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Strings in C

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This discussion of string handling in C presumes that the following compiler directive is used.

#include <string.h>

On Linux, this file is located in /usr/include. Only the basic functions will be discussed here, so you may want to investigate string.h.

In C, a string is stored as a null-terminated char array. This means that after the last truly usable char there is a null, hex 00, which is represented in C by '\0'. The subscripts used for the array start with zero (0). The following line declares a char array called str. C provides fifteen consecutive bytes of memory. N.B. Only the first fourteen bytes are usable for character storage, because one must be used for the string-terminating null.

char str[15];

The following is a representation of what would be in RAM, if the string "Hello, world!" is stored in this array.

Characters:  H  e  l  l  o  ,     w  o  r  l  d  !
Hex values: 48 65 6C 6C 6F 2C 20 77 6F 71 6C 64 21 00
Subscripts:  0  1  2  3  4  5  6  7  8  9 10 11 12 13 14

The name of the array is treated as a pointer to the array. The subscript serves as the offset into the array, i.e., the number of bytes from the starting memory location of the array. Thus, both of the following will save the address of the 0th character in the pointer variable ptr.

ptr = str;
ptr = &str[0];

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strlen()

Syntax: len = strlen(ptr);
    where len is an integer and
    ptr is a pointer to char

strlen() returns the length of a string, excluding the null. The following code will result in len having the value 13.

int len;
char str[15];

strcpy(str, "Hello, world!");
len = strlen(str);

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strcpy()

Syntax: strcpy(ptr1, ptr2);
    where ptr1 and ptr2 are pointers to char

strcpy() is used to copy a null-terminated string into a variable. Given the following declarations, several things are possible.

char S[25];
char D[25];

N.B. If you fail to ensure that the source string is null-terminated, very strange and sometimes very ugly things may result.

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strncpy()

Syntax: strncpy(ptr1, ptr2, n);
    where n is an integer and
    ptr1 and ptr2 are pointers to char

strncpy() is used to copy a portion of a possibly null-terminated string into a variable. Care must be taken because the '\0' is put at the end of destination string only if it is within the part of the string being copied. Given the following declarations, several things are possible.

char S[25];
char D[25];

Assume that the following statement has been executed before each of the remaining code fragments.

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strcat()

Syntax: strcat(ptr1, ptr2);
    where ptr1 and ptr2 are pointers to char

strcat() is used to concatenate a null-terminated string to end of another string variable. This is equivalent to pasting one string onto the end of another, overwriting the null terminator. There is only one common use for strcat().

char S[25] = "world!";
char D[25] = "Hello, ";

N.B. If you fail to ensure that the source string is null-terminated, very strange and sometimes very ugly things may result.

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strncat()

Syntax: strncat(ptr1, ptr2, n);
    where n is an integer and
    ptr1 and ptr2 are pointers to char

strncat() is used to concatenate a portion of a possibly null-terminated string onto the end of another string variable. Care must be taken because some earlier implementations of C do not append the '\0' at the end of destination string. Given the following declarations, several things are possible, but only one is commonly used.

char S[25] = "world!";
char D[25] = "Hello, ";

N.B. If you fail to ensure that the source string is null-terminated, very strange and sometimes very ugly things may result.

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strcmp()

Syntax: diff = strcmp(ptr1, ptr2);
    where diff is an integer and
    ptr1 and ptr2 are pointers to char

strcmp() is used to compare two strings. The strings are compared character by character starting at the characters pointed at by the two pointers. If the strings are identical, the integer value zero (0) is returned. As soon as a difference is found, the comparison is halted and if the ASCII value at the point of difference in the first string is less than that in the second (e.g. 'a' 0x61 vs. 'e' 0x65) a negative value is returned; otherwise, a positive value is returned. Examine the following examples.

char s1[25] = "pat";
char s2[25] = "pet";

diff will have a negative value after the following statement is executed.

diff = strcmp(s1, s2);

diff will have a positive value after the following statement is executed.

diff = strcmp(s2, s1);

diff will have a value of zero (0) after the execution of the following statement, which compares s1 with itself.

diff = strcmp(s1, s1);

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strncmp()

Syntax: diff = strncmp(ptr1, ptr2, n);
    where diff and n are integers
    ptr1 and ptr2 are pointers to char

strncmp() is used to compare the first n characters of two strings. The strings are compared character by character starting at the characters pointed at by the two pointers. If the first n strings are identical, the integer value zero (0) is returned. As soon as a difference is found, the comparison is halted and if the ASCII value at the point of difference in the first string is less than that in the second (e.g. 'a' 0x61 vs. 'e' 0x65) a negative value is returned; otherwise, a positive value is returned. Examine the following examples.

char s1[25] = "pat";
char s2[25] = "pet";

diff will have a negative value after the following statement is executed.

diff = strncmp(s1, s2, 2);

diff will have a positive value after the following statement is executed.

diff = strncmp(s2, s1, 3);

diff will have a value of zero (0) after the following statement.

diff = strncmp(s1, s2, 1);

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Miscellaneous Notes

Single characters can be replaced in a string. Given the following declarations, several things are possible.

char str[25] = "cot";
char ch = 'u';
char D[25] = "pat";

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© 1998-2000 DFStermole
Created 13 Oct 1998
Revised 18 Jan 2000