strncpy's problem

[Xiaoming Zhang]

I ran the following c code:

#include
#include
main(){
char str1[50]="aaaaaaaaaa", str2[50]="bbbbbbbbbbb", str3[50]="ccccccccccccc",str4[50];

strncpy(str4,str2,8); printf("str4 is %s \n", str4);
strncpy(str4,str3,4); printf("str4 is %s \n", str4);
strncpy(str4,str1,8); printf("str4 is %s \n", str4);
strncpy(str4,str3,4); printf("str4 is %s \n", str4);
}

and got following output:
str4 is bbbbbbbb
str4 is ccccbbbb
str4 is aaaaaaaa
str4 is ccccaaaa

However, I acutally want to get output like this:
tstr4 is bbbbbbbb
str4 is cccc
str4 is aaaaaaaa
str4 is cccc

What should I do to get the output I want?

Thanks!

[Patrick Wallek]

I haven't got a whole lot of experience with C, but I would say you should probably set your str4 to a null in between copies. I think that will then give you the results you are looking for.

[Dan Hetzel]

Hi,

This is the right behavior for 'strncpy(s1,s2,n)' as it will copy exactly n bytes. The result will not be automatically null terminated if if the length of s2 is n or more.

After strncpy(str4,str3,4), you should set str[4] to null to properly terminate your string so that the printf statement will find the end of string where you want it to be.

Best regards,

Dan

[Dan Hetzel]

Hi again,

Instead of strncpy(), you could use sprintf() to copy your data across strings.
This will allow formatting of the data.

Example:
#include
#include

main()
{
char a[20]="aaaaaaaaaaaaaaa";
char b[20]="bbbbbbbbbbbbbbb";

sprintf(b,"%4.4s",a);
printf("b is %s\n",b);
}

This prints 'b is aaaa' as it copies exactly 4 bytes.

See 'man sprintf' for information about all formatting capabilities.

Best regards,

Dan

[Andreas Voss]

Hi,

you could make a procedure that adds the NULL termination ie:

void mystrncpy(dest, src, len)
char *dest;
char *src;
int len;
{
strncpy(dest, src, len);
dest[len] = 0;
}

Regards