How to append string using snprint() function

#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);
 p开发者_如何学Gorintf("dst = %s\n",dst);
}

please suggest is it possible using snprintf()

Thanks Surya

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*"; //<--!!!
[...]

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;
}

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.

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);