April 8, 2001 BM 3

### NAME

bm_comp, bm_exec, bm_free - Boyer-Moore string search

### LIBRARY

Standard C Library (libc, -lc)

### SYNOPSIS

```bm_pat``*` bm_comp(```u_char *pattern``, size_t patlen``, u_char freq[256]`)
```u_char``*` bm_exec(```bm_pat *pdesc``, u_char *text``, size_t len`)
```void` bm_free(```bm_pat *pdesc`)

### DESCRIPTION

These routines implement an efficient mechanism to find an occurrence of a byte string within another byte string.

bm_comp(``) evaluates the ```patlen` bytes starting at ```pattern`, and returns a pointer to a structure describing them. The bytes referenced by ```pattern` may be of any value.

The search takes advantage of the frequency distribution of the bytes in the text to be searched. If specified, ```freq` should be an array of 256 values, with higher values indicating that the corresponding character occurs more frequently. (A less than optimal frequency distribution can only result in less than optimal performance, not incorrect results.) If ```freq` is ```NULL`, a system default table is used.

bm_exec(``) returns a pointer to the leftmost occurrence of the string given to bm_comp(``) within ```text`, or ```NULL` if none occurs. The number of bytes in ```text` must be specified by ```len`.

Space allocated for the returned description is discarded by calling bm_free(``) with the returned description as an argument.

The asymptotic speed of bm_exec(``) is O(len/patlen).

### SEE ALSO

regexp(3), strstr(3)