Sams Teach Yourself C in 21 Days

Enter the number of characters to copy (1-26)

15

Before strncpy destination = ..........................

After strncpy destination = abcdefghijklmno...........

In addition to demonstrating the strncpy()function, this program also illustrates

an effective way to ensure that only correct information is entered by the user.

Lines 13–20 contain a whileloop that prompts the user for a number from 1–26. The

loop continues until a valid value is entered, so the program can’t continue until the user

enters a valid value. When a number between 1 and 26 is entered, line 22 prints the origi-

nal value of dest, line 24 copies the number of characters specified by the user from

sourcetodest, and line 26 prints the final value of dest.

486 Day 17

INPUT/

OUTPUT

ANALYSIS

Be sure that the number of characters copied doesn’t exceed the allocated

size of the destination. Also, be aware that strncpy()does not add a null

terminator.

Caution

Thestrdup()Function ................................................................................

There is another string copying function worth mentioning. The strdup()function is

similar to strcpy(), except that strdup()performs its own memory allocation for the

destination string with a call to malloc(). In effect, it does what you did in Listing 17.2,

allocating space with malloc()and then calling strcpy().

You should note that strdup()is not an ANSI-standard function. It is included with

many compilers including the Microsoft, Borland, and Symantec C libraries, but it might

not be present (or it might be different) in other C compilers.

The prototype for strdup()is

char *strdup( char *source );

The argument source is a pointer to the source string. The function returns a pointer to

the destination string—the space allocated by malloc()—orNULLif the needed memory

can’t be allocated. Listing 17.4 demonstrates the use of strdup().

LISTING17.4 strdup.c. Using strdup()to copy a string with automatic memory

allocation

1: /* The non-ANSI strdup() function. */

2: #include <stdlib.h>

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