Use mmap to read file and output strings to stdout

mystrings.c

/*
gcc mystrings.c -O3 -o mystrings
gcc -DSTRING_LENGTH 99 -DBUFFER_SIZE 0x10000 mystrings.c -O3 -o mystrings
*/

#include <string.h>
#include <fcntl.h>
#include <sys/stat.h>
#include <unistd.h>
#include <sys/mman.h>

#ifndef STRING_LENGTH
#define STRING_LENGTH 8
#endif
#ifndef BUFFER_SIZE
#define BUFFER_SIZE 4096
#endif

char buffer[BUFFER_SIZE];
size_t buff_used = 0;

void flush_buffer()
{
        if(buff_used != 0) {
                write(1, buffer, buff_used);
        }
        buff_used = 0;
}

void report_string(const char *str, size_t len)
{
        if(len >= sizeof(buffer)) {
                flush_buffer();
                write(1, str, len);

                buffer[0] = '\n';
                buff_used = 1;
                return;
        }
        if(len + buff_used + 1 > sizeof(buffer)) {
                flush_buffer();
        }

        memcpy(buffer + buff_used, str, len);
        buffer[buff_used + len] = '\n';
        buff_used += len + 1;
}

void error(const char *s)
{
        write(2, s, strlen(s));
        write(2, "\n", 1);
        _exit(1);
}

int main(int argc, char const *argv[])
{
        if(argc < 2) {
                error("usage: ./mystrings file-to-read.bin");
        }
        int fd = open(argv[1], O_RDONLY);
        if(fd < 0)
        {
                error("error: can not open file");
        }

        struct stat s;
        int status = fstat(fd, &s);
        size_t size = s.st_size;

        const char *mapped = (const char *)mmap(0, size, PROT_READ, MAP_PRIVATE, fd, 0);
        if(mapped == NULL) {
                error("error: can not mmap file");
        }
        size_t begin = 0, length = 0;

        for (size_t i = 0; i < size; i++) {
                if((0x20 <= mapped[i] && mapped[i] <= 0x7e) || mapped[i] == '\r' || mapped[i] == '\n' || mapped[i] == '\t') {
                        if(begin == 0) {
                                begin = i;
                                length = 1;
                        } else {
                                ++length;
                        }

                } else {
                        if(begin != 0) {
                                if(length >= STRING_LENGTH) {
                                        report_string(mapped + begin, length);
                                }
                                begin = 0;
                        }
                }
        }
        if(length >= STRING_LENGTH) {
                report_string(mapped + begin, length);
        }

        flush_buffer();

        return 0;
}