NAME

Strncpy(), 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 strcat 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-terminate. 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 unacceptable.

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 strcpy 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);

NAME

SYNOPSIS

DESCRIPTION

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