C examples

Introduction

Here we present and describe the examples included in the w-data library.

In order to compile examples (optionally you will need to modify Makefile):

[wtools@dell ~]$ cd wdata
[wtools@dell wdata]$ make examples

Folder wdata contains the library that provides support for w-data processing.
A list of examples demonstrating how to use this library is presented below.

example-write.c: code creates an artificial set of variables and writes them to wdat files.
example-write-many.c: code creates an artificial set of variables and writes them to wdat files. The difference between the above is the non-repeating parts of the code, however, it introduces some limitations.
example-write-many-t_varying.c: code creates artificial set of variables and writes them do wdat files.
example-read.c: code reads data from wdat files (created by example-write.c).
example-addvar.c: code adds new variable to existing data set (created by example-write.c).

Low-level reading and writing of data

Reading

Code snippet demonstrating how to load datablock from wdat file:

double *dataR; // pointer to real data
int cycleid;  // id of cycle for loading 

char file_name[128]="test_density_a.wdat";

FILE *pFile;

pFile= fopen (file_name, "rb"); // open file in `read` and `binary` mode
if(pFile==NULL) 
    printf("ERROR: Cannot open file\n");   
    
// set pointer to correct location
size_t blocklength = nx*ny*nz;  // for datadim=3
size_t blocksize = blocklength*sizeof(double); // for real variable
size_t ptr_shift = 0;
// in case of other formats, like npy:
// ptr_shift += size_of_header; // skip header
ptr_shift += blocksize*cycleid; // set pointer to correct location
if(fseek(pFile, ptr_shift, SEEK_SET) != 0)
    printf("ERROR: Cannot set pointer to datablock\n");

size_t test_ele = fread(data, blocksize, 1, pFile); // read datablock
if(test_ele != 1)
    printf("ERROR: Cannot read datablock\n");

fclose(pFile); // close file

The above code demonstrates the idea behind the library, however, we want to introduce some abstraction, and the code that uses the library function looks like that:

double *dataR;  // pointer to real data
int cycleid; // id of cycle to be loaded

wdata_metadata md; // WData metadata struct

wdata_parse_metadata_file("test.wtxt", &md); // parse the metadata `*wtxt` file

wdata_read_cycle(&md, "dataR", cycleid, dataR); // read the block of the code

Writing

double *dataR; // pointer to real data
 
char file_name[128] = "test_density_a.wdat";

FILE *pFile;

pFile = fopen (file_name, "ab"); // open file 
if(pFile==NULL)
    printf("ERROR: Cannot open file\n");
      
size_t blocklength = nx*ny*nz;                         // for datadim=3
size_t blocksize = blocklength*sizeof(double);         // for real variable
size_t test_ele = fwrite (dataR, blocksize, 1, pFile); // add datablock
if(test_ele != 1)
    printf("ERROR: Cannot add datablock\n");

// in case of other formats, like npy, additional change of header is required.
//    ...

fclose(pFile); // close file

Using W-data library:

double *dataR;  // real data

wdata_metadata md; // WData metadata struct

wdata_write_cycle(&md, "density_a", dataR);