Safe, efficient way to access unaligned data in a network packet from C?

To avoid alignment issues in this case, access all data as an unsigned char So: unsigned char *p; //... uint16_t id = p0 | (p1.

To avoid alignment issues in this case, access all data as an unsigned char *. So: unsigned char *p; //... uint16_t id = p0 | (p1.

You should have functions (inline and/or templated if the language you're using supports those features) that will read the potentially unaligned data and return the data type you're interested in. Something like: unit16_t unaligned_uint16( void* p) { // this assumes big-endian values in data stream // (which is common, but not universal in network // communications) - this may or may not be // appropriate in your case unsigned char* pByte = (unsigned char*) p; uint16_t val = (pByte0.

Typo in the return type, should be uint16_t. Otherwise, great function, I just used it to solve an alignment problem on an UltraSparc64 and it worked fine. +1 – bortzmeyer Mar 5 '09 at 8:04.

The easy way is to manually rebuild the uint16_ts, at the expense of speed: uint8_t *packet = ...; uint16_t fieldID = (packet0 archive. Org if you really don't want to click through. Uint16_t fieldID = htons(*(uint16_t *)packet); uint16_t length = htons(*(uint16_t *)(packet + 2)); Note that unaligned access aren't always supported (e.g. They might generate a fault of some sort), and on other architectures, they're supported, but there's a performance penalty.

If the packet isn't aligned, you could always copy it into a static buffer and then read it: static char static_buffer65540; memcpy(static_buffer, packet, packet_size); // make sure packet_size.

Alignment is always going to be fine, although perhaps not super-efficient, if you go through a byte pointer. Setting aside issues of endian-ness, you can memcpy from the 'real' byte pointer into whatever you want/need that is properly aligned and you will be fine. (this works because the generated code will load/store the data as bytes, which is alignment safe.It's when the generated assembly has instructions loading and storing 16/32/64 bits of memory in a mis-aligned manner that it all falls apart).

Setting aside endian-ness is ill-advised in a network. – Jonathan Leffler Feb 13 '09 at 7:00.

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