trying tom make my linked list implementation in c11 (gcc6), threadsafe.
only thing i do not get how many mutex lock and unlocks i should go with?
/**
* adds a new node to head of the list, alocation of node is done dynamically
* #param list address of list
* #param data pointer to data
*/
void add_head(Linked_list* list, void* data)
{
Node *node = (Node*) malloc(sizeof(Node));
//lock
node->data = data;
if (list->head == NULL) {
list->tail = node;
node->next = NULL;
} else {
node->next = list->head;
}
list->head = node;
list->current = node;
list_size ++;
//unlock
}
or
/**
* adds a new node to head of the list, alocation of node is done dynamically
* #param list address of list
* #param data pointer to data
*/
void add_head(Linked_list* list, void* data)
{
Node *node = (Node*) malloc(sizeof(Node));
//lock
node->data = data;
if ( list->head == NULL ) {
list->tail = node;
node->next = NULL;
} else {
node->next = list->head;
}
//unlock
//lock
list->head = node;
list->current = node;
list_size ++;
//unlock
}
or
/**
* adds a new node to head of the list, alocation of node is done dynamically
* #param list address of list
* #param data pointer to data
*/
void add_head (Linked_list* list, void* data)
{
Node *node = (Node*) malloc(sizeof(Node));
//lock
node->data = data;
if (list->head == NULL) {
list->tail = node;
node->next = NULL;
} else {
node->next = list->head;
}
//unlock
//lock
list->head = node;
//unlock
//lock
list->current = node;
//unlock
//lock
list_size ++;
//unlock
}
looking for a way to not make other thread wait too much, since i will have many task of tiny time duration read up to 10 bytes from file, change 10 bytes in memory, write 10 bytes file.
Because you want to support threadsafe for your implementation of function add_head(), you need to guarantee that all shared data accesses have to be atomic.
So I think you should use the first one, i.e use one lock/unlock pair call for whole function implementation.
Related
This is a tail-recursive solution for reversing a singly linked list. What is the auxiliary space occupied by this solution?
void reverseUtil(Node* curr, Node* prev, Node** head)
{
if (curr->next==NULL) {
*head = curr;
curr->next = prev;
return;
}
Node* next = curr->next;
curr->next = prev;
reverseUtil(next, curr, head);
}
void reverse(Node** head)
{
if (head==NULL)
return;
reverseUtil(*head, NULL, head);
}
First, your code should check that *head is not NULL before making the initial call to reverseUtil, so add this condition here:
if (head==NULL || *head==NULL)
return;
This is indeed a case of tail recursion. All current mainstream C compilers perform tail call optimisation when the relevant option is set, and then the tail recursive calls are actually not increasing the stack space usage.
So provided that the compiler is instructed to perform this optimisation, the auxiliary space usage is O(1).
After the optimisation, the execution and space usage will be quite similar to what this iterative code would do:
void reverse(Node** head)
{
if (head==NULL || *head==NULL)
return;
Node* curr = *head;
Node* prev = NULL;
while (true) {
if (curr->next==NULL) {
*head = curr;
curr->next = prev;
return;
}
Node* next = curr->next;
curr->next = prev;
prev = curr;
curr = next;
}
}
I found a way to enumerate other programs handles, but I have problem now. I can not see Process type threads. I need to check which programs open handles for my process.
When I check the output, it is "unnamed", I don't know how to fix it.
Should I do this via dirver? or any other way to do this without driver?
pid = _wtoi(argv[1]);
if (!(processHandle = OpenProcess(PROCESS_DUP_HANDLE, FALSE, pid)))
{
printf("Could not open PID %d! (Don't try to open a system process.)\n", pid);
return 1;
}
handleInfo = (PSYSTEM_HANDLE_INFORMATION)malloc(handleInfoSize);
/* NtQuerySystemInformation won't give us the correct buffer size,
so we guess by doubling the buffer size. */
while ((status = NtQuerySystemInformation(
SystemHandleInformation,
handleInfo,
handleInfoSize,
NULL
)) == STATUS_INFO_LENGTH_MISMATCH)
handleInfo = (PSYSTEM_HANDLE_INFORMATION)realloc(handleInfo, handleInfoSize *= 2);
/* NtQuerySystemInformation stopped giving us STATUS_INFO_LENGTH_MISMATCH. */
if (!NT_SUCCESS(status))
{
printf("NtQuerySystemInformation failed!\n");
return 1;
}
for (i = 0; i < handleInfo->HandleCount; i++)
{
SYSTEM_HANDLE handle = handleInfo->Handles[i];
HANDLE dupHandle = NULL;
POBJECT_TYPE_INFORMATION objectTypeInfo;
PVOID objectNameInfo;
UNICODE_STRING objectName;
ULONG returnLength;
/* Check if this handle belongs to the PID the user specified. */
if (handle.ProcessId != pid)
continue;
/* Duplicate the handle so we can query it. */
if (!NT_SUCCESS(NtDuplicateObject(
processHandle,
handle.Handle,
GetCurrentProcess(),
&dupHandle,
0,
0,
0
)))
{
printf("[%#x] Error!\n", handle.Handle);
continue;
}
/* Query the object type. */
objectTypeInfo = (POBJECT_TYPE_INFORMATION)malloc(0x1000);
if (!NT_SUCCESS(NtQueryObject(
dupHandle,
ObjectTypeInformation,
objectTypeInfo,
0x1000,
NULL
)))
{
printf("[%#x] Error!\n", handle.Handle);
CloseHandle(dupHandle);
continue;
}
/* Query the object name (unless it has an access of
0x0012019f, on which NtQueryObject could hang. */
if (handle.GrantedAccess == 0x0012019f)
{
/* We have the type, so display that. */
printf(
"[%#x] %.*S: (did not get name)\n",
handle.Handle,
objectTypeInfo->Name.Length / 2,
objectTypeInfo->Name.Buffer
);
free(objectTypeInfo);
CloseHandle(dupHandle);
continue;
}
objectNameInfo = malloc(0x1000);
if (!NT_SUCCESS(NtQueryObject(
dupHandle,
ObjectNameInformation,
objectNameInfo,
0x1000,
&returnLength
)))
{
/* Reallocate the buffer and try again. */
objectNameInfo = realloc(objectNameInfo, returnLength);
if (!NT_SUCCESS(NtQueryObject(
dupHandle,
ObjectNameInformation,
objectNameInfo,
returnLength,
NULL
)))
{
/* We have the type name, so just display that. */
printf(
"[%#x] %.*S: (could not get name)\n",
handle.Handle,
objectTypeInfo->Name.Length / 2,
objectTypeInfo->Name.Buffer
);
free(objectTypeInfo);
free(objectNameInfo);
CloseHandle(dupHandle);
continue;
}
}
/* Cast our buffer into an UNICODE_STRING. */
objectName = *(PUNICODE_STRING)objectNameInfo;
/* Print the information! */
if (objectName.Length)
{
/* The object has a name. */
printf(
"[%#x] %.*S: %.*S\n",
handle.Handle,
objectTypeInfo->Name.Length / 2,
objectTypeInfo->Name.Buffer,
objectName.Length / 2,
objectName.Buffer
);
}
else
{
/* Print something else. */
printf(
"[%#x] %.*S: (unnamed)\n",
handle.Handle,
objectTypeInfo->Name.Length / 2,
objectTypeInfo->Name.Buffer
);
}
free(objectTypeInfo);
free(objectNameInfo);
CloseHandle(dupHandle);
}
free(handleInfo);
CloseHandle(processHandle);
return 0;
void SearchMyProcessHandles()
{
ULONG UniqueProcessId = GetCurrentProcessId();
if (HANDLE hProcess = OpenProcess(MAXIMUM_ALLOWED, FALSE, UniqueProcessId))
{
NTSTATUS status;
union {
PSYSTEM_HANDLE_INFORMATION_EX pshi;
PVOID buf;
};
ULONG cb = 0x10000;
do
{
status = STATUS_INSUFFICIENT_RESOURCES;
if (buf = new UCHAR[cb += PAGE_SIZE])
{
if (0 <= (status = ZwQuerySystemInformation(SystemExtendedHandleInformation, buf, cb, &cb)))
{
if (ULONG_PTR NumberOfHandles = pshi->NumberOfHandles)
{
SYSTEM_HANDLE_TABLE_ENTRY_INFO_EX* Handles = pshi->Handles;
do
{
if (Handles->UniqueProcessId == UniqueProcessId &&
Handles->HandleValue == (ULONG_PTR)hProcess)
{
PVOID Object = Handles->Object;
Handles = pshi->Handles;
NumberOfHandles = pshi->NumberOfHandles;
do
{
if (Handles->Object == Object &&
Handles->UniqueProcessId != UniqueProcessId)
{
DbgPrint("%p %p %08x\n",
Handles->UniqueProcessId,
Handles->HandleValue,
Handles->GrantedAccess);
}
} while (Handles++, --NumberOfHandles);
break;
}
} while (Handles++, --NumberOfHandles);
}
}
delete [] buf;
}
} while (status == STATUS_INFO_LENGTH_MISMATCH);
CloseHandle(hProcess);
}
}
The code below does compile, but it doesn't run as it should.
I'm not sure what am I doing wrong, so would someone be willing to tell me what I did wrong and what I should have done better.
What do I need to change to make it run properly?
#include<stdio.h>
#include<stdlib.h>
typedef struct sub_Node
{
int value;
struct sub_Node *next;
}sub_Node;
typedef struct Node
{
char *name;
struct Node *next;
struct sub_Node *sub_start;
}Node;
Node *start;
void add_player(char *name)
{
Node *temp;
temp = (Node *)malloc(sizeof(Node));
temp->next = start;
temp->name = name;
temp->sub_start = (sub_Node *)malloc(sizeof(sub_Node));
temp->sub_start->next = NULL;
temp->sub_start->value = -1;
start = temp;
}
void initialize()
{
char *p;
p = "\0";
add_player(p);
}
void remove_player(char *name)
{
Node *p;
for(p = start; p!= NULL; p = p->next)
if(p->name == name)
{
p->name = p->next->name;
p->next = p->next->next;
}
}
sub_Node* add_descending(sub_Node* sub_start, int piece_value)
{
sub_Node *temp, *prev, *next;
temp = (sub_Node *)malloc(sizeof(sub_Node));
temp->value = piece_value;
temp->next = NULL;
prev = NULL;
next = sub_start;
while(next && next->value >= piece_value)
{
prev = next;
next = next->next;
}
if(!next)
{
prev->next = temp;
}
else
{
if(prev)
{
temp->next = prev->next;
prev->next = temp;
}
else
{
temp->next = sub_start;
sub_start = temp;
}
}
return sub_start;
}
void add_piece(char *name, int piece_value)
{
Node *p;
int c;
for(p = start; p!=NULL; p = p->next)
if(p->name == name)
p->sub_start = add_descending(p->sub_start, piece_value);
}
void print_pieces(char *name)
{
Node *p;
sub_Node *q;
for(p = start; p!=NULL; p = p->next)
if(p->name == name)
{
printf("The values of the owned pieces are:");
for(q = p->sub_start; q->value != -1; q = q->next)
printf(" %d", q->value);
}
}
int lose_piece(char *name)
{
Node *p;
sub_Node *q;
int aux;
for(p = start; p!=NULL; p = p->next)
if(p->name == name)
{
for(q = p->sub_start; q->next->value != -1; q = q->next) {}
aux = q->value;
q->value = q->next->value;
q->next = q->next->next;
return aux;
}
}
void print_players()
{
Node *p;
printf("The players are: ");
for(p = start; p->name != "\0"; p = p->next)
printf("%s ", p->name);
printf("\n");
}
int main()
{
initialize();
int y, value;
char name[20];
printf("Insert a digit to execute the desired task:\n"
"<0> end the program\n"
"<1> add a player, who doesn't own any piece yet\n"
"<2> remove a player and all his pieces\n"
"<3> print the name of all the players\n"
"<4> a player gets a piece\n"
"<5> a player loses the piece with the lowest value out of the ones that he has\n"
"<6> prints the pieces of a player in a descending order by value\n\n");
do
{
printf("digit: ");
scanf("%d", &y);
switch(y)
{
case 1:
printf("Insert the player's name: ");
scanf("%s", name);
add_player(name);
break;
case 2:
printf("Insert the player's name: ");
scanf("%s", name);
remove_player(name);
break;
case 3:
print_players();
break;
case 4:
printf("Insert the player's name: ");
scanf("%s", name);
printf("Insert the value of the piece: ");
scanf("%d", value);
add_piece(name, value);
break;
case 5:
printf("Insert the player's name: ");
scanf("%s", name);
printf("\nThe player loses the piece: %d\n", lose_piece(name));
break;
case 6:
printf("Insert the player's name: ");
scanf("%s", name);
print_pieces(name);
}
} while(y != 0);
return 0;
}
your two main problems where this scanf("%d", value); value should be passed by reference like this scanf("%d", &value); and the second is string comparison in c as in your code p->name != "\0" and if(p->name == name) this is wrong because actually you are making comparison between addresses of strings (where it resides in memory) not strings values. to compare strings in c you have to use strcmp and families.
Actually 3 main problems. for setting string values as you did in temp->name = name; is little bit more complicated than that. because you are assigning to temp->name a string from the stack that is volatile (the stack will be more likely invalid soon you return from the function) . in your case you have to alloc a new string by using malloc (and friends) or just by using strdup.
here is as a bonus a slightly rewrite of your program, you will find many advises and is a good starting point for how to structure your code for an easy maintenance.
still want to advise you to change members and variables to more declarative names as in sub_start and sub_Node can be PieceNode and pieces respectively.
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
typedef struct sub_Node
{
int value;
struct sub_Node *next;
}sub_Node;
typedef struct Node
{
char *name;
struct Node *next;
struct Node *prev; // this to make life easyer
struct sub_Node *sub_start;
}Node;
Node *start = NULL;
Node *find_player(char *name){
Node *tmp = start;
while( tmp ){
if(strcmp(tmp->name,name) == 0 )
break;
tmp = tmp->next;
}
return tmp;
}
// int to return Error Code
//
int add_player(char *name)
{
Node *temp;
if( find_player(name)) {
printf("player %s already exists\n", name);
return 1;
}
// do not cast malloc
temp = malloc(sizeof(Node));
if( !temp ){
printf ("not enough memory\n");
return 2;
}
temp->name = strdup ( name); // here was your error
temp->sub_start = NULL; // keep it simple
temp->prev = NULL;
temp->next = start;
if(start)
start->prev = temp;
start = temp;
return 0; // no error
}
void DestroyPieces(sub_Node* piece){
if( piece ) {
DestroyPieces( piece->next );
free( piece );
}
}
// as usual use int to return error code
int remove_player(char *name)
{
Node *player = find_player(name);
if ( !player ){
return 1; // player not found
}
if ( player->next ){
player->next->prev = player->prev;
}
if ( player->prev ){
player->prev->next = player->next;
}
DestroyPieces(player->sub_start);
free(player->name);
free(player);
return 0; // success
}
sub_Node* new_piece(int value){
sub_Node *temp = malloc( sizeof(sub_Node) );
if(temp){
temp->value = value;
temp->next = NULL;
}
return temp;
}
// int to return error code
// pass sub_start as pointer to pointer, as it might be updated
int add_descending(sub_Node** psub_start, int piece_value)
{
sub_Node *piece, *current, *prev = NULL;
if( !psub_start){
return 5; // this should not happen
}
current = *psub_start;
piece = new_piece( piece_value );
if( !piece ) return 1; // no mem
if(!current){
// this is the first and only one
*psub_start = piece;
return 0; // OK
}
while(current && current->value >= piece_value)
{
prev = current;
current = current->next;
}
if( prev )
prev->next = piece;
piece->next = current;
if( current == *psub_start ){
*psub_start = piece;
}
return 0 ; // OK
}
void add_piece(Node * player, int piece_value)
{
if ( !player) {
return ;
}
if(add_descending (&(player->sub_start), piece_value) == 0 )
return ; //OK
printf("an error occured while adding a piece (%d) to player '%s'\n",piece_value,player->name);
}
void print_pieces(Node *player)
{
sub_Node *q;
if( !player ){
return;
}
if( !player->sub_start ){
printf("Player '%s' has no pieces\n",player->name);
return;
}
printf("The values of the owned pieces are:");
for(q = player->sub_start; q != NULL; q = q->next)
printf(" %d", q->value);
printf("\n");
}
void lose_piece(Node *player)
{
if( !player ){
return;
}
sub_Node *q, *prev = NULL;
int aux;
if( !player->sub_start ){
printf("Player '%s' has no pieces\n",player->name);
return;
}
// i think you want drop the last one
for(q = player->sub_start; q->next != NULL ;prev = q, q = q->next) {
;
}
if(prev)
prev->next = NULL;
else
player->sub_start = NULL;
aux = q->value;
free(q);
printf("\nThe player loses the piece: %d\n", aux);
return;
}
void print_players()
{
Node *p;
if( !start ){
printf("there are no players, try to add some\n");
return;
}
printf("The players are: ");
for(p = start; p != NULL; p = p->next)
printf("%s ", p->name);
printf("\n");
}
void print_menu(void){
printf("Insert a digit to execute the desired task:\n"
"<0> end the program\n"
"<1> add a player, who doesn't own any piece yet\n"
"<2> remove a player and all his pieces\n"
"<3> print the name of all the players\n"
"<4> a player gets a piece\n"
"<5> a player loses the piece with the lowest value out of the ones that he has\n"
"<6> prints the pieces of a player in a descending order by value\n\n");
}
Node * get_player(char *name){
Node *player = find_player(name);
if(!player)
printf("Player '%s' do not exists\n",name);
return player;
}
int main()
{
// initialize(); no more needed
int y, value;
char name[20];
Node *player;
print_menu();
do
{
printf("digit: ");
scanf("%d", &y);
switch(y)
{
case 1:
printf("Insert the player's name: ");
scanf("%s", name);
add_player(name);
break;
case 2:
printf("Insert the player's name: ");
scanf("%s", name);
player = get_player(name);
if( player )
break;
case 3:
print_players();
break;
case 4:
printf("Insert the player's name: ");
scanf("%s", name);
player = get_player(name);
if( player ){
printf("Insert the value of the piece: ");
scanf("%d", &value);
add_piece(player, value);
}
break;
case 5:
printf("Insert the player's name: ");
scanf("%s", name);
player = get_player(name);
lose_piece(player);
break;
case 6:
printf("Insert the player's name: ");
scanf("%s", name);
player = get_player(name);
print_pieces(player);
}
} while(y != 0);
return 0;
}
I'm learning libev however the code is so hard to understand, so I choose to learn libevent first whose code is relatively clearer. But I encounter a problem when try the example (http://www.wangafu.net/~nickm/libevent-book/01_intro.html).
How is the code event_add(state->write_event, NULL) in do_read() make do_write() function invoked?
/* For sockaddr_in */
#include <netinet/in.h>
/* For socket functions */
#include <sys/socket.h>
/* For fcntl */
#include <fcntl.h>
#include <event2/event.h>
#include <assert.h>
#include <unistd.h>
#include <string.h>
#include <stdlib.h>
#include <stdio.h>
#include <errno.h>
#define MAX_LINE 16384
void do_read(evutil_socket_t fd, short events, void *arg);
void do_write(evutil_socket_t fd, short events, void *arg);
char
rot13_char(char c)
{
return c;
/* We don't want to use isalpha here; setting the locale would change
* which characters are considered alphabetical. */
if ((c >= 'a' && c <= 'm') || (c >= 'A' && c <= 'M'))
return c + 13;
else if ((c >= 'n' && c <= 'z') || (c >= 'N' && c <= 'Z'))
return c - 13;
else
return c;
}
struct fd_state {
char buffer[MAX_LINE];
size_t buffer_used;
size_t n_written;
size_t write_upto;
struct event *read_event;
struct event *write_event;
};
struct fd_state *
alloc_fd_state(struct event_base *base, evutil_socket_t fd)
{
struct fd_state *state = malloc(sizeof(struct fd_state));
if (!state)
return NULL;
state->read_event = event_new(base, fd, EV_READ|EV_PERSIST, do_read, state);
if (!state->read_event) {
free(state);
return NULL;
}
state->write_event =
event_new(base, fd, EV_WRITE|EV_PERSIST, do_write, state);
if (!state->write_event) {
event_free(state->read_event);
free(state);
return NULL;
}
state->buffer_used = state->n_written = state->write_upto = 0;
assert(state->write_event);
return state;
}
void
free_fd_state(struct fd_state *state)
{
event_free(state->read_event);
event_free(state->write_event);
free(state);
}
void
do_read(evutil_socket_t fd, short events, void *arg)
{
struct fd_state *state = arg;
char buf[1024];
int i;
ssize_t result;
while (1) {
assert(state->write_event);
result = recv(fd, buf, sizeof(buf), 0);
if (result <= 0)
break;
for (i=0; i < result; ++i) {
if (state->buffer_used < sizeof(state->buffer))
state->buffer[state->buffer_used++] = rot13_char(buf[i]);
if (buf[i] == '\n') {
assert(state->write_event);
**event_add(state->write_event, NULL);**
state->write_upto = state->buffer_used;
}
}
}
if (result == 0) {
free_fd_state(state);
} else if (result < 0) {
if (errno == EAGAIN) // XXXX use evutil macro
return;
perror("recv");
free_fd_state(state);
}
}
void
**do_write(evutil_socket_t fd, short events, void *arg)**
{
struct fd_state *state = arg;
while (state->n_written < state->write_upto) {
ssize_t result = send(fd, state->buffer + state->n_written,
state->write_upto - state->n_written, 0);
if (result < 0) {
if (errno == EAGAIN) // XXX use evutil macro
return;
free_fd_state(state);
return;
}
assert(result != 0);
state->n_written += result;
}
if (state->n_written == state->buffer_used)
state->n_written = state->write_upto = state->buffer_used = 1;
event_del(state->write_event);
}
void
do_accept(evutil_socket_t listener, short event, void *arg)
{
struct event_base *base = arg;
struct sockaddr_storage ss;
socklen_t slen = sizeof(ss);
int fd = accept(listener, (struct sockaddr*)&ss, &slen);
if (fd < 0) { // XXXX eagain??
perror("accept");
} else if (fd > FD_SETSIZE) {
close(fd); // XXX replace all closes with EVUTIL_CLOSESOCKET */
} else {
struct fd_state *state;
evutil_make_socket_nonblocking(fd);
state = alloc_fd_state(base, fd);
assert(state); /*XXX err*/
assert(state->write_event);
event_add(state->read_event, NULL);
}
}
void
run(void)
{
evutil_socket_t listener;
struct sockaddr_in sin;
struct event_base *base;
struct event *listener_event;
base = event_base_new();
if (!base)
return; /*XXXerr*/
sin.sin_family = AF_INET;
sin.sin_addr.s_addr = 0;
sin.sin_port = htons(40713);
listener = socket(AF_INET, SOCK_STREAM, 0);
evutil_make_socket_nonblocking(listener);
#ifndef WIN32
{
int one = 1;
setsockopt(listener, SOL_SOCKET, SO_REUSEADDR, &one, sizeof(one));
}
#endif
if (bind(listener, (struct sockaddr*)&sin, sizeof(sin)) < 0) {
perror("bind");
return;
}
if (listen(listener, 16)<0) {
perror("listen");
return;
}
listener_event = event_new(base, listener, EV_READ|EV_PERSIST, do_accept, (void*)base);
/*XXX check it */
event_add(listener_event, NULL);
event_base_dispatch(base);
}
int
main(int c, char **v)
{
setvbuf(stdout, NULL, _IONBF, 0);
run();
return 0;
}
I'm not sure if I'm answering the same question you asked - I understand it as:
How does calling event_add(state->write_event, NULL) in do_read() lead to do_write() being invoked?
The key to figuring this out is understanding what the do_read() function is actually doing. do_read() is a callback function associated with a socket which has data to be read: this is set up with allocate_fd_state():
struct fd_state *
alloc_fd_state(struct event_base *base, evutil_socket_t fd)
{
/*
* Allocate a new fd_state structure, which will hold our read and write events
* /
struct fd_state *state = malloc(sizeof(struct fd_state));
[...]
/*
* Initialize a read event on the given file descriptor: associate the event with
* the given base, and set up the do_read callback to be invoked whenever
* data is available to be read on the file descriptor.
* /
state->read_event = event_new(base, fd, EV_READ|EV_PERSIST, do_read, state);
[...]
/*
* Set up another event on the same file descriptor and base, which invoked the
* do_write callback anytime the file descriptor is ready to be written to.
*/
state->write_event =
event_new(base, fd, EV_WRITE|EV_PERSIST, do_write, state);
[...]
return state;
}
At this point, though, neither of these events have been event_add()'ed to the event_base base. The instructions for what to do are all written out, but no one is looking at them. So how does anything get read? state->read_event is event_add()'ed to the base after an incoming connection is made. Look at do_accept():
void
do_accept(evutil_socket_t listener, short event, void *arg)
{
[ ... accept a new connection and give it a file descriptor fd ... ]
/*
* If the file descriptor is invalid, close it.
*/
if (fd < 0) { // XXXX eagain??
perror("accept");
} else if (fd > FD_SETSIZE) {
close(fd); // XXX replace all closes with EVUTIL_CLOSESOCKET */
/*
* Otherwise, if the connection was successfully accepted...
*/
} else {
[ ... allocate a new fd_state structure, and make the file descriptor non-blocking ...]
/*
* Here's where the magic happens. The read_event created back in alloc_fd_state()
* is finally added to the base associated with it.
*/
event_add(state->read_event, NULL);
}
}
So right after accepting a new connection, the program tells libevent to wait until there's data available on the connection, and then run the do_read() callback. At this point, it's still impossible for do_write() to be called. It needs to be event_add()'ed. This happens in do_read():
void
do_read(evutil_socket_t fd, short events, void *arg)
{
/* Create a temporary buffer to receive some data */
char buf[1024];
while (1) {
[ ... Receive the data, copying it into buf ... ]
[ ... if there is no more data to receive, or there was an error, exit this loop... ]
[ ... else, result = number of bytes received ... ]
for (i=0; i < result; ++i) {
[ ... if there's room in the buffer, copy in the rot13() encoded
version of the received data ... ]
/*
* Boom, headshot. If we've reached the end of the incoming data
* (assumed to be a newline), then ...
*/
if (buf[i] == '\n') {
[...]
/*
* Have libevent start monitoring the write_event, which calls do_write
* as soon as the file descriptor is ready to be written to.
*/
event_add(state->write_event, NULL);
[...]
}
}
}
[...]
}
So, after reading in some data from a file descriptor, the program starts waiting until
the file descriptor is ready to be written to, and then invokes do_write(). Program
flow looks like this:
[ set up an event_base and start waiting for events ]
[ if someone tries to connect ]
[ accept the connection ]
[ ... wait until there is data to read on the connection ... ]
[ read in data from the connection until there is no more left ]
[ ....wait until the connection is ready to be written to ... ]
[ write out our rot13() encoded response ]
I hope that a) that was the correct interpretation of your question, and b) this was a helpful answer.
I have a short program that generates a linked list by adding nodes to it, then frees the memory allocated by the linked list.
Valgrind does not report any memory leak errors, but the process continues to hold the allocated memory.
I was only able to fix the error after I changed the memory allocated from sizeof(structure_name) to fixed number 512. (see commented code)
Is this a bug or normal operation?
Here is the code:
#include <execinfo.h>
#include <stdlib.h>
#include <stdio.h>
typedef struct llist_node {
int ibody;
struct llist_node * next;
struct llist_node * previous;
struct llist * list;
}llist_node;
typedef struct llist {
struct llist_node * head;
struct llist_node * tail;
int id;
int count;
}llist;
llist_node * new_lnode (void) {
llist_node * nnode = (llist_node *) malloc ( 512 );
// llist_node * nnode = (llist_node *) malloc ( sizeof(llist_node) );
nnode->next = NULL;
nnode->previous = NULL;
nnode->list = NULL;
return nnode;
}
llist * new_llist (void) {
llist * nlist = (llist *) malloc ( 512 );
// llist * nlist = (llist *) malloc ( sizeof(llist) );
nlist->head = NULL;
nlist->tail = NULL;
nlist->count = 0;
return nlist;
}
void add_int_tail ( int ibody, llist * list ) {
llist_node * nnode = new_lnode();
nnode->ibody = ibody;
list->count++;
nnode->next = NULL;
if ( list->head == NULL ) {
list->head = nnode;
list->tail = nnode;
}
else {
nnode->previous = list->tail;
list->tail->next = nnode;
list->tail = nnode;
}
}
void destroy_list_nodes ( llist_node * nodes ) {
llist_node * llnp = NULL;
llist_node * llnpnext = NULL;
llist_node * llnp2 = NULL;
if ( nodes == NULL )
return;
for ( llnp = nodes; llnp != NULL; llnp = llnpnext ) {
llnpnext = llnp->next;
free (llnp);
}
return;
}
void destroy_list ( llist * list ) {
destroy_list_nodes ( list->head );
free (list);
}
int main () {
int i = 0;
int j = 0;
llist * list = new_llist ();
for ( i = 0; i < 100; i++ ) {
for ( j = 0; j < 100; j++ ) {
add_int_tail ( i+j, list );
}
}
printf("enter to continue and free memory...");
getchar();
destroy_list ( list );
printf("memory freed. enter to exit...");
getchar();
printf( "\n");
return 0;
}
If by "the process continues to hold the allocated memory" you mean that ps doesn't report a decrease in the process's memory usage, that's perfectly normal. Returning memory to your process's heap doesn't necessarily make the process return it to the operating system, for all sorts of reasons. If you create and destroy your list over and over again, in a big loop, and the memory usage of your process doesn't grow without limit, then you probably haven't got a real memory leak.
[EDITED to add: See also Will malloc implementations return free-ed memory back to the system? ]
[EDITED again to add: Incidentally, the most likely reason why allocating 512-byte blocks makes the problem go away is that your malloc implementation treats larger blocks specially in some way that makes it easier for it to notice when there are whole pages that are no longer being used -- which is necessary if it's going to return any memory to the OS.]
I discovered the answer to my question here:
http://linuxupc.upc.es/~pep/OLD/man/malloc.html
The memory after expanding the heap can be returned back to kernel if the conditions configured by __noshrink are satisfied. Only then the ps will notice that the memory is freed.
It is important to configure it sometimes particularly when the memory usage is small, but the heap size is bigger than the main memory available. Thus the program trashes even if the required memory is less than the available main memory.