1 String and Array Utilities

Operations on strings (or arrays of characters) are an important part of many programs. The GNU C library provides an extensive set of string utility functions, including functions for copying, concatenating, comparing, and searching strings. Many of these functions can also operate on arbitrary regions of storage; for example, the memcpy function can be used to copy the contents of any kind of array.

It’s fairly common for beginning C programmers to “reinvent the wheel” by duplicating this functionality in their own code, but it pays to become familiar with the library functions and to make use of them, since this offers benefits in maintenance, efficiency, and portability.

For instance, you could easily compare one string to another in two lines of C code, but if you use the built-in strcmp function, you’re less likely to make a mistake. And, since these library functions are typically highly optimized, your program may run faster too.

1.1 Representation of Strings

This section is a quick summary of string concepts for beginning C programmers. It describes how character strings are represented in C and some common pitfalls. If you are already familiar with this material, you can skip this section.

A string is an array of char objects. But string-valued variables are usually declared to be pointers of type char *. Such variables do not include space for the text of a string; that has to be stored somewhere else—in an array variable, a string constant, or dynamically allocated memory (@pxref{Memory Allocation}). It’s up to you to store the address of the chosen memory space into the pointer variable. Alternatively you can store a null pointer in the pointer variable. The null pointer does not point anywhere, so attempting to reference the string it points to gets an error.

By convention, a null character, '\0', marks the end of a string. For example, in testing to see whether the char * variable p points to a null character marking the end of a string, you can write !*p or *p == '\0'.

A null character is quite different conceptually from a null pointer, although both are represented by the integer 0.

String literals appear in C program source as strings of characters between double-quote characters (‘"’). In ANSI C, string literals can also be formed by string concatenation: "a" "b" is the same as "ab". Modification of string literals is not allowed by the GNU C compiler, because literals are placed in read-only storage.

Character arrays that are declared const cannot be modified either. It’s generally good style to declare non-modifiable string pointers to be of type const char *, since this often allows the C compiler to detect accidental modifications as well as providing some amount of documentation about what your program intends to do with the string.

The amount of memory allocated for the character array may extend past the null character that normally marks the end of the string. In this document, the term allocation size is always used to refer to the total amount of memory allocated for the string, while the term length refers to the number of characters up to (but not including) the terminating null character.

A notorious source of program bugs is trying to put more characters in a string than fit in its allocated size. When writing code that extends strings or moves characters into a pre-allocated array, you should be very careful to keep track of the length of the text and make explicit checks for overflowing the array. Many of the library functions do not do this for you! Remember also that you need to allocate an extra byte to hold the null character that marks the end of the string.

1.2 String and Array Conventions

This chapter describes both functions that work on arbitrary arrays or blocks of memory, and functions that are specific to null-terminated arrays of characters.

Functions that operate on arbitrary blocks of memory have names beginning with ‘mem’ (such as memcpy) and invariably take an argument which specifies the size (in bytes) of the block of memory to operate on. The array arguments and return values for these functions have type void *, and as a matter of style, the elements of these arrays are referred to as “bytes”. You can pass any kind of pointer to these functions, and the sizeof operator is useful in computing the value for the size argument.

In contrast, functions that operate specifically on strings have names beginning with ‘str’ (such as strcpy) and look for a null character to terminate the string instead of requiring an explicit size argument to be passed. (Some of these functions accept a specified maximum length, but they also check for premature termination with a null character.) The array arguments and return values for these functions have type char *, and the array elements are referred to as “characters”.

In many cases, there are both ‘mem’ and ‘str’ versions of a function. The one that is more appropriate to use depends on the exact situation. When your program is manipulating arbitrary arrays or blocks of storage, then you should always use the ‘mem’ functions. On the other hand, when you are manipulating null-terminated strings it is usually more convenient to use the ‘str’ functions, unless you already know the length of the string in advance.

1.3 String Length

You can get the length of a string using the strlen function. This function is declared in the header file ‘string.h’.

Function: size_t strlen (const char *s)

The strlen function returns the length of the null-terminated string s. (In other words, it returns the offset of the terminating null character within the array.)

For example,

strlen ("hello, world")
    ⇒ 12

When applied to a character array, the strlen function returns the length of the string stored there, not its allocation size. You can get the allocation size of the character array that holds a string using the sizeof operator:

char string[32] = "hello, world"; 
sizeof (string)
    ⇒ 32
strlen (string)
    ⇒ 12

1.4 Copying and Concatenation

You can use the functions described in this section to copy the contents of strings and arrays, or to append the contents of one string to another. These functions are declared in the header file ‘string.h’.

A helpful way to remember the ordering of the arguments to the functions in this section is that it corresponds to an assignment expression, with the destination array specified to the left of the source array. All of these functions return the address of the destination array.

Most of these functions do not work properly if the source and destination arrays overlap. For example, if the beginning of the destination array overlaps the end of the source array, the original contents of that part of the source array may get overwritten before it is copied. Even worse, in the case of the string functions, the null character marking the end of the string may be lost, and the copy function might get stuck in a loop trashing all the memory allocated to your program.

All functions that have problems copying between overlapping arrays are explicitly identified in this manual. In addition to functions in this section, there are a few others like sprintf (@pxref{Formatted Output Functions}) and scanf (@pxref{Formatted Input Functions}).

Function: void * memcpy (void *to, const void *from, size_t size)

The memcpy function copies size bytes from the object beginning at from into the object beginning at to. The behavior of this function is undefined if the two arrays to and from overlap; use memmove instead if overlapping is possible.

The value returned by memcpy is the value of to.

Here is an example of how you might use memcpy to copy the contents of an array:

struct foo *oldarray, *newarray;
int arraysize;
…
memcpy (new, old, arraysize * sizeof (struct foo));

Function: void * memmove (void *to, const void *from, size_t size)

memmove copies the size bytes at from into the size bytes at to, even if those two blocks of space overlap. In the case of overlap, memmove is careful to copy the original values of the bytes in the block at from, including those bytes which also belong to the block at to.

Function: void * memccpy (void *to, const void *from, int c, size_t size)

This function copies no more than size bytes from from to to, stopping if a byte matching c is found. The return value is a pointer into to one byte past where c was copied, or a null pointer if no byte matching c appeared in the first size bytes of from.

Function: void * memset (void *block, int c, size_t size)

This function copies the value of c (converted to an unsigned char) into each of the first size bytes of the object beginning at block. It returns the value of block.

Function: char * strcpy (char *to, const char *from)

This copies characters from the string from (up to and including the terminating null character) into the string to. Like memcpy, this function has undefined results if the strings overlap. The return value is the value of to.

Function: char * strncpy (char *to, const char *from, size_t size)

This function is similar to strcpy but always copies exactly size characters into to.

If the length of from is more than size, then strncpy copies just the first size characters.

If the length of from is less than size, then strncpy copies all of from, followed by enough null characters to add up to size characters in all. This behavior is rarely useful, but it is specified by the ANSI C standard.

The behavior of strncpy is undefined if the strings overlap.

Using strncpy as opposed to strcpy is a way to avoid bugs relating to writing past the end of the allocated space for to. However, it can also make your program much slower in one common case: copying a string which is probably small into a potentially large buffer. In this case, size may be large, and when it is, strncpy will waste a considerable amount of time copying null characters.

Function: char * strdup (const char *s)

This function copies the null-terminated string s into a newly allocated string. The string is allocated using malloc; see @ref{Unconstrained Allocation}. If malloc cannot allocate space for the new string, strdup returns a null pointer. Otherwise it returns a pointer to the new string.

Function: char * stpcpy (char *to, const char *from)

This function is like strcpy, except that it returns a pointer to the end of the string to (that is, the address of the terminating null character) rather than the beginning.

For example, this program uses stpcpy to concatenate ‘foo’ and ‘bar’ to produce ‘foobar’, which it then prints.

This function is not part of the ANSI or POSIX standards, and is not customary on Unix systems, but we did not invent it either. Perhaps it comes from MS-DOG.

Its behavior is undefined if the strings overlap.

Function: char * strcat (char *to, const char *from)

The strcat function is similar to strcpy, except that the characters from from are concatenated or appended to the end of to, instead of overwriting it. That is, the first character from from overwrites the null character marking the end of to.

An equivalent definition for strcat would be:

char *
strcat (char *to, const char *from)
{
  strcpy (to + strlen (to), from);
  return to;
}

This function has undefined results if the strings overlap.

Function: char * strncat (char *to, const char *from, size_t size)

This function is like strcat except that not more than size characters from from are appended to the end of to. A single null character is also always appended to to, so the total allocated size of to must be at least size + 1 bytes longer than its initial length.

The strncat function could be implemented like this:

char *
strncat (char *to, const char *from, size_t size)
{
  strncpy (to + strlen (to), from, size);
  return to;
}

The behavior of strncat is undefined if the strings overlap.

Here is an example showing the use of strncpy and strncat. Notice how, in the call to strncat, the size parameter is computed to avoid overflowing the character array buffer.

The output produced by this program looks like:

hello
hello, wo

Function: void * bcopy (void *from, const void *to, size_t size)

This is a partially obsolete alternative for memmove, derived from BSD. Note that it is not quite equivalent to memmove, because the arguments are not in the same order.

Function: void * bzero (void *block, size_t size)

This is a partially obsolete alternative for memset, derived from BSD. Note that it is not as general as memset, because the only value it can store is zero. Some machines have special instructions for zeroing memory, so bzero might be more efficient than memset.

1.5 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. @xref{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”.

If you are using these functions only to check for equality, you might find it makes for a cleaner program to hide them behind a macro definition, like this:

#define str_eq(s1,s2)  (!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.

1.6 Collation Functions

In some locales, the conventions for lexicographic ordering differ from the strict numeric ordering of character codes. For example, in Spanish most glyphs with diacritical marks such as accents are not considered distinct letters for the purposes of collation. On the other hand, the two-character sequence ‘ll’ is treated as a single letter that is collated immediately after ‘l’.

You can use the functions strcoll and strxfrm (declared in the header file ‘string.h’) to compare strings using a collation ordering appropriate for the current locale. The locale used by these functions in particular can be specified by setting the locale for the LC_COLLATE category; see @ref{Locales}.

In the standard C locale, the collation sequence for strcoll is the same as that for strcmp.

Effectively, the way these functions work is by applying a mapping to transform the characters in a string to a byte sequence that represents the string’s position in the collating sequence of the current locale. Comparing two such byte sequences in a simple fashion is equivalent to comparing the strings with the locale’s collating sequence.

The function strcoll performs this translation implicitly, in order to do one comparison. By contrast, strxfrm performs the mapping explicitly. If you are making multiple comparisons using the same string or set of strings, it is likely to be more efficient to use strxfrm to transform all the strings just once, and subsequently compare the transformed strings with strcmp.

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

The strcoll function is similar to strcmp but uses the collating sequence of the current locale for collation (the LC_COLLATE locale).

Here is an example of sorting an array of strings, using strcoll to compare them. The actual sort algorithm is not written here; it comes from qsort (@pxref{Array Sort Function}). The job of the code shown here is to say how to compare the strings while sorting them. (Later on in this section, we will show a way to do this more efficiently using strxfrm.)

/* This is the comparison function used with qsort. */

int
compare_elements (char **p1, char **p2)
{
  return strcoll (*p1, *p2);
}

/* This is the entry point---the function to sort
   strings using the locale's collating sequence. */

void
sort_strings (char **array, int nstrings)
{
  /* Sort temp_array by comparing the strings. */
  qsort (array, sizeof (char *),
         nstrings, compare_elements);
}

Function: size_t strxfrm (char *to, const char *from, size_t size)

The function strxfrm transforms string using the collation transformation determined by the locale currently selected for collation, and stores the transformed string in the array to. Up to size characters (including a terminating null character) are stored.

The behavior is undefined if the strings to and from overlap; see Copying and Concatenation.

The return value is the length of the entire transformed string. This value is not affected by the value of size, but if it is greater than size, it means that the transformed string did not entirely fit in the array to. In this case, only as much of the string as actually fits was stored. To get the whole transformed string, call strxfrm again with a bigger output array.

The transformed string may be longer than the original string, and it may also be shorter.

If size is zero, no characters are stored in to. In this case, strxfrm simply returns the number of characters that would be the length of the transformed string. This is useful for determining what size string to allocate. It does not matter what to is if size is zero; to may even be a null pointer.

Here is an example of how you can use strxfrm when you plan to do many comparisons. It does the same thing as the previous example, but much faster, because it has to transform each string only once, no matter how many times it is compared with other strings. Even the time needed to allocate and free storage is much less than the time we save, when there are many strings.

struct sorter { char *input; char *transformed; };

/* This is the comparison function used with qsort
   to sort an array of struct sorter. */

int
compare_elements (struct sorter *p1, struct sorter *p2)
{
  return strcmp (p1->transformed, p2->transformed);
}

/* This is the entry point---the function to sort
   strings using the locale's collating sequence. */

void
sort_strings_fast (char **array, int nstrings)
{
  struct sorter temp_array[nstrings];
  int i;

  /* Set up temp_array.  Each element contains
     one input string and its transformed string. */
  for (i = 0; i < nstrings; i++)
    {
      size_t length = strlen (array[i]) * 2;

      temp_array[i].input = array[i];

      /* Transform array[i].
         First try a buffer probably big enough. */
      while (1)
        {
          char *transformed = (char *) xmalloc (length);
          if (strxfrm (transformed, array[i], length) < length)
            {
              temp_array[i].transformed = transformed;
              break;
            }
          /* Try again with a bigger buffer. */
          free (transformed);
          length *= 2;
        }
    }

  /* Sort temp_array by comparing transformed strings. */
  qsort (temp_array, sizeof (struct sorter),
         nstrings, compare_elements);

  /* Put the elements back in the permanent array
     in their sorted order. */
  for (i = 0; i < nstrings; i++)
    array[i] = temp_array[i].input;

  /* Free the strings we allocated. */
  for (i = 0; i < nstrings; i++)
    free (temp_array[i].transformed);
}

Compatibility Note: The string collation functions are a new feature of ANSI C. Older C dialects have no equivalent feature.

1.7 Search Functions

This section describes library functions which perform various kinds of searching operations on strings and arrays. These functions are declared in the header file ‘string.h’.

Function: void * memchr (const void *block, int c, size_t size)

This function finds the first occurrence of the byte c (converted to an unsigned char) in the initial size bytes of the object beginning at block. The return value is a pointer to the located byte, or a null pointer if no match was found.

Function: char * strchr (const char *string, int c)

The strchr function finds the first occurrence of the character c (converted to a char) in the null-terminated string beginning at string. The return value is a pointer to the located character, or a null pointer if no match was found.

For example,

strchr ("hello, world", 'l')
    ⇒ "llo, world"
strchr ("hello, world", '?')
    ⇒ NULL

The terminating null character is considered to be part of the string, so you can use this function get a pointer to the end of a string by specifying a null character as the value of the c argument.

Function: char * index (const char *string, int c)

index is another name for strchr; they are exactly the same.

Function: char * strrchr (const char *string, int c)

The function strrchr is like strchr, except that it searches backwards from the end of the string string (instead of forwards from the front).

For example,

strrchr ("hello, world", 'l')
    ⇒ "ld"

Function: char * rindex (const char *string, int c)

rindex is another name for strrchr; they are exactly the same.

Function: char * strstr (const char *haystack, const char *needle)

This is like strchr, except that it searches haystack for a substring needle rather than just a single character. It returns a pointer into the string haystack that is the first character of the substring, or a null pointer if no match was found. If needle is an empty string, the function returns haystack.

For example,

strstr ("hello, world", "l")
    ⇒ "llo, world"
strstr ("hello, world", "wo")
    ⇒ "world"

Function: void * memmem (const void *needle, size_t needle_len,
const void *haystack, size_t haystack_len)

This is like strstr, but needle and haystack are byte arrays rather than null-terminated strings. needle_len is the length of needle and haystack_len is the length of haystack.

This function is a GNU extension.

Function: size_t strspn (const char *string, const char *skipset)

The strspn (“string span”) function returns the length of the initial substring of string that consists entirely of characters that are members of the set specified by the string skipset. The order of the characters in skipset is not important.

For example,

strspn ("hello, world", "abcdefghijklmnopqrstuvwxyz")
    ⇒ 5

Function: size_t strcspn (const char *string, const char *stopset)

The strcspn (“string complement span”) function returns the length of the initial substring of string that consists entirely of characters that are not members of the set specified by the string stopset. (In other words, it returns the offset of the first character in string that is a member of the set stopset.)

For example,

strcspn ("hello, world", " \t\n,.;!?")
    ⇒ 5

Function: char * strpbrk (const char *string, const char *stopset)

The strpbrk (“string pointer break”) function is related to strcspn, except that it returns a pointer to the first character in string that is a member of the set stopset instead of the length of the initial substring. It returns a null pointer if no such character from stopset is found.

For example,

strpbrk ("hello, world", " \t\n,.;!?")
    ⇒ ", world"

1.8 Finding Tokens in a String

It’s fairly common for programs to have a need to do some simple kinds of lexical analysis and parsing, such as splitting a command string up into tokens. You can do this with the strtok function, declared in the header file ‘string.h’.

Function: char * strtok (char *newstring, const char *delimiters)

A string can be split into tokens by making a series of calls to the function strtok.

The string to be split up is passed as the newstring argument on the first call only. The strtok function uses this to set up some internal state information. Subsequent calls to get additional tokens from the same string are indicated by passing a null pointer as the newstring argument. Calling strtok with another non-null newstring argument reinitializes the state information. It is guaranteed that no other library function ever calls strtok behind your back (which would mess up this internal state information).

The delimiters argument is a string that specifies a set of delimiters that may surround the token being extracted. All the initial characters that are members of this set are discarded. The first character that is not a member of this set of delimiters marks the beginning of the next token. The end of the token is found by looking for the next character that is a member of the delimiter set. This character in the original string newstring is overwritten by a null character, and the pointer to the beginning of the token in newstring is returned.

On the next call to strtok, the searching begins at the next character beyond the one that marked the end of the previous token. Note that the set of delimiters delimiters do not have to be the same on every call in a series of calls to strtok.

If the end of the string newstring is reached, or if the remainder of string consists only of delimiter characters, strtok returns a null pointer.

Warning: Since strtok alters the string it is parsing, you always copy the string to a temporary buffer before parsing it with strtok. If you allow strtok to modify a string that came from another part of your program, you are asking for trouble; that string may be part of a data structure that could be used for other purposes during the parsing, when alteration by strtok makes the data structure temporarily inaccurate.

The string that you are operating on might even be a constant. Then when strtok tries to modify it, your program will get a fatal signal for writing in read-only memory. @xref{Program Error Signals}.

This is a special case of a general principle: if a part of a program does not have as its purpose the modification of a certain data structure, then it is error-prone to modify the data structure temporarily.

The function strtok is not reentrant. @xref{Nonreentrancy}, for a discussion of where and why reentrancy is important.

Here is a simple example showing the use of strtok.

#include <string.h>
#include <stddef.h>

…

char string[] = "words separated by spaces -- and, punctuation!";
const char delimiters[] = " .,;:!-";
char *token;

…

token = strtok (string, delimiters);  /* token => "words" */
token = strtok (NULL, delimiters);    /* token => "separated" */
token = strtok (NULL, delimiters);    /* token => "by" */
token = strtok (NULL, delimiters);    /* token => "spaces" */
token = strtok (NULL, delimiters);    /* token => "and" */
token = strtok (NULL, delimiters);    /* token => "punctuation" */
token = strtok (NULL, delimiters);    /* token => NULL */

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