Strings

INFO

In C, strings are implemented as arrays of char values Not all character array is used as a C string, but every C string is a character array

For more information on C Arrays

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RECALL

Arrays in C might be defined with a larger size than a program ultimately uses → we cannot assume that a string’s length is equal to that of the array that stores it. Therefore strings in C must end with a special character, null character (‘\0’) to indicate the end of the string

Strings that ends with a null character are said to be null-terminated

• Common source of errors
• To store the string “hi” → char my_string[3] = {'h', 'i', '\0'};

IMPORTANT

Character arrays must be declared with enough capacity to store each character (including the null character)

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C String Libraries

• Contains functions for manipulating strings
• Needs to include the string.h header
• Library functions often locate the end of the string by searching ‘\0’ or add a ‘\0’ character to the end of the string modified

Printing Strings

When using printf to print a string, use the %s placeholder in the format string → prints all the characters in the array argument until encounters the null character

Example

# include <stdio.h>
# include <string.h> // Include the C string library
 
int main(void){
	char str1[10];
	char str2[10];
	int len;
	
	str1[0] = 'h';
	str1[1] = 'i';
	str1[2] = '\0';
	
	len = strlen(str1);
	
	printf("%s %d\n", str1, len); // Prints hi 2
	
	strcpy(str2, "hello"); // Copy the string "hello" to str2
	len = strlen(str2);
	printf("%s has %d chars\n", str2, len); // prints: hello has 5 characters
}

• strlen: returns the number of characters in its string argument (excluding ‘\0’)
• strcpy: copies one character at a time from a source string (second parameter) to a destination string (first parameter) until it reaches a null character in the source

WARNING

Most C string library functions expect the call to pass in a character array that has enough capacity for the function to perform its job → Don’t want to call strcpy with a destination string that isn’t large enough to contain the source → leads to undefined behavior in program

IMPORTANT

C string library functions require string values passed to them are correctly formed with a terminating ‘\0’ character. Programmer’s responsibility, not compiler’s

The strcpy call in the example is safe, but in general, strcpy poses a security risk because it assumes that its destination is large enough to store the entire string (may not always be the case)

String.h

• int strlen(char* str): return the length of the string
• void strcpy(char* dest, char* src): copies characters from src to dest
• void strcat(char* dest, char* src): append src to the end of dest

char[] str vs char* str In function arguments these do the same thing

C’s Support for Statically Allocated Strings