c - Geting xml data using xml parser expat

Question

I have managed to parse ok. But now I am having trouble getting the values that I need. I can get the element and the attributes. But cannot get the values. I would like to get the value of frame in this xml it is 20.

/* track the current level in the xml tree */
static int depth = 0;
/* first when start element is encountered */
void start_element(void *data, const char *element, const char **attribute)
{
int i;

for(i = 0; i < depth; i++)
{
    printf(" ");
}

printf("%s", element);

for(i = 0; attribute[i]; i += 2)
{
    printf(" %s= '%s'", attribute[i], attribute[i + 1]);
}

printf("
");
depth++;
}

/* decrement the current level of the tree */
void end_element(void *data, const char *el)
{
depth--;
}
int parse_xml(char *buff, size_t buff_size)
{
    FILE *fp;
    fp = fopen("start_indication.xml", "r");
    if(fp == NULL)
    {
    printf("Failed to open file
");
    return 1;
    }

    XML_Parser parser = XML_ParserCreate(NULL);
    int done;
    XML_SetElementHandler(parser, start_element, end_element);

    memset(buff, 0, buff_size);
    printf("strlen(buff) before parsing: %d
", strlen(buff));

    size_t file_size = 0;
    file_size = fread(buff, sizeof(char), buff_size, fp);

    /* parse the xml */
    if(XML_Parse(parser, buff, strlen(buff), XML_TRUE) == XML_STATUS_ERROR)
    {
        printf("Error: %s
", XML_ErrorString(XML_GetErrorCode(parser)));
    }

    fclose(fp);
    XML_ParserFree(parser);

    return 0;
}



<data>
    <header length="4">
            <item name="time" type="time">16</item>
            <item name="ref" type="string">3843747</item>
            <item name="port" type="int16">0</item>
            <item name="frame" type="int16">20</item>
    </header>
</data>

Output from parsing


Element: data
Element: header length= '4'
Element: item name= 'time' type= 'time'
Element: item name= 'ref' type= 'string'
Element: item name= 'port' type= 'int16'
Element: item name= 'frame' type= 'int16'

Answer 1

The 'value' 20 is the character data "20" in the element whose tagname is "item" and whose name attribute is "frame".

To receive character data events, register a callback with the XML_SetCharacterDataHandler function.

This callback will receive the character data. The parser may split character data - typically to handle reaching the end of a buffer, or for entities (so for foo&bar your handler will get three calls - "foo", "&" and "bar"), so you have to paste the string parts together again if you need the whole of the data.

You know when you have all the character data inside a node when you receive the next element start or close callback.

When you have all the character data, you can process it.

A stand-alone example simplified from your code:

#include <expat.h>
#include <stdio.h>
#include <stdbool.h>
#include <string.h>

static const char* xml =
    "<data>
"
    "    <header length="4">
"
    "            <item name="time" type="time">16</item>
"
    "            <item name="ref" type="string">3843747</item>
"
    "            <item name="port" type="int16">0</item>
"
    "            <item name="frame" type="int16">20</item>
"
    "    </header>
"
    "</data>
";

void reset_char_data_buffer ();
void process_char_data_buffer ();
static bool grab_next_value;

void start_element(void *data, const char *element, const char **attribute) {
    process_char_data_buffer();
    reset_char_data_buffer();

    if ( strcmp("item", element) == 0 ) {
        size_t matched = 0;

        for (size_t i = 0; attribute[i]; i += 2) {
            if ( ( strcmp("name", attribute[i]) == 0 ) && ( strcmp("frame", attribute[i+1]) == 0 ) )
                ++matched;

            if ( ( strcmp("type", attribute[i]) == 0 ) && ( strcmp("int16", attribute[i+1]) == 0 ) )
                ++matched;
        }

        if (matched == 2) {
            printf("this is the element you are looking for
");
            grab_next_value = true;
        }
    }
}

void end_element(void *data, const char *el) {
    process_char_data_buffer();
    reset_char_data_buffer();
}

static char char_data_buffer[1024];
static size_t offs;
static bool overflow;

void reset_char_data_buffer (void) {
    offs = 0;
    overflow = false;
    grab_next_value = false;
}

// pastes parts of the node together
void char_data (void *userData, const XML_Char *s, int len) {
    if (!overflow) {
        if (len + offs >= sizeof(char_data_buffer) ) {
            overflow = true;
        } else {
            memcpy(char_data_buffer + offs, s, len);
            offs += len;
        }
    }
}

// if the element is the one we're after, convert the character data to
// an integer value
void process_char_data_buffer (void) {
    if (offs > 0) {
        char_data_buffer[ offs ] = '';

        printf("character data: %s
", char_data_buffer);

        if ( grab_next_value ) {
            int value = atoi( char_data_buffer );

            printf("the value is %d
", value);
        }
    }
}

int main (void ) {
    XML_Parser parser = XML_ParserCreate(NULL);

    XML_SetElementHandler(parser, start_element, end_element);
    XML_SetCharacterDataHandler(parser, char_data);

    reset_char_data_buffer();

    if (XML_Parse(parser, xml, strlen(xml), XML_TRUE) == XML_STATUS_ERROR)
        printf("Error: %s
", XML_ErrorString(XML_GetErrorCode(parser)));

    XML_ParserFree(parser);

    return 0;
}