How to read FORTRAN binary file with c or c++? - c++

Question

I have FORTRAN 77 binary file (created on Sun Sparc machine,big endian). I want to read it on my little endian machine. I have come across this
http://paulbourke.net/dataformats/reading/
Paul has written these macros for C or C++, but I do not understand what they really do.
#define SWAP_2(x) ( (((x) & 0xff) << 8) | ((unsigned short)(x) >> 8) )
#define SWAP_4(x) ( ((x) << 24) | (((x) << 8) & 0x00ff0000) | \
(((x) >> 8) & 0x0000ff00) | ((x) >> 24) )
#define FIX_SHORT(x) (*(unsigned short *)&(x) = SWAP_2(*(unsigned short *)&(x)))
#define FIX_LONG(x) (*(unsigned *)&(x) = SWAP_4(*(unsigned *)&(x)))
#define FIX_FLOAT(x) FIX_LONG(x)
I know that every record of the file contains contains
x,y,z,t,d,i
i is integer*2,all other variables are real*4.
First 512 bytes hexdump
0000000 0000 1800 0000 0000 0000 0000 0000 0000
0000010 0000 0000 0000 0000 ffff ffff 0000 1800
0000020 0000 1800 003f 0000 0000 0000 233c 0ad7
0000030 0000 0000 233c 0ad7 0000 0100 0000 1800
0000040 0000 1800 803f 0000 0000 0000 233c 0ad7
0000050 0000 0000 233c 0ad7 0000 0100 0000 1800
0000060 0000 1800 c03f 0000 0000 0000 233c 0ad7
0000070 0000 0000 233c 0ad7 0000 0100 0000 1800
0000080 0000 1800 0040 0000 0000 0000 233c 0ad7
0000090 0000 0000 233c 0ad7 0000 0100 0000 1800
00000a0 0000 1800 2040 0000 0000 0000 233c 0ad7
00000b0 0000 0000 233c 0ad7 0000 0100 0000 1800
00000c0 0000 1800 4040 0000 0000 0000 233c 0ad7
00000d0 0000 0000 233c 0ad7 0000 0100 0000 1800
00000e0 0000 1800 6040 0000 0000 0000 233c 0ad7
00000f0 0000 0000 233c 0ad7 0000 0100 0000 1800
0000100 0000 1800 8040 0000 0000 0000 233c 0ad7
0000110 0000 0000 233c 0ad7 0000 0100 0000 1800
0000120 0000 1800 9040 0000 0000 0000 233c 0ad7
0000130 0000 0000 233c 0ad7 0000 0100 0000 1800
0000140 0000 1800 a040 0000 0000 0000 233c 0ad7
0000150 0000 0000 233c 0ad7 0000 0100 0000 1800
0000160 0000 1800 b040 0000 0000 0000 233c 0ad7
0000170 0000 0000 233c 0ad7 0000 0100 0000 1800
0000180 0000 1800 c040 0000 0000 0000 233c 0ad7
0000190 0000 0000 233c 0ad7 0000 0100 0000 1800
00001a0 0000 1800 d040 0000 0000 0000 233c 0ad7
00001b0 0000 0000 233c 0ad7 0000 0100 0000 1800
00001c0 0000 1800 e040 0000 0000 0000 233c 0ad7
00001d0 0000 0000 233c 0ad7 0000 0100 0000 1800
00001e0 0000 1800 f040 0000 0000 0000 233c 0ad7
00001f0 0000 0000 233c 0ad7 0000 0100 0000 1800
0000200
My code to read file
#include <endian.h>
#include <stdint.h>
#include <stdio.h>
#include <stdlib.h>
int main()
{
FILE *file;
char *buffer;
char *rec;
long fileLen;
file = fopen("rec.in", "rb");
fseek(file, 0, SEEK_END);
fileLen=ftell(file);
fseek(file, 0, SEEK_SET);
buffer=(char *)malloc(fileLen+1);
fread(buffer, fileLen, 1, file);
fclose(file);
free(buffer);
char *curr = buffer;
char *end = buffer + fileLen;
constexpr int LINE_SIZE = sizeof(float)*5 + sizeof(uint16_t); //based upon your "x,y,z,t,d,i" description
while(curr < end) {
uint32_t temp = be32toh(*reinterpret_cast<uint32_t*>(*curr));
float x = *reinterpret_cast<float*>(&temp);
temp = be32toh(*reinterpret_cast<uint32_t*>(*(curr+sizeof(float))));
float y = *reinterpret_cast<float*>(&temp);
temp = be32toh(*reinterpret_cast<uint32_t*>(*(curr+2*sizeof(float))));
float z = *reinterpret_cast<float*>(&temp);
temp = be32toh(*reinterpret_cast<uint32_t*>(*(curr+3*sizeof(float))));
float t = *reinterpret_cast<float*>(&temp);
temp = be32toh(*reinterpret_cast<uint32_t*>(*(curr+4*sizeof(float))));
float d = *reinterpret_cast<float*>(&temp);
uint16_t i = be16toh(*reinterpret_cast<uint16_t*>(*(curr+5*sizeof(float))));
curr += LINE_SIZE;
}
}
I got two errors
r.cc: In function ‘int main()’:
r.cc:29:1: error: ‘constexpr’ was not declared in this scope
constexpr int LINE_SIZE = sizeof(float)*5 + sizeof(uint16_t); //based upon your "x,y,z,t,d,i" description
^
r.cc:49:13: error: ‘LINE_SIZE’ was not declared in this scope
curr += LINE_SIZE;

Answer 1

If you're reading the file on a linux machine, there are some library functions provided for this purpose in the endian.h header (documentation here). To convert a 16-bit integer to host order (little-endian in your case):
uint16_t hostInteger = be16toh(bigEndianIntegerFromFile);
For floats, you can do something similar but incorporate reinterpretation:
float hostFloat = reinterpret_cast<float>(be32toh(reinterpret_cast<uint32_t>(bigEndianFloatFromFile)));
Or, if you read it as an unsigned int in the first place, you don't need the inner reinterpret_cast:
float hostFloat = reinterpret_cast<float>(be32toh(bigEndianUint32FromFile));
UPDATE: Given your code, you could read the file by inserting this between your fclose and free calls:
char *curr = buffer;
char *end = buffer + fileLen;
constexpr int LINE_SIZE = sizeof(float)*5 + sizeof(uint16_t); //based upon your "x,y,z,t,d,i" description
while(curr < end) {
uint32_t temp = be32toh(*reinterpret_cast<uint32_t*>(*curr));
float x = *reinterpret_cast<float*>(&temp);
temp = be32toh(*reinterpret_cast<uint32_t*>(*(curr+sizeof(float))));
float y = *reinterpret_cast<float*>(&temp);
temp = be32toh(*reinterpret_cast<uint32_t*>(*(curr+2*sizeof(float))));
float z = *reinterpret_cast<float*>(&temp);
temp = be32toh(*reinterpret_cast<uint32_t*>(*(curr+3*sizeof(float))));
float t = *reinterpret_cast<float*>(&temp);
temp = be32toh(*reinterpret_cast<uint32_t*>(*(curr+4*sizeof(float))));
float d = *reinterpret_cast<float*>(&temp);
uint16_t i = be16toh(*reinterpret_cast<uint16_t*>(*(curr+5*sizeof(float))));
curr += LINE_SIZE;
...
//do something with these values
...
}