confusion with char *a and char a[NUM]

[Bob Martin]

In article <7656 at umd5.umd.edu> jjk at astro.umd.edu( Jim Klavetter) writes:
>
>I think part of the problem is with an inconsistency in c
>and part is my understanding.
>
>I can have two identical files except that one I declare a to be
>	char a[NUM]
>and the other has
>	char *a with a malloc of NUM+1 characters.
>
>I guess I can stop there and ask the general question, "what is the
>difference between those two?"  If done properly, they will both be
>NUM+1 bytes (or whatever a char is) of memory and should be accessible
>either by a[3] or *(a+3) for the forth element, for example.  Yet,
>there are differences.

	a[NUM] is an array of NUM chars. (NOT NUM+1 as the text implies).
	char *a is a pointer to a character, period.  In the first case
	the compiler has knowledge about the number of bytes in 'a' and
	reserves those bytes for you.  In the second case the compiler 
	knows nothing about the size of the object you are pointing to
	and depends on you to explicitly allocate the bytes.
>
>In both cases, the following is accepted by both my sun4 compiler and
>gcc
>	strcpy(string, a)
>and there is no problem.

	Right.  This is because of C's convention of treating the name
	of an array like a pointer to the first element of the array
	in an rvalue context.  
>
>However, if I have
>	a=strchr(string, ":");
>I get the error message
>	121: incompatible types in assignment
>or some such thing (that one is from gcc).

	Right again.  This is because 'a' is an lvalue in this statement.
	If 'a' was declared as an array, then in an lvalue context the name
	a represents the array, not a pointer at all.

	If you declared a as "char a[NUM]" you have set aside NUM bytes
	for your use.  Ask yourself what you mean when you say:
		"a=strchr(string, ':');"
	'a' is an array of NUM bytes, do you want to throw away those 
	bytes and now ask 'a' to be a pointer to the innards of 'string'?
>
>The man page treats both the arguement of strcpy() and the return value
>of strchr() as type (char *).  So why this inconsistency?  Am I using
>strcpy() wrong above and just getting away with a flaw in the
>compiler, or is there actually an inconsistency here.

	There is no inconsistency.  The return value of strchr is char*,
	but 'a' (if declared as char a[NUM]) in the lvalue context is
	not a char*, it is a char[NUM].
>
>Thanks in advance.
>
>jjk at astro.umd.edu
>Jim Klavetter (accepting mail for Athabasca and Reudi)
>Astronomy
>UMD
>College Park, MD  20742


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