DA.asn and SEMI_v2.3.0_070616.asn

3. Generate the C files:

asn1c -gen-OER -gen-PER -fcompound-names -pdu=BasicSafetyMessage J2735_201603DA.asn SEMI_v2.3.0_070616.asn

• -gen-OER generates OER encoder/decoders
• -gen-PER generates [U]PER encoder/decoders
• -fcompound-names resolves the name conflicts presented by the J2735 schema

4. Write a C main method to decode and print using the compiler library:

Copy and paste the following code into a file such as main.c:

Note that this program was taken from the sample decoder file in Lev Walkin's user guide.

#include <stdio.h>
#include <sys/types.h>
#include "BasicSafetyMessage.h" /* Basic safety message J2735 ASN.1 type */
int main(int ac, char **av) {
  char buf[1024]; /* Temporary buffer */
  asn_dec_rval_t rval; /* Decoder return value */
  BasicSafetyMessage_t *bsm = 0; /* Type to decode. Note this 01! */
  FILE *fp; /* Input file handler */
  size_t size; /* Number of bytes read */
  char *filename; /* Input file name */
  /* Require a single filename argument */
  if(ac != 2) {
    fprintf(stderr, "Usage: %s <file.ber>\n", av[0]);
    exit(1);
  } else {
    filename = av[1];
  }
  /* Open input file as read-only binary */
  fp = fopen(filename, "rb");
  if(!fp) {
    perror(filename);
    exit(1);
  }
  /* Read up to the buffer size */
  size = fread(buf, 1, sizeof(buf), fp);
  fclose(fp);
  if(!size) {
    fprintf(stderr, "%s: Empty or broken\n", filename);
    exit(1);
  }
  /* Decode the input buffer as basic safety message type */
  rval = per_decode(0, &asn_DEF_BasicSafetyMessage, (void **)&bsm, buf, size);
  if(rval.code != RC_OK) {
    fprintf(stderr, "%s: Broken bsm encoding at byte %ld\n", filename,
      (long)rval.consumed);
      exit(1);
    }
    /* Print the decoded basic safety message type as XML */
    xer_fprint(stdout, &asn_DEF_BasicSafetyMessage, bsm);
    return 0; /* Decoding finished successfully */
  }

5. Fix the include statements to use quotes instead of angle brackets:

Note: this step may not be necessary for all compilers.

Make a script to replace the C include statements' <> with "":

bracketReplacer.sh

#/bin/bash
find ./asn/ \
 -name '*.[ch]' \
 -exec \
    sed -i '/^#include[[:blank:]]/ s/<\([^>]*\)>/"\1"/g' "{}" \
 \;

(This script will not work on OSX unless you have installed the gnu-sed tool)

6. Use the generated makefile to compile the classes:

make -f Makefile.am.sample

7. Run the generated program and provide the bsm.uper file as a command line argument

./progname bsm.uper

If it works, the BSM will be decoded and printed to standard out in XML format:

<BasicSafetyMessage>
    <coreData>
        <msgCnt>102</msgCnt>
        <id>BE A1 00 00</id>
        <secMark>36799</secMark>
        <lat>411641851</lat>
        <long>-1048434230</long>
        <elev>18969</elev>
        <accuracy>
            <semiMajor>255</semiMajor>
            <semiMinor>255</semiMinor>
            <orientation>65535</orientation>
        </accuracy>
        <transmission><unavailable/></transmission>
        <speed>0</speed>
        <heading>25681</heading>
        <angle>127</angle>
        <accelSet>
            <long>2001</long>
            <lat>2001</lat>
            <vert>-127</vert>
            <yaw>0</yaw>
        </accelSet>
        <brakes>
            <wheelBrakes>
                10000
            </wheelBrakes>
            <traction><unavailable/></traction>
            <abs><unavailable/></abs>
            <scs><unavailable/></scs>
            <brakeBoost><unavailable/></brakeBoost>
            <auxBrakes><unavailable/></auxBrakes>
        </brakes>
        <size>
            <width>0</width>
            <length>0</length>
        </size>
    </coreData>
    <partII>
        <PartIIcontent>
            <partII-Id>0</partII-Id>
            <partII-Value>
                <VehicleSafetyExtensions>
                    <pathHistory>
                        <crumbData>
                            <PathHistoryPoint>
                                <latOffset>35</latOffset>
                                <lonOffset>131071</lonOffset>
                                <elevationOffset>95</elevationOffset>
                                <timeOffset>3320</timeOffset>
                            </PathHistoryPoint>
                            <PathHistoryPoint>
                                <latOffset>740</latOffset>
                                <lonOffset>131071</lonOffset>
                                <elevationOffset>46</elevationOffset>
                                <timeOffset>4460</timeOffset>
                            </PathHistoryPoint>
                            <PathHistoryPoint>
                                <latOffset>944</latOffset>
                                <lonOffset>51</lonOffset>
                                <elevationOffset>35</elevationOffset>
                                <timeOffset>4930</timeOffset>
                            </PathHistoryPoint>
                            <PathHistoryPoint>
                                <latOffset>1826</latOffset>
                                <lonOffset>637</lonOffset>
                                <elevationOffset>2047</elevationOffset>
                                <timeOffset>7170</timeOffset>
                            </PathHistoryPoint>
                            <PathHistoryPoint>
                                <latOffset>1313</latOffset>
                                <lonOffset>606</lonOffset>
                                <elevationOffset>2047</elevationOffset>
                                <timeOffset>8020</timeOffset>
                            </PathHistoryPoint>
                            <PathHistoryPoint>
                                <latOffset>1535</latOffset>
                                <lonOffset>748</lonOffset>
                                <elevationOffset>2047</elevationOffset>
                                <timeOffset>9290</timeOffset>
                            </PathHistoryPoint>
                            <PathHistoryPoint>
                                <latOffset>1650</latOffset>
                                <lonOffset>1171</lonOffset>
                                <elevationOffset>2047</elevationOffset>
                                <timeOffset>12640</timeOffset>
                            </PathHistoryPoint>
                            <PathHistoryPoint>
                                <latOffset>1363</latOffset>
                                <lonOffset>1000</lonOffset>
                                <elevationOffset>2047</elevationOffset>
                                <timeOffset>13610</timeOffset>
                            </PathHistoryPoint>
                            <PathHistoryPoint>
                                <latOffset>1196</latOffset>
                                <lonOffset>1028</lonOffset>
                                <elevationOffset>2047</elevationOffset>
                                <timeOffset>14190</timeOffset>
                            </PathHistoryPoint>
                            <PathHistoryPoint>
                                <latOffset>1111</latOffset>
                                <lonOffset>980</lonOffset>
                                <elevationOffset>2047</elevationOffset>
                                <timeOffset>14300</timeOffset>
                            </PathHistoryPoint>
                            <PathHistoryPoint>
                                <latOffset>131071</latOffset>
                                <lonOffset>95</lonOffset>
                                <elevationOffset>2047</elevationOffset>
                                <timeOffset>34800</timeOffset>
                            </PathHistoryPoint>
                            <PathHistoryPoint>
                                <latOffset>131071</latOffset>
                                <lonOffset>131071</lonOffset>
                                <elevationOffset>126</elevationOffset>
                                <timeOffset>43780</timeOffset>
                            </PathHistoryPoint>
                            <PathHistoryPoint>
                                <latOffset>131071</latOffset>
                                <lonOffset>131071</lonOffset>
                                <elevationOffset>115</elevationOffset>
                                <timeOffset>46820</timeOffset>
                            </PathHistoryPoint>
                            <PathHistoryPoint>
                                <latOffset>131071</latOffset>
                                <lonOffset>131071</lonOffset>
                                <elevationOffset>100</elevationOffset>
                                <timeOffset>47400</timeOffset>
                            </PathHistoryPoint>
                            <PathHistoryPoint>
                                <latOffset>131071</latOffset>
                                <lonOffset>70</lonOffset>
                                <elevationOffset>102</elevationOffset>
                                <timeOffset>48130</timeOffset>
                            </PathHistoryPoint>
                        </crumbData>
                    </pathHistory>
                    <pathPrediction>
                        <radiusOfCurve>32767</radiusOfCurve>
                        <confidence>0</confidence>
                    </pathPrediction>
                </VehicleSafetyExtensions>
            </partII-Value>
        </PartIIcontent>
    </partII>
</BasicSafetyMessage>