How to append string using snprint() function

Question

#include<stdio.h>

main()
{
 char str[50] = "Wel %s";
 char dst[50];

 snprintf(dst,50,str,"Come");
 //Now i want to append "*" to dst string ie "Wel Come*" using snprintf() 
 printf("str = %s\n",str);
 printf("dst = %s\n",dst);
}

please suggest is it possible using snprintf()

Thanks Surya

Answer 1

you can use the %s format for this:

snprintf(dst, 50, "%s*", dst);

EDIT: This seems to have some undefined behaviors. The best thing would be to ask if it is really necessary to use snprintf instead of strncat.

Answer 2

The obvious solution:

snprintf(dst,50,"%s*",dst);

is inefficient, because it makes an unnecessary copy of dst (into itself).

invokes undefined behavior as R. pointed out, because the arguments may not overlap (from man snprintf(3) on MacOSX):

"[...]or those routines that write to a user-provided character string, that string and the format strings should not overlap, as the behavior is undefined."

Posix says:

• http://www.opengroup.org/onlinepubs/000095399/functions/printf.html

"If copying takes place between objects that overlap as a result of a call to sprintf() or snprintf(), the results are undefined."

snprintf returns the number of characters it has written, so you can do this instead:

 int k=snprintf(dst,50,str,"Come");
 // make sure that we do not pass potential disastrous values to snprintf, because 
 // the size argument is unsigned (size_t, 50-52 is a large positive number!) 
 // and we want 50-k to be in the range 0-50
 // k<0 means output error and k>50 means "output truncated". There is no point in 
 // appending anything in these cases anyway. 
 if (k<0 || k>50) 
 {
  fprintf(stderr,"output error or buffer too small");
 }    
 else k=snprintf(dst+k,50-k,"*");
 // check k for truncation here.

And then there's always strcat...And just in case, you overlooked it. You can have the * attached right in the first place:

main()
{
 char str[50] = "Wel %s*"; //<--!!!
[...]

Answer 3

This should work:

#include<stdio.h>

int main()
{
 char str[50] = "Wel %s";
 char dst[50];
 int len;

 snprintf(dst,50,str,"Come");

 //get size of current string
 len = strlen(dst);

 //add character to the end
 snprintf(dst + len, sizeof(dst) - len, "*");

 printf("str = %s\n",str);
 printf("dst = %s\n",dst);

 return 0;
}

Answer 4

All the information is already available to you:

snprintf(dst + 8, sizeof(dst) - 8, "%s", "*");

You'd be better off doing:

strncat(dst, "*", sizeof(dst) - strlen(dst) - 1);