Quick Navigator

Search Site

Unix VPS
A - Starter
B - Basic
C - Preferred
D - Commercial
MPS - Dedicated
Previous VPSs
* Sign Up! *

Contact Us
Online Help
Domain Status
Man Pages

Virtual Servers

Topology Map

Server Agreement
Year 2038

USA Flag



Man Pages

Manual Reference Pages  -  STRN (3)


Strncpy(), Strncat() - Safe string copying functions



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);


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);


strcpy(3), strncpy(3), strcat(3), strncat(3), sprintf(3).
Search for    or go to Top of page |  Section 3 |  Main Index

West Interactive Corporation STRN (3)

Powered by GSP Visit the GSP FreeBSD Man Page Interface.
Output converted with manServer 1.07.