How to play with strings in C


Examples in this lesson modify a string/character array.

Copying one string to another - strcpy

strcpy can be used to copy one string to another. Remember that C strings are character arrays. You must pass character array, or pointer to character array to this function where string will be copied.

The following example will overwrite the contents of dest with the content of src:

#define DEST_SIZE 40

char src[] = "Look Here";
char dest[DEST_SIZE] = "Unimaginable";

strcpy(dest, src);
printf(dest); /* Output: Look Here */

The destination character array is the first parameter to strcpy. The source character array is the second parameter to strcpy. The terminating NULL character is automatically appended at the end of the copy.

Warning: The destination character array must be large enough to hold all characters in source character array, plus a NULL character. If the source array has 100 characters, the destination array must be at least 101 character long. Following code snippet will result in undefined behaviour:

char src[] = "Look Here";
char dest[4] = "A";

/* strcpy(dest, src); */ /* Fatal: dest doesn't have enough space to hold all characters of src plus a NULL character */

The destination character array doesn't have to be initialized. It can be left uninitialized and can be passed to strcpy. Still, it must have enough space to hold the source array and a NULL character.

#define DEST_SIZE 40

char src[] = "Look Here";
char dest[DEST_SIZE];

strcpy(dest, src);
printf(dest); /* Output: Look Here */

Also possible:

#define DEST_SIZE 40

char dest[DEST_SIZE];

strcpy(dest, "Look Here");
printf(dest); /* Output: Look Here */

Character pointers can also be passed as parameters of strcpy.

What happens in the example above is that p points to the 6th character of dest. When p is passed as the first parameter of strcpy, it becomes the first character to be overwritten by the function. So src gets copied starting at the 6th character of dest leaving first 5 characters untouched.

Of course, the second parameter could also be pointer to character:

In the example above ps points to the space character of src. pd points to the NULL character of dest. So the call to strcpy overwrites the NULL character of dest with space character of src, followed by other characters of src, finally a NULL character is appended.

Warning: Care must be taken when passing character pointers to strcpy. The source and destination aren't allowed to overlap. For example, the following is forbidden:

#define DEST_SIZE 40

char dest[DEST_SIZE] = "Unimaginable";

char *sp = dest + 5;
char *dp = dest + 8;

/* strcpy(dp, sp); */ /* Fatal: source and destination aren't allowed to overlap */

In the example above, sp points to the 6th character of dest and dp points to the 9th character of dest. Both of them share the same array (dest). This is not allowed and executing code like this may produce unexpected results.

Copying first n characters from one string to another - strncpy

strncpy is used to copy first several characters from source string to destination string. It doesn't append any NULL character when the copying finishes.

In the following example, the first 5 characters are copied. No NULL character is appended.

#define DEST_SIZE 40

char src[]    = "Look Here";
char dest[DEST_SIZE] = "Unimaginable";

strncpy(dest, src, 5);
printf(dest); /* Output: Look ginable */

Warning: Care must be taken when using strncpy to make sure that strings are NULL terminated. Following is an example of an ill-formed code which doesn't take NULL character into account:

#define DEST_SIZE 9

char src[]  = "Look Here"; /* src has 9 + 1 = 10 characters */
char dest[DEST_SIZE]; /* dest can only hold 9 characters */

strncpy(dest, src, 9); /* First 9 characters are copied to dest, where is the NULL character? */
/* printf(dest); */ /* Fatal: undefined behaviour - dest doesn't have a NULL character */

But it's fine if each characters are accessed individually:

If the number of characters to copy is more than the characters in the source string, strncpy will stop copying when it will encounter the NULL character in source. In the example below strncpy stops copying as soon as it reaches the NULL character of src:

#define DEST_SIZE 40

char src[] = "Look Here";
char dest[DEST_SIZE] = "Unimaginable";

char *ps = src + 3;

strncpy(dest, ps, 10); /* The third parameter is 10, but there are 6 characters before the NULL character from ps */
printf(dest); /* Output: k Here */
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