Read a file written in bigendian

Question

My machine works in little endian. I have to read using C++ std::ios::binary some files with big endian encoding. Is there any standard and fast way for doing it? For the moment, after having read raw data I do the following:

(reading a double)

char raw [8];
double d = 0; //maybe initialization is not needed
file.read(raw,8); //4 for an int
for(int k = 0; k < 8; k++) {
    memcpy((void *)(((char *)(&d))+k), (const void *)(raw+j+7-k), 1);
}

(reading an integer)

char raw [4];
file.read(raw,4); //4 for an int
int i = (raw[j] << 24) | (raw[j+1] << 16) | (raw[j+2] << 8) | raw[j+3];

Also, `sizeof(int)` instead of the (very likely incorrect) `4`, and `sizeof(double)` instead of `8`. — William Pursell, Sep 17 '16 at 12:47
@WilliamPursell Since he reads a file, the 4 is fine. It needs to be converted to something appropriate on the target architecture though, I agree. — JustSid, Sep 17 '16 at 13:18

Chris · Accepted Answer · 2016-09-24T03:27:25.773

Use the htonl and ntohl ('host to network long" and "network to host long") functions to convert between host and network byte order. Network byte order is big endian, and on your architecture host byte order is little endian.

You should also always use explicitly-sized unsigned integer types when doing this kind of bit-mangling, not int. You can reinterpret_cast to the desired type of the data later, e.g. start with uint64_t to read in a double.

uint32_t little_endian_thing;
uint32_t big_endian_thing;
file.read(&big_endian_thing, sizeof(uint32_t));
little_endian_thing = ntohl(big_endian_thing);

Read a file written in bigendian

1 Answers1