String/Array Comparison

You can use the functions in this section to perform comparisons on the contents of strings and arrays. As well as checking for equality, these functions can also be used as the ordering functions for sorting operations. See section Searching and Sorting, for an example of this.

Unlike most comparison operations in C, the string comparison functions return a nonzero value if the strings are not equivalent rather than if they are. The sign of the value indicates the relative ordering of the first characters in the strings that are not equivalent: a negative value indicates that the first string is "less" than the second, while a positive value indicates that the first string is "greater".

The most common use of these functions is to check only for equality. This is canonically done with an expression like `! strcmp (s1, s2)'.

All of these functions are declared in the header file `string.h'.

Function: int memcmp (const void *a1, const void *a2, size_t size)

The function memcmp compares the size bytes of memory beginning at a1 against the size bytes of memory beginning at a2. The value returned has the same sign as the difference between the first differing pair of bytes (interpreted as unsigned char objects, then promoted to int).

If the contents of the two blocks are equal, memcmp returns 0.

On arbitrary arrays, the memcmp function is mostly useful for testing equality. It usually isn't meaningful to do byte-wise ordering comparisons on arrays of things other than bytes. For example, a byte-wise comparison on the bytes that make up floating-point numbers isn't likely to tell you anything about the relationship between the values of the floating-point numbers.

You should also be careful about using memcmp to compare objects that can contain "holes", such as the padding inserted into structure objects to enforce alignment requirements, extra space at the end of unions, and extra characters at the ends of strings whose length is less than their allocated size. The contents of these "holes" are indeterminate and may cause strange behavior when performing byte-wise comparisons. For more predictable results, perform an explicit component-wise comparison.

For example, given a structure type definition like:

struct foo
  {
    unsigned char tag;
    union
      {
        double f;
        long i;
        char *p;
      } value;
  };

you are better off writing a specialized comparison function to compare struct foo objects instead of comparing them with memcmp.

Function: int strcmp (const char *s1, const char *s2)

The strcmp function compares the string s1 against s2, returning a value that has the same sign as the difference between the first differing pair of characters (interpreted as unsigned char objects, then promoted to int).

If the two strings are equal, strcmp returns 0.

A consequence of the ordering used by strcmp is that if s1 is an initial substring of s2, then s1 is considered to be "less than" s2.

Function: int strcasecmp (const char *s1, const char *s2)

This function is like strcmp, except that differences in case are ignored.

strcasecmp is derived from BSD.

Function: int strncasecmp (const char *s1, const char *s2, size_t n)

This function is like strncmp, except that differences in case are ignored.

strncasecmp is a GNU extension.

Function: int strncmp (const char *s1, const char *s2, size_t size)

This function is the similar to strcmp, except that no more than size characters are compared. In other words, if the two strings are the same in their first size characters, the return value is zero.

Here are some examples showing the use of strcmp and strncmp. These examples assume the use of the ASCII character set. (If some other character set--say, EBCDIC--is used instead, then the glyphs are associated with different numeric codes, and the return values and ordering may differ.)

strcmp ("hello", "hello")
    => 0    /* These two strings are the same. */
strcmp ("hello", "Hello")
    => 32   /* Comparisons are case-sensitive. */
strcmp ("hello", "world")
    => -15  /* The character 'h' comes before 'w'. */
strcmp ("hello", "hello, world")
    => -44  /* Comparing a null character against a comma. */
strncmp ("hello", "hello, world", 5)
    => 0    /* The initial 5 characters are the same. */
strncmp ("hello, world", "hello, stupid world!!!", 5)
    => 0    /* The initial 5 characters are the same. */

Function: int bcmp (const void *a1, const void *a2, size_t size)

This is an obsolete alias for memcmp, derived from BSD.

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