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man pages section 3: Basic Library Functions

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Updated: Wednesday, July 27, 2022



lsearch, lfind - linear search and update


#include <search.h>

void *lsearch(const void *key, void *base, size_t *nelp,
     size_t width, int (*compar)(const void *, const void *));
void *lfind(const void *key, const void *base, size_t *nelp,
     size_t width, int (*compar)(const void *, const void *));


The lsearch() function is a linear search routine generalized from Knuth (6.1) Algorithm S. (see The Art of Computer Programming, Volume 3, Section 6.1, by Donald E. Knuth.). It returns a pointer to a table indicating where a datum can be found. If the datum does not occur, it is added at the end of the table. The key argument points to the datum to be sought in the table. The base argument points to the first element in the table. The nelp argument points to an integer containing the current number of elements in the table. The integer is incremented if the datum is added to the table. The width argument is the size of an element in bytes. The compar argument is a pointer to the comparison function that the user must supply (strcmp(3C) for example). It is called with two arguments that point to the elements being compared. The function must return zero if the elements are equal and non-zero otherwise.

The lfind() function is the same as lsearch() except that if the datum is not found, it is not added to the table. Instead, a null pointer is returned.

It is important to note the following:

  • The pointers to the key and the element at the base of the table can be pointers to any type.

  • The comparison function need not compare every byte, so arbitrary data can be contained in the elements in addition to the values being compared.

  • The value returned should be cast into type pointer-to-element.

Return Values

If the searched-for datum is found, both lsearch() and lfind() return a pointer to it. Otherwise, lfind() returns NULL and lsearch() returns a pointer to the newly added element.


Undefined results can occur if there is not enough room in the table to add a new item.

The lsearch() and lfind() functions safely allows concurrent access by multiple threads to disjoint data, such as overlapping subtrees or tables.


Example 1 A sample code using the lsearch() function.

This program will read in less than TABSIZE strings of length less than ELSIZE and store them in a table, eliminating duplicates, and then will print each entry.

#include <search.h>
#include <string.h>
#include <stdlib.h>
#include <stdio.h>

#define TABSIZE 50
#define ELSIZE 120

    char line[ELSIZE];          /* buffer to hold input string */
    char tab[TABSIZE][ELSIZE];  /* table of strings */
    size_t nel = 0;             /* number of entries in tab */
    int i;

    while (fgets(line, ELSIZE, stdin) != NULL &&
            nel < TABSIZE)
            (void) lsearch(line, tab, &nel, ELSIZE, mycmp);
    for (i = 0; i < nel; i++)
            (void) fputs(tab[i], stdout);
    return 0;


See attributes(7) for descriptions of the following attributes:

Interface Stability

See Also

bsearch(3C), hsearch(3C), strcmp(3C), tsearch(3C), attributes(7), standards(7)

The Art of Computer Programming, Volume 3, Sorting and Searching by Donald E. Knuth, published by Addison-Wesley Publishing Company, 1973.