how to use strncpy

[Barbara Baby]

Hi, I have the following codewhich is supposed to use strncpy. Can anyone help, please:

int i
for( i = 0; i <= StringCapacity; i++ )
StringArray[i] = s[i];
StringArray[i] = '\0';
StringSize = StringCapacity;

These lines of code should use strncpy in order to get a much better solution.

Thank you very much!

[weaknessforcats]

To use strncpy you need to knowthe number of characters to copy.

strlen will tell you that. So StringCapacity - stren(s) will tell you the number of characters to copy. If negative, copy the entire s otherwise strncpy only the difference.

Be aware that strncpy does not provide a null terminator unless the entire string is copied. You will need to provide the terminator yourself for the portion that was copied.

Another solution is to delete the current StringArray and allocate a new StringArray large enough to hold the entire new string. Then you would strcpy and increase your StringCapacity value.

[donbock]

To use strncpy you need to knowthe number of characters to copy.

strlen will tell you that. So StringCapacity - stren(s) will tell you the number of characters to copy. If negative, copy the entire s otherwise strncpy only the difference.

You don't need to know the number of characters to copy, you just need to know the size of the destination buffer. Pass the size of the destination buffer as the len parameter and strncpy will copy the source string into the destination buffer, stopping either when the null terminator is copied or the destination buffer is filled up, whichever happens first.

[donbock]

Your starting code is in error. Suppose the s array is smaller than StringCapacity. Your loop will read past the end of s. Accessing beyond the end of an array provokes undefined behavior.

[Barbara Baby]

Thank you very much for the advise.
Here is the complete code:

MyString::MyStr ing( const char* s )
{
int len = strlen(s);
if( len <= StringCapacity ) //size <= 80
{
strcpy( StringArray, s );
StringSize = len;
}
else
{
int i;
for ( i = 0; i <= StringCapacity; i++ );
StringArray[i] = s[i];
StringArray[i] = '\0';
StringSize = len;
}
}

[donbock]

Your code assumes StringArray is defined equivalently to:

char StringArray[StringCapacity+1];

All of your code can be replaced by three lines if you use strncpy.

[Barbara Baby]

I need to use strncpy for these lines:

else
{
int i;
for ( i = 0; i <= StringCapacity; i++ );
StringArray[i] = s[i];
StringArray[i] = '\0';
StringSize = len;
}

How can I chage this?

[donbock]

No, you need to replace all of the lines (except for using strlen to set StringSize) with a call to strncpy instead of strcpy.

How to replace strcpy with strncpy?
First, consider the differences in their function prototypes:

char *strcpy(char *destination, const char *source);
char *strncpy(char *destination, const char *source, size_t num);

Then consider the differences in their functionality:

• strncpy needs a num argument to specify the maximum number of characters to be copied from source.
• For strncpy, if the source is longer than the destination (that is, strlen(source)+1 > num), then the destination is not null-terminated.