How can I read an XML file into a buffer in C?

Question

I want to read an XML file into a char *buffer using C.

What is the best way to do this?

How should I get started?

Answer 1

And if you want to parse XML, not just reading it into a buffer (something which would not be XML-specific, see Christoph's and Baget's answers), you can use for instance libxml2:

#include <stdio.h>
#include <string.h>
#include <libxml/parser.h>

int main(int argc, char **argv) {
   xmlDoc *document;
   xmlNode *root, *first_child, *node;
   char *filename;

   if (argc < 2) {
     fprintf(stderr, "Usage: %s filename.xml\n", argv[0]);
     return 1;
   }
   filename = argv[1];

  document = xmlReadFile(filename, NULL, 0);
  root = xmlDocGetRootElement(document);
  fprintf(stdout, "Root is <%s> (%i)\n", root->name, root->type);
  first_child = root->children;
  for (node = first_child; node; node = node->next) {
     fprintf(stdout, "\t Child is <%s> (%i)\n", node->name, node->type);
  }
  fprintf(stdout, "...\n");
  return 0;
}

On an Unix machine, you typically compile the above with:

% gcc -o read-xml $(xml2-config --cflags) -Wall $(xml2-config --libs) read-xml.c

Answer 2

Is reading the contents of the file into a single, simple buffer really what you want to do? XML files are generally there to be parsed, and you can do this with a library like libxml2, just to give one example (but notably, is implemented in C).

Answer 3

Hopefully bug-free ISO-C code to read the contents of a file and add a '\0' char:

#include <stdlib.h>
#include <stdio.h>

long fsize(FILE * file)
{
    if(fseek(file, 0, SEEK_END))
        return -1;

    long size = ftell(file);
    if(size < 0)
        return -1;

    if(fseek(file, 0, SEEK_SET))
        return -1;

    return size;
}

size_t fget_contents(char ** str, const char * name, _Bool * error)
{
    FILE * file = NULL;
    size_t read = 0;
    *str = NULL;
    if(error) *error = 1;

    do
    {
        file = fopen(name, "rb");
        if(!file) break;

        long size = fsize(file);
        if(size < 0) break;

        if(error) *error = 0;

        *str = malloc((size_t)size + 1);
        if(!*str) break;

        read = fread(*str, 1, (size_t)size, file);
        (*str)[read] = 0;
        *str = realloc(*str, read + 1);

        if(error) *error = (size != (long)read);
    }
    while(0);

    if(file) fclose(file);
    return read;
}