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Yxml Manual


Yxml is a small non-validating and mostly conforming XML parser written in C.

The latest version of yxml and this document can be found on https://dev.yorhel.nl/yxml.

Compiling yxml

Due to the small size of yxml, the recommended way to use it is to copy the yxml.c and yxml.h from the git repository into your project directory, and compile and link yxml.c as part of your program or library.

The git repository also includes a Makefile. Running make without specifying a target will compile a .a file for easy static linking. A test suite is available under make test.

API documentation


Yxml is designed to be very flexible and efficient, and thus offers a relatively low-level stream-based API. The entire API consists of two typedefs and three functions:

typedef enum { /* .. */ } yxml_ret_t;
typedef struct { /* .. */ } yxml_t;

void yxml_init(yxml_t *x, void *buf, size_t bufsize);
yxml_ret_t yxml_parse(yxml_t *x, int ch);
yxml_ret_t yxml_eof(yxml_t *x);

The values of yxml_ret_t and the public fields of yxml_t are explained in detail below. Parsing a file using yxml involves three steps:

  1. Initialization, using yxml_init().
  2. Parsing. This is performed in a loop where yxml_parse() is called on each character of the input file.
  3. Finalization, using yxml_eof().


#define BUFSIZE 4096
void *buf = malloc(BUFSIZE);
yxml_t x;
yxml_init(&x, buf, BUFSIZE);

The parsing state for an input document is remembered in the yxml_t structure. This structure needs to be allocated and initialized before parsing a new XML document.

Allocating space for the yxml_t structure is the responsibility of the application. Allocation can be done on the stack, but it is also possible to embed the struct inside a larger object or to allocate space for the struct separately.

yxml_init() takes a pointer to an (uninitialized) yxml_t struct as first argument and performs the necessary initialization. The two additional arguments specify a pointer to a buffer and the size of this buffer. The given buffer must be writable, but does not have to be initialized by the application.

The buffer is used internally by yxml to keep a stack of opened XML element names, property names and PI targets. The size of the buffer determines both the maximum depth in which XML elements can be nested and the maximum length of element names, property names and PI targets. Each name consumes strlen(name)+1 bytes in the buffer, and the first byte of the buffer is reserved for the \0 byte. This means that in order to parse an XML document with an element name of 100 bytes, a property name or PI target of 50 bytes and a nesting depth of 10 levels, the buffer must be at least 1+10*(100+1)+(50+1)=1062 bytes. Note that properties and PIs don’t nest, so the max(PI_name, property_name) only needs to be counted once.

It is not currently possibly to dynamically grow the buffer while parsing, so it is important to choose a buffer size that is large enough to handle all the XML documents that you want to parse. Since element names, property names and PI targets are typically much shorter than in the previous example, a buffer size of 4 or 8 KiB will give enough headroom even for documents with deep nesting.

As a useful hack, it is possible to merge the memory for the yxml_t struct and the stack buffer in a single allocation:

yxml_t *x = malloc(sizeof(yxml_t) + BUFSIZE);
yxml_init(x, x+1, BUFSIZE);

This way, the complete parsing state can be passed around with a single pointer, and both the struct and the buffer can be freed with a single call to free(x).


yxml_t *x; /* An initialized state */
char *doc; /* The XML document as a zero-terminated string */
for(; *doc; doc++) {
  yxml_ret_t r = yxml_parse(x, *doc);
  if(r < 0)
    exit(1); /* Handle error */
  /* Handle any tokens we are interested in */

The actual parsing of an XML document is facilitated by the yxml_parse() function. It accepts a pointer to an initialized yxml_t struct as first argument and a byte as second argument. The byte is passed as an int, and values in the range of -128 to 255 (both inclusive) are accepted. This way you can pass either signed char or unsigned char values, yxml will work fine with both. To parse a complete document, yxml_parse() needs to be called for each byte of the document in sequence, as done in the above example.

For each byte, yxml_parse() will return either YXML_OK (0), a token (>0) or an error (<0). YXML_OK is returned if the given byte has been parsed/consumed correctly but that otherwise nothing worthy of note has happened. The application should then continue processing and pass the next byte of the document.

Public State Variables

After each call to yxml_parse(), a number of interesting fields in the yxml_t struct are updated. The fields documented here are part of the API, and are considered as extra return values of yxml_parse(). All of these fields should be considered read-only.

char *elem;
Name of the currently opened XML element. Points into the buffer given to yxml_init(). Described in “Elements”.
char *attr;
Name of the currently opened attribute. Points into the buffer given to yxml_init(). Described in “Attributes”.
char *pi;
Target of the currently opened PI. Points into the buffer given to yxml_init(). Described in “Processing Instructions”.
char data[8];
Character data of element contents, attribute values or PI contents. Described in “Character Data”.
uint32_t line;
Number of the line in the XML document that is currently being parsed.
uint64_t byte;
Byte offset into the current line the XML document.
uint64_t total;
Byte offset into the XML document.

The values of the elem, attr, pi and data elements depend on the parsing context, and only remain valid within that context. The exact contexts in which these fields contain valid information is described in their respective sections below.

The line, byte and total fields are mainly useful for error reporting. When yxml_parse() reports an error, these fields can be used to generate a useful error message. For example:

printf("Parsing error at %s:%"PRIu32":%"PRIu64" byte offset %"PRIu64",
  filename, x->line, x->byte, x->total);

Error Handling

Errors are not recoverable. No further calls to yxml_parse() or yxml_eof() should be performed on the same yxml_t struct. Re-initializing the same struct using yxml_init() to start parsing a new document is possible, however. The following error values may be returned by yxml_parse():

Invalid character or entity reference. E.g. &whatever; or &#ABC;.
Close tag does not match open tag. E.g. <Tag> .. </SomeOtherTag>.
Stack overflow. This happens when the buffer given to yxml_init() was not large enough to parse this document. E.g. when elements are too deeply nested or an element name, attribute name or PI target is too long.
Miscellaneous syntax error.

Handling Tokens

The yxml_parse() function will return tokens as they are found. When loading an XML document, it is important to know which tokens are returned in which situation and how to handle them.

The following graph shows the (simplified) state machine of the parser to illustrate the order in which tokens are returned. The labels on the edge indicate the tokens that are returned by yxml_parse(), with their YXML_ prefix removed. The special return value YXML_OK and error returns are not displayed.

Tokens that the application is not interested in can be ignored safely. For example, if you are not interested in handling processing instructions, then the YXML_PISTART, YXML_PICONTENT and YXML_PIEND tokens can be handled exactly as if they were an alias for YXML_OK.


The YXML_ELEMSTART and YXML_ELEMEND tokens are returned when an XML element is opened and closed, respectively. When YXML_ELEMSTART is returned, the elem struct field will hold the name of the element. This field will be valid (i.e. keeps pointing to the name of the opened element) until the end of the attribute list. That is, until any token other than those described in “Attributes” is returned. Although the elem pointer itself may be reused and modified while parsing the contents of the element, the buffer that elem points to will remain valid up to and including the corresponding YXML_ELEMEND.

Yxml will verify that elements properly nest and that the name of each closing tag properly matches that of the corresponding opening tag. The application may safely assume that each YXML_ELEMSTART is properly matched with a YXML_ELEMEND, or that otherwise an error is returned. Furthermore, only a single root element is allowed. When the root element is closed, no further YXML_ELEMSTART tokens will be returned.

No distinction is made between self-closing tags and elements with empty content. For example, both <a/> and <a></a> will result in the YXML_ELEMSTART token (with elem="a") followed by YXML_ELEMEND.

Element contents are returned in the form of the YXML_CONTENT token and the data field. This is described in more detail in “Character Data”.


Element attributes are passed using the YXML_ATTRSTART, YXML_ATTRVAL and YXML_ATTREND tokens. The name of the attribute is available in the attr field, which is available when YXML_ATTRSTART is returned and valid up to and including the next YXML_ATTREND.

Yxml does not verify that attribute names are unique within a single element. It is thus possible that the same attribute will appear twice, possibly with a different value. The correct way to handle this situation is to stop parsing the rest of the document and to report an error, but if the application is not interested in all attributes, detecting duplicates in them may complicate the code and possibly even introduce security vulnerabilities (e.g. algorithmic complexity attacks in a hash table). As such, the best solution is to report an error when you can easily detect a duplicate attribute, but ignore duplicates that require more effort to be detected.

The attribute value is returned with the YXML_ATTRVAL token and the data field. This is described in more detail in “Character Data”.

Processing Instructions

Processing instructions are passed in similar fashion to attributes, and are passed using YXML_PISTART, YXML_PICONTENT and YXML_PIEND. The target of the PI is available in the pi field after YXML_PISTART and remains valid up to (but excluding) the next YXML_PIEND token.

PI contents are returned as YXML_PICONTENT tokens and using the data field, described in more detail in “Character Data”.

Character Data

Element contents (YXML_CONTENT), attribute values (YXML_ATTRVAL) and PI contents (YXML_PICONTENT) are all passed to the application in small chunks through the data field. Each time that yxml_parse() returns one of these tokens, the data field will contain one or more bytes of the element contents, attribute value or PI content. The string is zero-terminated, and its value is only valid until the next call to yxml_parse().

Typically only a single byte is returned after each call, but multiple bytes can be returned in the following special cases:

Note that yxml_parse() operates on bytes rather than characters. If the document is encoded in a multi-byte character encoding such as UTF-8, then each Unicode character that occupies more than a single byte will be broken up and its bytes processed individually. As a result, the bytes returned in the data field may not necessarily represent a single Unicode character. To ensure that multi-byte characters are not broken up, the application can concatenate multiple data tokens to a single buffer before attempting to do further processing on the result.

To make processing easier, an application may want to combine all the tokens into a single buffer. This can be easily implemented as follows:

SomeString attrval;
while(..) {
  yxml_ret_t r = yxml_parse(x, ch);
  switch(r) {
    somestring_append(attrval, x->data);
    /* Now we have a full attribute. Its name is in x->attr, and its value is
     * in the string 'attrval'. */

The SomeString type and somestring_ functions are stubs for any string handling library of your choosing. When using Glib, for example, one could use the GString type and the g_string_new(), g_string_append() and g_string_free() functions. For a more lighter-weight string library there is also kstring.h in klib, but the functionality required in the above example can easily be implemented in a few lines of pure C, too.

When buffering data into an ever-growing string, as done in the previous example, one should be careful to protect against memory exhaustion. This can be done trivially by limiting the size of the total XML document or the maximum length of the buffer. If you want to extract information from an XML document that might not fit into memory, but you know that the information you care about is limited in size and is only stored in specific attributes or elements, you can choose to ignore data you don’t care about. For example, if you only want to extract the “Size” attribute and you know that its value is never larger than 63 bytes, you can limit your code to read only that value and store it into a small pre-allocated buffer:

char sizebuf[64], *sizecur = NULL, *tmp;
while(..) {
  yxml_ret_t r = yxml_parse(x, ch);
  switch(r) {
    if(strcmp(x->attr, "Size") == 0)
      sizecur = sizebuf;
    if(!sizecur) /* Are we in the "Size" attribute? */
    /* Append x->data to sizecur while there is space */
    tmp = x->data;
    while(*tmp && sizecur < sizebuf+sizeof(sizebuf))
      *(sizecur++) = *(tmp++);
    if(sizecur == sizebuf+sizeof(sizebuf))
      exit(1); /* Too long attribute value, handle error */
    *sizecur = 0;
    if(sizecur) {
      /* Now we have the value of the "Size" attribute in sizebuf */
      sizecur = NULL;


yxml_t *x; /* An initialized state */
yxml_ret_t r = yxml_eof(x);
if(r < 0)
  exit(1); /* Handle error */
  /* No errors in the XML document */

Because yxml_parse() does not know when the end of the XML document has been reached, it is unable to detect certain errors in the document. This is why, after successfully parsing a complete document with yxml_parse(), the application should call yxml_eof() to perform some extra checks.

yxml_eof() will return YXML_OK if the parsed XML document is well-formed, YXML_EEOF otherwise. The following errors are not detected by yxml_parse() but will result in an error on yxml_eof():

Utility functions

size_t yxml_symlen(yxml_t *, const char *);

yxml_symlen() returns the length of the element name (x->elem), attribute name (x->attr), or PI name (x->pi). When used correctly, it gives the same result as strlen(), except without having to scan through the string. This function should ONLY be used directly after the YXML_ELEMSTART, YXML_ATTRSTART or YXML_PISTART (respectively) tokens have been returned by yxml_parse(), calling this function at any other time may not give the correct results. This function should NOT be used on strings other than x->elem, x->attr or x->pi.

Encoding considerations

Yxml operates on bytes and assumes that the input XML document has been encoded in an ASCII-compatible encoding (i.e. byte values below 128 are interpreted as ASCII). Beyond that, yxml does not validate that the input XML document conforms to any specific encoding - in this sense it follows the garbage-in-garbage-out principle. All strings returned by the yxml API will have the same encoding as the input XML document, with the exception of character references outside of the ASCII character range: those are encoded and returned as UTF-8 strings.

If character encoding matters to your application (it likely does), you should either make sure that the input XML document as a whole is valid UTF-8 or you should perform UTF-8 validation on individual strings returned by the yxml API. The former approach is the safest and easiest, but the latter approach allows you to still extract useful information out of a document with an invalid encoding.