Introduction to Expat XML Parser Library

The Expat XML Parser is a fast, non-validating, stream-oriented XML parsing library written in C. It is widely used for efficient processing of XML data in applications. The library provides various APIs for handling XML elements, attributes, and character data, making it highly versatile for different use cases.

Getting Started with Expat

First, you need to include the necessary headers and link against the Expat library:

#include <expat.h>

Example Usage of Expat APIs

1. Creating an XML Parser

XML_Parser parser = XML_ParserCreate(NULL); if (!parser) {
  fprintf(stderr, "Couldn't allocate memory for parser\n");
  exit(-1);
}

2. Setting Up Handlers

void startElement(void *userData, const char *name, const char **atts) {
  printf("Start element: %s\n", name);
}
void endElement(void *userData, const char *name) {
  printf("End element: %s\n", name);
}
XML_SetElementHandler(parser, startElement, endElement);

3. Parsing XML Data

FILE *file = fopen("example.xml", "r"); if (!file) {
  fprintf(stderr, "Unable to open file.\n");
  exit(-1);
}
char buffer[BUFSIZ]; int done;
do {
  size_t len = fread(buffer, 1, sizeof(buffer), file);
  done = len < sizeof(buffer);
  if (XML_Parse(parser, buffer, len, done) == XML_STATUS_ERROR) {
    fprintf(stderr, "Parse error at line %lu:\n%s\n",
            XML_GetCurrentLineNumber(parser),
            XML_ErrorString(XML_GetErrorCode(parser)));
    exit(-1);
  }
} while (!done);
  
fclose(file); XML_ParserFree(parser);

4. Error Handling

if (XML_Parse(parser, buffer, len, done) == XML_STATUS_ERROR) {
  fprintf(stderr, "Parse error at line %lu:\n%s\n",
          XML_GetCurrentLineNumber(parser),
          XML_ErrorString(XML_GetErrorCode(parser)));
  exit(-1);
}

Comprehensive App Example

Let's put it all together in a simple application that reads an XML file and prints out the start and end tags of elements:

#include <stdio.h> #include <stdlib.h> #include <expat.h>
void startElement(void *userData, const char *name, const char **atts) {
  printf("Start element: %s\n", name);
}
void endElement(void *userData, const char *name) {
  printf("End element: %s\n", name);
}
int main(int argc, char *argv[]) {
  if (argc != 2) {
    fprintf(stderr, "Usage: %s filename.xml\n", argv[0]);
    return 1;
  }

  XML_Parser parser = XML_ParserCreate(NULL);
  if (!parser) {
    fprintf(stderr, "Couldn't allocate memory for parser\n");
    return -1;
  }

  XML_SetElementHandler(parser, startElement, endElement);

  FILE *file = fopen(argv[1], "r");
  if (!file) {
    fprintf(stderr, "Unable to open file.\n");
    XML_ParserFree(parser);
    return -1;
  }

  char buffer[BUFSIZ];
  int done;

  do {
    size_t len = fread(buffer, 1, sizeof(buffer), file);
    done = len < sizeof(buffer);
    if (XML_Parse(parser, buffer, len, done) == XML_STATUS_ERROR) {
      fprintf(stderr, "Parse error at line %lu:\n%s\n",
              XML_GetCurrentLineNumber(parser),
              XML_ErrorString(XML_GetErrorCode(parser)));
      XML_ParserFree(parser);
      fclose(file);
      return -1;
    }
  } while (!done);

  fclose(file);
  XML_ParserFree(parser);
  return 0;
}

Conclusion

The Expat library is a powerful tool for XML parsing, providing efficient and fast processing capabilities. With a variety of APIs available, developers can handle XML data in various ways, making it suitable for many applications.