Generally searching for a specific element in an array means that potentially all elements must be checked. The GNU C Library contains functions to perform linear search. The prototypes for the following two functions can be found in search.h.
void * lfind (const void *key, const void *base, size_t *nmemb, size_t size, comparison_fn_t compar) ¶Preliminary: | MT-Safe | AS-Safe | AC-Safe | See POSIX Safety Concepts.
The lfind function searches in the array with *nmemb
elements of size bytes pointed to by base for an element
which matches the one pointed to by key. The function pointed to
by compar is used to decide whether two elements match.
The return value is a pointer to the matching element in the array
starting at base if it is found. If no matching element is
available NULL is returned.
The mean runtime of this function is proportional to *nmemb/2,
assuming random elements of the array are searched for. This
function should be used only if elements often get added to or deleted from
the array in which case it might not be useful to sort the array before
searching.
void * lsearch (const void *key, void *base, size_t *nmemb, size_t size, comparison_fn_t compar) ¶Preliminary: | MT-Safe | AS-Safe | AC-Safe | See POSIX Safety Concepts.
The lsearch function is similar to the lfind function. It
searches the given array for an element and returns it if found. The
difference is that if no matching element is found the lsearch
function adds the object pointed to by key (with a size of
size bytes) at the end of the array and it increments the value of
*nmemb to reflect this addition.
This means for the caller that if it is not sure that the array contains
the element one is searching for the memory allocated for the array
starting at base must have room for at least size more
bytes. If one is sure the element is in the array it is better to use
lfind so having more room in the array is always necessary when
calling lsearch.
To search a sorted or partially sorted array for an element matching the key,
use the bsearch function. The prototype for this function is in
the header file stdlib.h.
void * bsearch (const void *key, const void *array, size_t count, size_t size, comparison_fn_t compare) ¶Preliminary: | MT-Safe | AS-Safe | AC-Safe | See POSIX Safety Concepts.
The bsearch function searches array for an element
that is equivalent to key. The array contains count elements,
each of which is of size size bytes.
The compare function is used to perform the comparison. This function is called with arguments that point to the key and to an array element, in that order, and should return an integer less than, equal to, or greater than zero corresponding to whether the key is considered less than, equal to, or greater than the array element. The function should not alter the array’s contents, and the same array element should always compare the same way with the key.
Although the array need not be completely sorted, it should be partially sorted with respect to key. That is, the array should begin with elements that compare less than key, followed by elements that compare equal to key, and ending with elements that compare greater than key. Any or all of these element sequences can be empty.
The return value is a pointer to a matching array element, or a null pointer if no match is found. If the array contains more than one element that matches, the one that is returned is unspecified.
This function derives its name from the fact that it is implemented using the binary search algorithm.