Not populating a string

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

It depends how you declare the string...

Code:

---

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

---

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. :)

I am defining the string in a structure. like below

Code:

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struct RunParam
{

char redEyeExcp[RED_EYE];
int upperTimeLimit;
}

---

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

Code:

---


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

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.

Code:

---

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

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

---

Thank you

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

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.

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.

Code:

---

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

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

Quote:

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Originally Posted by 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.

---

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.)

Quote:

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Originally Posted by tabstop

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

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