C Custom Database writing errors - c

Question

I have an assignment for class that I have to write a program to read and write key, value pairs to disk. I am using a linked list to store the keys, and read in values whenever I need to from disk. However, I am having trouble changing and deleting values. I am using this to test it: http://gaming.jhu.edu/~phf/2010/fall/cs120/src/sdbm-examples.tar.gz. Code below. Basically, I need some help figuring out errors, because this is the first assignment we have had to use pointers on, and I am just dying in all the segfaults and everything else. Just some advice would be greatly appreciated.
#include <stdio.h>
#include <stdlib.h>
#include <assert.h>
#include <string.h>
#include <stdbool.h>
#include "sdbm.h"
FILE *db;
bool opened = false, needNewDB = false;
int err = 0, keyLen = 0;
char *filename;
typedef struct Key_{
char *name;
char *val;
long offset;
struct Key_ *next;
} Key;
Key *head = NULL,*tail = NULL, *lastHas = NULL, *beforeLastHas = NULL;
/**
* Create new database with given name. You still have
* to sdbm_open() the database to access it. Return true
* on success, false on failure.
*/
void listAdd() {
if (tail != NULL) {
tail->next = (Key *) malloc(sizeof(Key));
tail = tail->next;
}
else {
tail = (Key *)malloc(sizeof(Key));
head = tail;
}
tail->next = NULL;
tail->name = NULL;
tail->val = NULL;
}
bool sdbm_create( const char *name ) { //Errors: 1) fopen failed 2) fclose failed on new db
filename = malloc(sizeof(*name));
strcpy(filename,name);
FILE *temp = fopen(name, "w");
if (temp == NULL) {
printf("Couldn't create file %s\n",name);
err = 1;
return false;
}
if (fclose(temp) == EOF) {
printf("Couldn't close created file %s\n",name);
err = 2;
return false;
}
return true;
}
/**
* Open existing database with given name. Return true on
* success, false on failure.
*/
bool sdbm_open( const char *name ) { //Errors: 3) couldn't open database
db = fopen(name,"r+");
if (db == NULL) {
err = 3;
printf("Couldn't open database file %s\n",name);
return false;
}
opened = true;
int c;
bool inKey = true;
char currKey[MAX_KEY_LENGTH];
while ((c = getc(db)) != EOF) {
if (!inKey && c == '\0') {
inKey = true;
}
else if (inKey && c == '\0') {
currKey[keyLen] = '\0';
listAdd();
tail->offset = ftell(db);
tail->name = malloc(sizeof(*currKey));
strcpy(tail->name,currKey);
keyLen = 0;
inKey = false;
}
else if (inKey) {
currKey[keyLen] = c;
keyLen++;
}
}
Key *curr = head;
while (curr != NULL) {
printf("Key: %s\n",curr->name);
curr = curr->next;
}
return true;
}
void readVal(char *value, long offset) {
fseek(db,offset,SEEK_SET);
int c;
for (int i = 0; (c = getc(db)) != '\0'; i++) {
*(value + i) = c;
}
}
/**
* Synchronize all changes in database (if any) to disk.
* Useful if implementation caches intermediate results
* in memory instead of writing them to disk directly.
* Return true on success, false on failure.
*/
bool sdbm_sync() {
if (!needNewDB) {
Key *curr = head;
fseek(db,0,SEEK_END);
while (curr != NULL) {
if (curr->val != NULL) {
fprintf(db,"%s%c%s%c",curr->name,'\0',curr->val,'\0');
}
curr = curr->next;
}
}
else {
FILE *temp;
sdbm_create("tRpdxD.p4ed");
temp = fopen("tRpdxD.p4ed","w");
Key *curr = head;
while (curr != NULL) {
if (curr->val != NULL) {
fprintf(temp,"%s%c%s%c",curr->name, '\0', curr->val, '\0');
}else {
char *tempS = malloc(MAX_VALUE_LENGTH);
readVal(tempS, curr->offset);
fprintf(temp,"%s%c%s%c",curr->name,'\0',tempS,'\0');
free(tempS);
}
fflush(temp);
fflush(db);
curr = curr->next;
}
fclose(db);
remove(filename);
rename("tRpdxD.p4ed",filename);
db = fopen(filename,"r+");
}
fflush(db);
return true;
}
/**
* Close database, synchronizing changes (if any). Return
* true on success, false on failure.
*/
bool sdbm_close() { // Errors: 5) Couldn't close database
sdbm_sync();
Key *tmp = head;
while (head->next != NULL) {
tmp = head;
head = head->next;
free(tmp->name);
if (tmp->val != NULL) {
free(tmp->val);
}
free(tmp);
}
if (fclose(db) == EOF) {
err = 5;
printf("Couldn't close database.\n");
return false;
}
return true;
}
/**
* Return error code for last failed database operation.
*/
int sdbm_error() {
return err;
}
/**
* Is given key in database?
*/
bool sdbm_has( const char *key ) {
if (head == NULL) {
return false;
}
Key *curr = head;
lastHas = NULL;
beforeLastHas = NULL;
while (curr != NULL) {
if (!strcmp(curr->name,key)) {
lastHas = curr;
return true;
}
beforeLastHas = curr;
curr = curr->next;
}
return false;
}
/**
* Get value associated with given key in database.
* Return true on success, false on failure.
*
* Precondition: sdbm_has(key)
*/
bool sdbm_get( const char *key, char *value ) { //Errors: 6)Don't have key
if (!sdbm_has(key)) {
printf("Precondition sdbm_has(%s) failed", key);
err = 6;
return false;
}
readVal(value, lastHas->offset);
return true;
}
/**
* Update value associated with given key in database
* to given value. Return true on success, false on
* failure.
*
* Precondition: sdbm_has(key)
*/
bool sdbm_put( const char *key, const char *value ) {
if (!sdbm_has(key)) {
printf("Precondition !sdbm_has(%s) failed",key);
err = 7;
return false;
}
sdbm_remove(key);
sdbm_insert(key,value);
return true;
}
/**
* Insert given key and value into database as a new
* association. Return true on success, false on
* failure.
*
* Precondition: !sdbm_has(key)
*/
bool sdbm_insert( const char *key, const char *value ) { //Errors: 7)Already have key 8)Invalid key or value length
if (sdbm_has(key)) {
printf("Precondition !sdbm_has(%s) failed",key);
err = 7;
return false;
}
if (strlen(key) < MIN_KEY_LENGTH || strlen(key) > MAX_KEY_LENGTH || strlen(value) < MIN_VALUE_LENGTH || strlen(value) > MAX_VALUE_LENGTH) {
printf("Invalid key or value length");
err = 8;
return false;
}
listAdd();
tail->name = (char *)key;
tail->val = malloc(sizeof(*value));
strcpy(tail->val,value);
return true;
}
/**
* Remove given key and associated value from database.
* Return true on success, false on failure.
*
* Precondition: sdbm_has(key)
*/
bool sdbm_remove( const char *key ) {
if (!sdbm_has(key)) {
printf("Precondition !sdbm_has(%s) failed",key);
err = 7;
return false;
}
needNewDB = true;
if (beforeLastHas == NULL) {
head = lastHas->next;
}
else if (lastHas->next == NULL) {
tail = beforeLastHas;
}
else {
beforeLastHas->next = lastHas->next;
}
if (lastHas->val != NULL) {
free(lastHas->val);
}
free(lastHas->name);
free(lastHas);
return true;
}

Answer 1

There's a lot of errors in this code. To name just one:
filename = malloc(sizeof(*name));
*name is the first element of name, so it's a char, so sizeof(*name) == 1. To get the size of a string, use strlen(name) + 1. Better yet, use strdup if your system has it.

Answer 2

i would advise against global variables. you can not change the program (in the future) to use two of your databases in parallel.
so all your sdbm_xxx functions should get (or infer) all necessary values on their own.