very bad key/value database

#include <errno.h>
#include <fcntl.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <unistd.h>

static const char *dpath = "db.db";
static int db_fd;
static char *db_buf;
static size_t db_len;
static int keys(void);
static int get(const char *key);
static int set(const char *key, const char *value);
static int delete(const char *key);
static int readdb(void);
static int openerr(void);
static void setval(const char *value);

int main(int argc, const char **argv) {
    if(argc >= 2) goto nousage;
    usage:
    fprintf(stderr,
        "usage: %s\n"
        " get <key>\n"
        " set <key> <value>\n"
        " delete <key>\n"
        " keys\n",
        argv[0]);
    return 1;
    nousage:
    if(!strcmp(argv[1], "keys")) return keys();
    if(argc < 3) goto usage;
    if(!strcmp(argv[1], "get")) return get(argv[2]);
    if(!strcmp(argv[1], "set")) return set(argv[2], argc >= 4 ? argv[3] : NULL);
    if(!strcmp(argv[1], "delete")) return delete(argv[2]);
    goto usage;
}

static int keys(void) {
    int ret;
    if(ret = readdb()) return ret;
    char *pos = db_buf;
    char *end = db_buf + db_len;
    loop:;
    if(pos >= end) goto end;
    size_t keylen = *(size_t *)pos;
    pos += 8;
    fwrite(pos, keylen, 1, stdout);
    putchar('\n');
    pos += keylen;
    size_t vallen = *(size_t *)pos;
    pos += 8 + vallen;
    goto loop;
    end:;
    return 0;
}

static int get(const char *key) {
    int ret;
    if(ret = readdb()) return ret;
    char *pos = db_buf;
    char *end = db_buf + db_len;
    size_t keylen, vallen;
    loop:;
    if(pos >= end) goto end;
    keylen = *(size_t *)pos;
    if(keylen != strlen(key)) goto next;
    pos += 8;
    if(memcmp(key, pos, keylen)) goto next2;
    pos += keylen;
    vallen = *(size_t *)pos;
    pos += 8;
    fwrite(pos, vallen, 1, stdout);
    return 0;
    next:;
    pos += 8;
    next2:;
    pos += keylen;
    vallen = *(size_t *)pos;
    pos += 8 + vallen;
    goto loop;
    end:;
    fprintf(stderr, "no such key\n");
    return 1;
}

static int set(const char *key, const char *value) {
    int ret;
    if(ret = readdb()) return ret;
    char *pos = db_buf;
    char *end = db_buf + db_len;
    char *last;
    size_t keylen, vallen;
    loop:;
    if(pos >= end) goto new;
    last = pos;
    keylen = *(size_t *)pos;
    if(keylen != strlen(key)) goto next;
    pos += 8;
    if(memcmp(key, pos, keylen)) goto next2;
    pos += keylen;
    ftruncate(db_fd, pos - db_buf);
    lseek(db_fd, 0, SEEK_END);
    setval(value);
    vallen = *(size_t *)pos;
    pos += 8 + vallen;
    write(db_fd, pos, end - pos);
    return 0;
    next:;
    pos += 8;
    next2:;
    pos += keylen;
    vallen = *(size_t *)pos;
    pos += 8 + vallen;
    goto loop;
    new:;
    keylen = strlen(key);
    write(db_fd, &keylen, 8);
    write(db_fd, key, keylen);
    setval(value);
    return 0;
}

static int delete(const char *key) {
    int ret;
    if(ret = readdb()) return ret;
    char *pos = db_buf;
    char *end = db_buf + db_len;
    char *last;
    size_t keylen, vallen;
    loop:;
    if(pos >= end) goto end;
    last = pos;
    keylen = *(size_t *)pos;
    if(keylen != strlen(key)) goto next;
    pos += 8;
    if(memcmp(key, pos, keylen)) goto next2;
    pos += keylen;
    ftruncate(db_fd, last - db_buf);
    lseek(db_fd, 0, SEEK_END);
    vallen = *(size_t *)pos;
    pos += 8 + vallen;
    write(db_fd, pos, end - pos);
    return 0;
    next:;
    pos += 8;
    next2:;
    pos += keylen;
    vallen = *(size_t *)pos;
    pos += 8 + vallen;
    goto loop;
    end:;
    fprintf(stderr, "no such key\n");
    return 1;
}

static int readdb(void) {
    db_fd = open(dpath, O_RDWR | O_CREAT, 0644);
    if(db_fd < 0) return openerr();
    db_len = lseek(db_fd, 0, SEEK_END);
    lseek(db_fd, 0, SEEK_SET);
    db_buf = malloc(db_len);
    if(!db_buf) return openerr();
    read(db_fd, db_buf, db_len);
    return 0;
}

static int openerr(void) {
    fputs(strerror(errno), stderr);
    putc('\n', stderr);
    return 2;
}

static void setval(const char *value) {
    size_t vallen;
    if(!value) goto fromstdin;
    vallen = strlen(value);
    write(db_fd, &vallen, 8);
    write(db_fd, value, vallen);
    return;
    fromstdin:;
    vallen = 0;
    char buf[512];
    size_t n;
    write(db_fd, &vallen, 8);
    off_t pos = lseek(db_fd, 0, SEEK_CUR);
    loop:;
    n = fread(buf, 1, 512, stdin);
    if(!n) goto end;
    write(db_fd, buf, n);
    goto loop;
    end:;
    off_t end = lseek(db_fd, 0, SEEK_CUR);
    lseek(db_fd, pos - 8, SEEK_SET);
    vallen = end - pos;
    write(db_fd, &vallen, 8);
    lseek(db_fd, end, SEEK_SET);
}