Not populating a string

[SasDutta]

Hi

I have declared a string. If i dont populate the string, can i check this string for NULL? If i dont populate the string, then with what value it contains? Does it contains gurbage value?

If i initialize the string with memset function the can i check the string with NULL?

Thanks
Sas

[CommonTater]

It depends how you declare the string...

char *string;     <--- will point to garbage

char *string = NULL;   <--- this will test true for null

char *string = malloc(100); <--- this will conatain garbage

char *string = calloc(1,100); <--- strlen() will return 0

char string[100];  <--- declared as global, strlen() will return 0

char string[100]; <--- declared in a function will contain garbage

char string [100] = {0};  <--- strlen will return 0

[Inanna]

How are you declaring the string? NULL is for pointers, but a string is just a convention that some sequence of characters ends with '\0'. If your string is actually an array, NULL will not be useful. If your string is a pointer, it is best to initialize it to NULL yourself. That is easier than remembering the rules of where variables are initialized to zero and where they are not.

[SasDutta]

I am defining the string in a structure. like below

struct RunParam
{

char redEyeExcp[RED_EYE];
int upperTimeLimit;
}

This structure is declared in header file. In the code, i am doing following things

struct RunParam *modelRunParams=( struct RunParams*)calloc(1, sizeof(struct RunParams));

.

Now what i am seeing is each element redEyeExcp char array is populated by '\0' and intgers and floats are automatically initialized with 0, 0.000.
Are they automatically initialized when calloc works

[Inanna]

calloc() just combines malloc() and memset() with a value of 0. It is not safe to use memset() on anything but int or char types. You have to initialize the members separately.

struct RunParam* modelRunParams = (struct RunParams*)malloc(sizeof(struct RunParams));

modelRunParams->redEyeExcp[0] = '\0'; /* Empty string, strlen() will return 0 */
modelRunParams->upperTimeLimit = 0;

[SasDutta]

Thank you

[SasDutta]

Why it is not safe to use memset apart from int and char?

[tabstop]

It is not guaranteed that all-bits-zero will give you the value 0 for floating-point types (it does for IEEE floats which are very common). It is also not guaranteed that NULL is all-bits-zero, or even that all-bits-zero is a valid pointer representation.

[nonoob]

Bah! You'd do fine on a C standards committee. Real world programming has been OK with zeros in each data type for every architecture/machine since the late 70s.
Please cite real-world examples of where floating point all-zero is not legit, or where a NULL pointer is not zero.

[Bayint Naung]

int main(void)
{
	
	double f;
	memset(&f,0,sizeof(f));
	
	printf("%f\n",f);
	
	char *p;
	memset(&p,0,sizeof(*p));
	
	if(p == NULL) {
		printf("ok null\n");
	} else {
		printf("nonoob is real noob\n");
	}
	
return 0;
}

0.000000
nonoob is real noob

http://c-faq.com/null/machnon0.html
http://c-faq.com/~scs/cgi-bin/faqcat...=malloc#calloc

[tabstop]

Bah! You'd do fine on a C standards committee. Real world programming has been OK with zeros in each data type for every architecture/machine since the late 70s.
Please cite real-world examples of where floating point all-zero is not legit, or where a NULL pointer is not zero.

I've not used them personally, but I thought the old-but-not-as-old-as-the-70s segment:pointer system didn't have an all-zero NULL. That might have been just a scary story told by my programming instructors though. (And I don't know what system Bayint was using.)

[adeyblue]

(And I don't know what system Bayint was using.)

Obviously one where sizeof(char**) != sizeof(char) and which didn't 0-fill stack space