bom.h File Reference

find unicode bom More...

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

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Classes
struct	bom_t
	Byte Order MArk (BOM) information of a file. This struct is created by detect_bom() More...

Typedefs
typedef struct bom_t	bom_t
	Byte Order MArk (BOM) information of a file. This struct is created by detect_bom()

Enumerations
enum	bom_endianness_t { UNKNOWN, LITTLE, BIG, SIGNATURE }
	endianness detected in bom More...

Functions
char *	bom_to_string (int length, unsigned char *bom, bom_endianness_t endianness)
	hex representation of the bom More...
void	print_bom (bom_t *bom)
	debug function to print bom More...
bom_t *	detect_bom (FILE *fd)
	check if the file has a bom More...

Detailed Description

find unicode bom

When parsing an HL7 file, the opened file pointer should be at the beginning of data (typically just at the beginning of MSH).

If the file contains a unicode BOM, and the file pointer points at the beginning of the file, the parser will fail. Therefore we first must skip the BOM bytes.

This is a crude method of detecting if the file has a BOM. Alternatively you may deploy you own method and just skip ahead until you know the file pointer is at the first character of data (at the beginning of MSH) before parsing the file.

how it's done:

we try to detect known BOM patterns and then place the pointer just after it. known patterns:

2 Bytes

• UTF-16 LE 0xFF 0xFE
• UTF-16 BE 0xFE 0xFF

3 Bytes

• UTF-8 0xEF 0xBB 0xBF
• UTF-1 0xF7 0x64 0x4C
• SCSU 0x0E 0xFE 0xFF
• BOCU-1 0xFB 0xEE 0xFF

4 Bytes

• UTF-7 0x2B 0x2F 0x76 // Followed by 38, 39, 2B, or 2F (ASCII 8, 9, + or /), depending on what the next character is.
• UTF-32 BE 0x00 0x00 0xFF 0xFF
• UTF-32 LE 0xFF 0xFE 0x00 0x00
• UTF-EBCDIC 0xDD 0x73 0x66 0x73
• GB-18030 0x84 0x31 0x95 0x33

usage

#include "bom.h"

FILE *fd = fopen(some/file, 'rb');
rewind(fd); // make sure the file pointer is at the beginning of the file
bom_t* detect_bom(fd);

Enumeration Type Documentation

bom_endianness_t

enum bom_endianness_t

endianness detected in bom

Enumerator
UNKNOWN	undetected
LITTLE	little endian
BIG	big endian
SIGNATURE	smaller than 16 bit, it doesn't matter

Function Documentation

bom_to_string()

char* bom_to_string	(	int	length,
		unsigned char *	bom,
		bom_endianness_t	endianness
	)

hex representation of the bom

Parameters

length	lenght of input buffer
bom	byte array with the bom
endianness	endianness to display

Returns

printable string

detect_bom()

bom_t* detect_bom

(

FILE *

fd

)

check if the file has a bom

if there is a bom, it will be copied to bom->bom. The file pointer will be set to the first character after the bom.

To check if a bom has been detected, bom->length is greater than 0. Length represents the number of bytes bom->bom contains.

Note

The file pointer must be at the beginning of the file or this will fail. Either run detect_bom() right after opening a file or rewind before using.

See also

https://en.wikipedia.org/wiki/Byte_order_mark

Parameters

fd	file handle to read data from

Returns

bom_t bom bytes are stored in bom->bom, length is indicated by bom->length

print_bom()

void print_bom

(

bom_t *

bom

)

debug function to print bom

Parameters

bom