xchange

Structured data exchange and JSON support for C/C++

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Free platform-agnostic data exchange framework for C/C++, including a built-in JSON parser and emitter.

Author: Attila Kovacs

Last Updated: 10 December 2024

Table of Contents

Introduction

The xchange library provides a free framework for platform independent data exchange for structured data in C/C++, and includes a JSON parser and emitter. It is free to use, in any way you like, without licensing restrictions.

While there are many excellent libraries out there that offer such capabilities for C++ and/or other object-oriented languages, support for structured data exchange is notably rare for standard C. The xchange library aims to fill that niche, by providing a data exchange framework with a C99-compatible API that supports the interchange of arbitrary structured data between different platforms and different serialization formats with ease. The xchange library also provides support for JSON formatting and parsing using the C99 standard out of the box. All that in a light-weight and fast package.

The xchange library was created, and is maintained, by Attila Kovács at the Center for Astrophysics | Harvard & Smithsonian, and it is available through the Smithsonian/xchange repository on GitHub.

There are no official releases of xchange yet. An initial 1.0.0 release is expected in late 2024. Before then the API may undergo slight changes and tweaks. Use the repository as is at your own risk for now.

Building

The xchange library can be built either as a shared (libxchange.so[.1]) and as a static (libxchange.a) library, depending on what suits your needs best.

You can configure the build, either by editing config.mk or else by defining the relevant environment variables prior to invoking make. The following build variables can be configured:

  • CC: The C compiler to use (default: gcc).

  • CPPFLAGS: C preprocessor flags, such as externally defined compiler constants.

  • CFLAGS: Flags to pass onto the C compiler (default: -g -Os -Wall). Note, -Iinclude will be added automatically.

  • CSTANDARD: Optionally, specify the C standard to compile for, e.g. c99 to compile for the C99 standard. If defined then -std=$(CSTANDARD) is added to CFLAGS automatically.

  • WEXTRA: If set to 1, -Wextra is added to CFLAGS automatically.

  • LDFLAGS: Extra linker flags (default: not set). Note, -lm will be added automatically.

  • CHECKEXTRA: Extra options to pass to cppcheck for the make check target

After configuring, you can simply run make, which will build the shared (lib/libxchange.so[.1]) and static (lib/libxchange.a) libraries, local HTML documentation (provided doxygen is available), and performs static analysis via the check target. Or, you may build just the components you are interested in, by specifying the desired make target(s). (You can use make help to get a summary of the available make targets).

After building the library you can install the above components to the desired locations on your system. For a system-wide install you may simply run:

  $ sudo make install

Or, to install in some other locations, you may set a prefix and/or DESTDIR. For example, to install under /opt instead, you can:

  $ sudo make prefix="/opt" install

Or, to stage the installation (to /usr) under a ‘build root’:

  $ make DESTDIR="/tmp/stage" install

Linking your application against xchange

Provided you have installed the shared (libxchange.so) or static (libxchange.a) library in a location that is in your LD_LIBRARY_PATH (e.g. in /usr/lib or /usr/local/lib) you can simply link your program using the -lxchange flag. Your Makefile may look like:

myprog: ...
	$(CC) -o $@ $^ $(LDFLAGS) -lxchange

(Or, you might simply add -lxchange to LDFLAGS and use a more standard recipe.) And, in if you installed the xchange library elsewhere, you can simply add the location to LD_LIBRARY_PATH prior to linking.

Structured data

The xchange library defines the XStructure type to represent structured data. It is defined in xchange.h, but as a user you really do not need to know much about its layout, as you probably want to avoid low-level direct access to its elements. Rather, you should be using the functions of the xchange API to create, modify, or access data within.

Under the hood, the XStructure contains a linked list of fields, each an XField data type to represent a single element, or an array of elements, of the above mentioned types, including embedded Xstructures. In this way, an Xstructure can easily represent a multi-level hierarchy of a composite data object. Each XField has a name/ID, an associated data type, a dimensionality, a shape (for multidimensional arrays).

Basic data types

The xchange library supports most basic (primitive) data types used across programming languages. The table below shows the xchange types recognized by the library and the corresponding pointer/array type values:

XType element type Comment / example
X_BOOLEAN boolean ‘true’ or ‘false’
X_BYTE char or int8_t ’-128’ to ‘127’
X_SHORT short or int16_t ’-32768’ to ‘32767’
X_INT int32_t (int) ’-2,147,483,648’ to ‘2,147,483,647’
X_LONG long long or int64_t ’-9,223,372,036,854,775,808’ to ‘9,223,372,036,854,775,807’
X_FLOAT float 1, 1.0, -1.234567e-33
X_DOUBLE double 1, 1.0, -1.2345678901234567e-111
X_STRING char * Hello world!, line1\nline2\n (0-terminated)
X_CHARS(n) char[n] Fixed-length character arrays (also w/o termination)
X_FIELD XField For irregular and/or heterogeneous arrays
X_STRUCT XStructure substructure

The boolean type is defined in xchange.h. The XField.value is a pointer / array of the given element type. So, an XField of type X_DOUBLE will have a value field that should be cast a (double *), while for type X_STRING the value field shall be cast as (char **).

Strings

Strings can be either fixed-width or else a 0-terminated sequence of ASCII characters. At its basic level the library does not impose any restriction of what ASCII characters may be used. However, we recommend that users stick to the JSON convention, and represent special characters in escaped form. E.g. carriage return (0xd) as \ followed by n, tabs as \ followed by t, etc. As a result a single backslash should also be escaped as two consecutive \ characters. You might use xjsonEscapeString() or xjsonUnescapeString() to perform the conversion to/from standard JSON representation.

Fixed-width strings of up to n characters are represented internally as the XCHAR(n) type. They may be 0-terminated as appropriate, or else represent exactly n ASCII characters without explicit termination. Alternatively, the X_STRING type represents ASCII strings of arbitrary length, up to the 0-termination character.

Scalars

You can create scalar fields easily, e.g.:

  // Create "is_ok" as a boolean field with TRUE
  XField *fb = xCreateBooleanField("is_ok", TRUE);

  // Create "serial-number" field with an integer value
  XField *fi = xCreateIntField("serial-number", 1001);

  // Create "my measurement" as a double-precision value 1.04
  XField *fd = xCreateDoubleField("my-measurement", 1.04);
  
  // Create "description" as a string
  XField *fs = xCreateStringField("description", "blah-blah-blah");

Under the hood, scalar values are a special case of arrays containing a single element. Scalars have dimension zero i.e., a shape defined by an empty integer array, e.g. int shape[0] in a corresponding XField element.

In this way scalars are distinguished from true arrays containing just a single elements, which have dimensionality <=1 and shapes e.g., int shape[1] = {1} or int shape[2] = {1, 1}. The difference, while subtle, becomes more obvious when serializing the array, e.g. to JSON. A scalar floating point value of 1.04, for example, will appear as 1.04 in JSON, whereas the 1D and 2D single-element arrays will be serialized as { 1.04 } or 1.04, respectively.

Arrays

The xchange library supports array data types in one or more dimensions (up to 20 dimensions). For example, to create a field for 2×3×4 array of doubles, you may have something along:

  double data[2][3][4] = ...;		// The native array in C
  int sizes[] = { 2, 3, 4 };		// An array containing the dimensions for xchange
  
  // Create a field for the 3-dimensional array with the specified shape.
  XField *f = xCreateField("my-array", X_DOUBLE, 3, sizes, data);

Note, that there is no requirement that the native array has the same dimensionality as it’s nominal format in the field. We could have declared data as a 1D array double data[2 * 3 * 4] = ..., or really any array (pointer) containing doubles with storage for at least 24 elements. It is the sizes array, along with the dimensionality, which together define the number of elements used from it, and the shape of the array for xchange.

Arrays of irregular shape or mixed element types can be represented by fields containing an array of XField entries:

  XField *row1, row2, ...  		   // Heterogeneous entries, each wrapped in an `XField`
  XField data[N] = { *row1, *row2, ... };  // The irregular / mixed-type array. 

  XField *f = xCreateMixed1DField("my_array", N);

Or, use xCreateMixedArrayField() to create a multi-dimensional array of heterogeneous elements the same way.

Creating structure

Structures should always be created by calling xCreateStruct() (or else by an appropriate de-serialization function such as xjsonParseAt(), or as a copy via xCopyStruct()). Once the structure is no longer used it should be explicitly destroyed (freed) by calling xDestroyStruct(). Named substructures can be added to any structure with xSetSubstruct(), and named fields via xSetField(). That is the gist of it. So for example, the skeleton structure from the example above can be created programatically as:

  XStructure *s, *sys, *sub;
  
  // Create the top-level structure
  s = xCreateStruct();
  
  // Create and add the "system" sub-structure
  sys = xCreateStruct();
  xSetSubstruct(s, "system", sys);
  
  // Create and add the "subsystem" sub-structure
  sub = xCreateStruct();
  xSetSubstruct(sys, "subsystem", sub);
  
  // Set the "property" field in "subsystem".
  xSetField(sub, "property", xCreateStringField("some value here"));

and then eventually destroyed after use as:

  // Free up all resources used by the structure 's'
  xDestroyStruct(s);

Aggregate IDs

Since the XStructure data type can represent hierarchies of arbitrary depth, and named at every level of the hierarchy, we can uniquely identify any particular field, at any level, with an aggregate ID, which concatenates the field names each every level, top-down, with a separator. The convention of xchange is to use colon (‘:’) as the separator. Consider an example structure (in JSON notation):

  {
    "system": {
      "subsystem": {
        "property": "some value here"
      }
    }
  }

Then, the leaf “property” entry can be ‘addressed’ with the aggregate ID of system:subsystem:property from the top level. The xGetAggregateID() function is provided to construct such aggregate IDs by gluing together a leading and trailing component.

Accessing substructures and elements

Once a structure is populated – either by having constructed it programatically, or e.g. by parsing a JSON definition of it from a string or file – you can access its content and/or modify it.

E.g., to retrieve the “property” field from the above example structure:

  XField *f = xGetField(s, "system:subsystem:property");

or to retrieve the “subsystem” structure from within:

  XStructure *sub = xGetSubstruct(s, "system:subsystem");

Conversely you can set / update fields in a structure using xSetField() / xSetSubstruct(), e.g.:

  XStructrure *newsub = ...     // The new substructure
  XField *newfield = ...        // A new field to set
  XField *oldfield, *oldsub;    // prior entries by the same field name/location (if any)
  
  oldfield = xSetField(s, newfield);        // Sets the a field in 's'
  oldsub = xSetSubstruct(s, "field", sub);  // Set a substructure named "bar" in 's'

The above calls return the old values (if any) for the “foo” and “bar” field in the structure, e.g. so we may dispose of them if appropriate:

  // Destroy the replaced fields if they are no longer needed.
  xDestroyField(oldfield);
  xDestroyField(oldsub);

You can also remove existing fields from structures using xRemoveField(), e.g.

  // Remove and then destroy the field named "blah" in structure 's'.
  xDestroyField(xRemoveField(s, "blah"));

Large structures

The normal xGetField() and xGetSubstruct() functions have computational costs that scale linearly with the number of direct fields in the structure. It is not much of an issue for structures that contain dozens of, or even a couple hundred, fields (per layer). For much larger structures, which have a fixed layout, there is an option for a potentially much more efficient hash-based lookup also. E.g. instead of xGetField() you may use xLookupField():

  XStructure *s = ...
  
  // Create a lookup table for all fields of 's' and all its substructures.
  XLookupTable *l = xCreateLookupTable(s, TRUE);
  
  // Now use a hash-based lookup to locate the field by name
  XField *f = xLookupField(l, "subsystem:property");
 
  ...
  
  // Once done with the lookup, destroy it.
  xDestroyLookup(l);

Note however, that preparing the lookup table has significant O(N) computational cost also. Therefore, a lookup table is practical only if you are going to use it repeatedly, many times over. As a rule of thumb, lookups may have the advantage if accessing fields in a structure by name hundreds of times, or more.

The same performance limitation also applies to building large structures, since the xSetField() and xSetSubstruct() functions iterate over the existing fields to check if a prior field by the same name was already present, and which should be removed before the new field is set (hence the time to build up a structure with N fields will scale as O(N2) in general). The user may consider using xInsertField() instead, which is much more scalable for building large structures, since it does not check for duplicates (hence scales as O(N) overall). However, xInsertField() also makes the ordering of fields less intuitive, and it is left up to the caller to ensure that field names added this way are never duplicated. (Tip: if you used InsertField() consistently, you may call xReverseFieldOrder() at the end, so the fields will appear in the same order in which they were inserted.)

Iterating over elements

You can easily iterate over the elements also. This is one application where you may want to know the internal layout of XStructure, namely that it contains a simple linked-list of XField fields. One way to iterate over a structures elements is with a for loop, e.g.:

  XStructure *s = ...
  XField *f;
  
  for (f = s->firstField; f != NULL; f = f->next) {
    // Process each field 'f' here...
    ...
  }

Sorting fields

You can easily sort fields by name using xSortFieldsByName(), or with using a custom comparator function with xSortFields(). You can also reverse the order with xReverseFieldOrder(). For example to sort fields in a structure (and its substructures) in descending alphabetical order:

  XStructure *s = ...
  
  // Sort in by names in ascending order, recursively
  xSortFieldsByName(s, TRUE);

  // Reverse the order, recursively
  xReverseFieldOrder(s, TRUE);

JSON parser and emitter

Once you have an XStructure data object, you can easily convert it to JSON string representation, as:

  XStructure *s = ...

  // Obtain a JSON string representation of the structure 's'.
  char *json = xjsonToString(s);

The above produces a proper JSON document. Or, you can do the reverse and create an XStructure from its JSON representation, either from a string (a 0-terminated char array):

  int lineNumber = 0;
  XStructure *s1 = xjsonParseAt(json, &lineNumber);
  if (s1 == NULL) {
     // Oops, there was some problem...
  }

or parse it from a file, which contains a JSON definition of the structured data:

  XStructure *s1 = xjsonParseFilename("my-data.json", &lineNumber);
  if (s1 == NULL) {
     // Oops, there was some problem...
  }

Snippets

Alternatively, you can also create partial JSON fragments for individual fields, e.g.:

  XField *f = ...
  
  // Obtain a JSON fragment for the field 'f'.
  char *json = xjsonFieldToString(f);

For example, for a numerical array field with 4 elements the above might generate something like:

  "my-numbers": [ 1, 2, 3, 4]

Escaped string representations

You might just want to use JSON-style escaping for strings, and xjsonEscape() / xjsonUnescape() can help with that too. Suppose you have a C string that you want to escape…

  char *string = "\"This has some\n\t special characters\"";
  
  // Escape the special character, e.g. replace `\n` with `\` + `n` etc...
  char *escaped = xjsonEscape(string);

If you print string to a file or the standard output, it will show up as 2 lines:

  "This has some
          special characters"

But if you now print escaped instead, that will show up as:

  \"This has some\n\t special characters\"

And the reverse, suppose you read back the above line from an input, containing the escaped form, and want to reconstruct from it the original C string with the special characters in it:

   // And reverse, from escaped form to ASCII (e.g. `\` + `n` --> `\n`)
   char *string = xjsonUnescape(escaped);

Error handling

The functions that can encounter an error will return either one of the error codes defined in xchange.h, or NULL pointers. String descriptions for the error codes can be produced by xErrorDescription(int). For example,

  char *text = ...
  int status = xParseDouble(text, NULL);
  if (status != X_SUCCESS) {
    // Ooops, something went wrong...
    fprintf(stderr, "WARNING! %s", xErrorDescription(status));
    ...
  }

The JSON parser can also sink its error messages to a designated file or stream, which can be set by xjsonSetErrorStream(FILE *).

Debugging support

You can enable verbose output of the xchange library with xSetVerbose(boolean). When enabled, it will produce status messages to stderrso you can follow what’s going on. In addition (or alternatively), you can enable debug messages with xSetDebug(boolean). When enabled, all errors encountered by the library (such as invalid arguments passed) will be printed to stderr, including call traces, so you can walk back to see where the error may have originated from. (You can also enable debug messages by default by defining the DEBUG constant for the compiler, e.g. by adding -DDEBUG to CFLAGS prior to calling make).

For helping to debug your application, the xchange library provides two macros: xvprintf() and xdprintf(), for printing verbose and debug messages to stderr. Both work just like printf(), but they are conditional on verbosity being enabled via xSetVerbose(boolean) and xSetDebug(boolean), respectively. Applications using xchange may use these macros to produce their own verbose and/or debugging outputs conditional on the same global settings.

Future plans

There are a number of ways this little library can evolve and grow in the not too distant future. Some of the obvious paths forward are:

  • Add regression testing and code coverage tracking (high priority)
  • Add support for BSON – MongoDB’s binary exchange format.
  • Add support for 128-bit floating point types (X_FLOAT128).

If you have an idea for a must have feature, please let me (Attila) know. Pull requests, for new features or fixes to existing ones are especially welcome!

Copyright (C) 2024 Attila Kovács