Strn(3)Strn(3)NAMEStrncpy(), Strncat() - Safe string copying functions
SYNOPSIS
cc -I/usr/src/include . . . -L/usr/src/lib -lStrn
#include <string.h>
#include <Strn.h>
char *Strncpy(char *dst, char *src, size_t max);
char *Strncat(char *dst, char *src, size_t max);
DESCRIPTION
The Strn library provides bounds-safe versions of string
copying and concatenation functions. These are useful
with fixed length character arrays because the functions
will not overwrite past the end of the array, unlike str_
cat or strncat. The functions also guarantee that a
string is nul-terminated, unlike strncpy or strncat.
For example:
char bar1[8], foo[8], bar2[8];
In the following block, Strncpy correctly sets this to
"hello w", using seven characters + a nul terminator, so
the array is full and nul-terminated.
Strncpy(foo, "hello world", sizeof(foo));
strncpy will write 8 characters, but it won't nul-termi
nate. If you tried to print "foo" you would get "hello
wo" and garbage.
strncpy(foo, "hello world", sizeof(foo));
These cooperate to guarantee that the bounds of "foo" are
not violated. Here, you would get "hello" and then "hello
w".
Strncpy(foo, "hello", sizeof(foo));
Strncat(foo, " world", sizeof(foo));
These two would overflow, with this usage. The strncpy
would work, but then the strncat would happily write past
the end of foo.
strncpy(foo, "hello", sizeof(foo));
strncat(foo, " world", sizeof(foo));
To get the result I want, I would have to do this, which
with all the extra overhead in the third argument is unac
ceptable.
strncpy(foo, "hello", sizeof(foo));
strncat(foo, " world", sizeof(foo) - strlen(foo) - 1);
In actual use, you don't usually call Strncpy or Strncat
directly. The library's header file includes two macros
which fill in the third argument with sizeof(dst) for you.
That way, they can be used as direct replacements of str_
cpy and strcat most of the time:
char fullName[32];
strcpy(fullName, firstName);
strcat(fullName, " ");
strcat(fullName, lastName);
This can be transformed into the following, and you can
rest assured that you won't trash the stack if for some
reason "firstName" + "lastName" would exceed the bounds of
"fullName."
STRNCPY(fullName, firstName);
STRNCAT(fullName, " ");
STRNCAT(fullName, lastName);
SEE ALSOstrcpy(3), strncpy(3), strcat(3), strncat(3), sprintf(3).
West Interactive CorporationStrn(3)
