typecast array to struct in c

Question

I have a structure like this

struct packet
{
int seqnum;
char type[1];
float time1;
float pri;
float time2;
unsigned char data[512];
}

I am receiving packet in an array

char buf[529];

I want to take the seqnum,data everything separately.Does the following typecast work.. It is giving junk value for me.

struct packet *pkt;
pkt=(struct packet *)buf;
printf(" %d",pkt->seqnum)

try to read the good number of bytes : char buf[sizeof (struct packet)]; and with : read (fd, buf, sizeof(struct packet)); — Marcassin
– Marcassin, Commented Apr 15, 2013 at 14:07
@Marcassin: That ensures the number of bytes read equals the size of the structure, but it does not change the incoming bytes and therefore does nothing to ensure they have padding or endianness that match the structure. If the incoming bytes do not have three bytes between the char type[1] and the float time1 but the structure does, then that read is completely broken. — Eric Postpischil
– Eric Postpischil, Commented Apr 15, 2013 at 14:09
For the sake of beauty, don't ever type the Please help me with the code. phrase again — Rerito
– Rerito, Commented Apr 15, 2013 at 14:11
Do you want a solution that works in standard C or do you want a solution for a specific C implementation? Which implementation (including what target platform)? — Eric Postpischil
– Eric Postpischil, Commented Apr 15, 2013 at 14:12
@ Eric Postpischil I am running this using gcc compiler in ubuntu — user1762571
– user1762571, Commented Apr 16, 2013 at 3:44

unwind · Accepted Answer · 2013-04-15 14:05:26Z

8

No, that likely won't work and is generally a bad and broken way of doing this.

You must use compiler-specific extensions to make sure there's no invisible padding between your struct members, for something like that to work. With gcc, for instance, you do this using the __attribute__() syntax.

It is, thus, not a portable idea.

It's much better to be explicit about it, and unpack each field. This also gives you a chance to have a well-defined endianness in your network protocol, which is generally a good idea for interoperability's sake.

answered Apr 15, 2013 at 14:05

unwind

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user1762571 Over a year ago

Hi thanks. can you please tell how to unpack each field in the above example.

Kerrek SB · Accepted Answer · 2013-04-15 14:06:28Z

5

No, that isn't generally valid code. You should make the struct first and then memcopy stuff into it:

packet p;

memcpy(&p.seqnum, buf + 0, 4);

memcpy(&p.type[0], buf + 4, 1);

memcpy(&p.time1, buf + 5, 4);

And so forth.

You must take great care to get the type sizes and endianness right.

answered Apr 15, 2013 at 14:06

Kerrek SB

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Comments

Rerito · Accepted Answer · 2013-04-15 14:18:29Z

2

First of all, you cannot know in advance where the compiler will insert padding bytes in your structure for performance optimization (cache line alignment, integer alignment etc) since this is platform-dependent. Except, of course, if you are considering building the app only on your platform.

Anyway, in your case it seems like you are getting data from somewhere (network ?) and it is highly probable that the data has been compacted (no padding bytes between fields).

If you really want to typecast your array to a struct pointer, you can still tell the compiler to remove the padding bytes it might add. Note that this depends on the compiler you use and is not a standard C implementation. With gcc, you might add this statement at the end of your structure definition :

struct my_struct {
    int blah;
    /* Blah ... */
} __attribute__((packed));

Note that it will affect the performance for member access, copy etc ...

Unless you have a very good reason to do so, don't ever use the __attribute__((packed)) thing !

The other solution, which is much more advisable is to make the parsing on your own. You just allocate an appropriate structure and fill its fields by seeking the good information from your buffer. A sequence of memcpy instructions is likely to do the trick here (see Kerrek's answer)

edited Apr 15, 2013 at 14:18

answered Apr 15, 2013 at 14:08

Rerito

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3 Comments

Rerito Over a year ago

The compiler-specific __attribute__((packed)) is not standard C, but will still be valid if the same compiler is used on several platforms. By the way, the OP's context seems to imply some network related code and thus, packets are packed hence the suggestion.

Rerito Over a year ago

@EricPostpischil I edited my answer to make it clearer. The __attribute__ thing limits the portability of a code snippet in the sense that some compilers might not integrate the required attributes, but if the same compiler is used all along on several platforms, is there still portability across platforms ? (I ask this to clear my mind :) )

Eric Postpischil Over a year ago

If it is supported, __attribute__((packed)) solves the padding issue but not endianness issues or other issues about how values are encoded/represented (e.g., whether the float type uses IEEE 754 or how big the int type is). There are two ways to solve this problem: Write in standard C, reading each byte individually and converting to native types using arithmetic et cetera, or write for a specific implementation and target with the bytes in the struct completely defined and implementation dependent.

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