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more elegant way to load characters?

Posted on 2009-04-14
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Last Modified: 2012-05-06
Hey, I'm a little rusty in C.  Is there a more elegant way to load a series of chars of the caller's choice into an outgoing array?  I don't want to use up RAM to allocate a separate char array, nor use string manipulation functions; but I'd prefer to write

outmsg_cmd("ABC");
instead of
outmsg_cmd('A', 'B', 'C');

to get data loaded into the characters 2-4 of my array.  Simply because the former is more readable.

Is there a way to achieve this?  

Thanks.

char out_buf[50];
 
void outmsg_cmd(char a, char b, char c)
{
  out_buf[2] = a;
  out_buf[3] = b;
  out_buf[4] = c;
}
 
void foo()
{
  outmsg_cmd('A', 'B', 'C');
}

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Question by:riceman0
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8 Comments
 
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Accepted Solution

by:
Zoppo earned 500 total points
ID: 24138331
Hi riceman0,

IMO you could do it like this:

> void outmsg_cmd( char* pBuf )
> {
>  strncpy( out_buf + 1, pBuf, 3 );
> }

Hope that helps,

ZOPPO
0
 

Author Comment

by:riceman0
ID: 24138394

For nuanced reasons (this is an interrupt service routine) I don't want to use string manipulation functions.  

But I guess this would work, wouldn't it:

> void outmsg_cmd( char* pBuf )
> {
  out_buf[2] = *pBuf;
  out_buf[3] = *(pBuf+1);
  out_buf[4] = *(pBuf+2);

> }
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LVL 40

Expert Comment

by:evilrix
ID: 24138446
Without using string functions, memcpy is your best bet.
http://www.cplusplus.com/reference/clibrary/cstring/memcpy/

Frankly, I'd use strncpy as Zoppo suggests.
#include <cstdio>
 
char out_buf[50];
 
void outmsg_cmd(char const * p)
{
	memcpy(&out_buf[2], p, sizeof(char) * 3);
}
 
void foo()
{
  outmsg_cmd("ABC");
}
 
int main()
{
	foo();
}

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Expert Comment

by:evilrix
ID: 24138486
out_buf[2] = *pBuf;
out_buf[3] = *(pBuf+1);
out_buf[4] = *(pBuf+2);

is identical to

out_buf[2] = pBuf[0];
out_buf[3] = pBuf[1];
out_buf[4] = pBuf[2];

as far as the compiler is concerned since the index operator is just syntactic sugar. To prove this to yourself try the little bit of code below, either look as the value of 'c' in the debugger or output it.

int main()
{
	char const a[] = "ABC";
	size_t x = 1;
 
	char c = x[a]; // c == B
}

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Author Comment

by:riceman0
ID: 24138567
Oops, my counterproposal doesn't work:

> void outmsg_cmd( char* pBuf )
> {
  out_buf[2] = *pBuf;
  out_buf[3] = *(pBuf+1);
  out_buf[4] = *(pBuf+2);

> }

If I call

outmsg_cmd("ABC");

I get the error
 function argument #1 of type 'flash unsigned char [4]' is incompatible with required parameter of type 'unsigned char *'
0
 

Author Comment

by:riceman0
ID: 24138600
evilrix: I agree strncpy is preferable to memcpy.  Also, yeah I'm not so rusty that I've forgotten those are identical.
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Author Comment

by:riceman0
ID: 24138939

Ah, I think the "flash" error is because I'm programming a chip with harvard architecture (i.e., volatile and nonvolatile memory are not compatible), am trying this on a desktop app.
0
 

Author Closing Comment

by:riceman0
ID: 31569906
I think the problem is specific to my chip, and using a pointer parameter as first suggested by Zoppo would otherwise be the right way to go.
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