使用pthread的C语言中的读取器/写入器问题
[英]Reader/Writer problem in C using pthreads
问题描述
当我遇到C语言中的读取器/写入器问题时，我正在学习pthread。问题非常简单，其中“写入器”线程将从外部源访问数据，而“读取器”线程将从共享缓冲区读取数据然后对数据进行伪处理。 读取器和写入器线程需要在while循环中连续运行。
我正在尝试使用POSIX接口在标准的UNIX系统上实现和编译此文件。
我已经看过一些堆栈溢出问题：
使用互斥和pthread的C语言中的Reader Writer程序
而我却一无所获。
这是我到目前为止的内容：
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <unistd.h>
#include <pthread.h>
#include <semaphore.h>
#include <assert.h>
#define BUFF_SIZE 50
#define M 10
#define N 20
int get_external_data(char *buffer, int bufferSizeInBytes);
void process_data(char *buffer, int bufferSizeInBytes);
int get_external_data(char *buffer, int bufferSizeInBytes){
int status;
int val;
char srcString[] = "0123456789abcdefghijklmnopqrstuvwxyxABCDEFGHIJKLMNOPQRSTUVWXYZ";
val = (int)(rand() % 62);
if (bufferSizeInBytes < val){
return (-1);
}
strncpy(buffer, srcString, val);
return val;
}
void process_data(char *buffer, int bufferSizeInBytes){
int i;
if(buffer) {
printf("thread %li - ", pthread_self());
for(i = 0; i < bufferSizeInBytes; i++) {
printf("%c", buffer[i]);
}
printf("\n");
memset(buffer, 0, bufferSizeInBytes);
} else {
printf("error in process data - %li\n", pthread_self());
}
return;
}
pthread_mutex_t data_lock = PTHREAD_MUTEX_INITIALIZER;
pthread_mutex_t print_lock = PTHREAD_MUTEX_INITIALIZER;
sem_t data_count;
typedef struct node {
struct node *next;
char *data;
int length;
} node_t;
node_t *head, *tail;
/**
* This thread is responsible for pulling data off of the shared data
* area and processing it using the process_data() API.
*/
void *reader_thread(void *arg) {
int rc;
node_t *removed_node;
while(1) {
rc = sem_wait(&data_count);
if (0 != rc) {
return NULL;
}
pthread_mutex_lock(&data_lock);
assert(NULL != head);
removed_node = head;
head = head->next;
pthread_mutex_unlock(&data_lock);
//Adding this lock for sake of readability at the cost of reduced consumption rate...will run out of memory eventually.
pthread_mutex_lock(&print_lock);
process_data(removed_node->data, removed_node->length);
pthread_mutex_unlock(&print_lock);
free(removed_node->data);
free(removed_node);
}
return NULL;
}
/**
* This thread is responsible for pulling data from a device using
* the get_external_data() API and placing it into a shared area
* for later processing by one of the reader threads.
*/
void *writer_thread(void *arg) {
int length;
char *buffer;
node_t *new_node;
new_node = (node_t*) malloc(sizeof(*new_node));
buffer = (char*) malloc(sizeof(*buffer) * BUFF_SIZE);
while(1) {
length = get_external_data(buffer, BUFF_SIZE);
if (length == -1) {
//data too big, discard it and try again;
continue;
}
new_node->next = NULL;
new_node->length = length;
new_node->data = buffer;
pthread_mutex_lock(&data_lock);
if (head == NULL) { //The linked list is completely empty
head = new_node;
tail = new_node;
} else { //There are items in the list and we're appending
tail->next = new_node;
tail = new_node;
}
pthread_mutex_unlock(&data_lock);
pthread_mutex_lock(&print_lock);
printf("thread %ld wrote - %s \n", pthread_self(), buffer);
pthread_mutex_unlock(&print_lock);
sem_post(&data_count);
buffer = (char*) malloc(sizeof(*buffer) * BUFF_SIZE);
new_node = (node_t*) malloc(sizeof(*new_node));
}
return NULL;
}
int main(int argc, char **argv) {
int i = sem_init(&data_count, 0, 0);
pthread_t dummy; //creating a dummy thread
for(i = 0; i < N; i++) {
pthread_create(&dummy, NULL, reader_thread, NULL);
}
for(i = 0; i < M; i++) {
pthread_create(&dummy, NULL, writer_thread, NULL);
}
sleep(100);
return 0;
}
那里有线程同步，没有任何编译错误，但是在编写器线程在缓冲区上写了几次之后，我的程序停止了。
读取器和写入器线程需要在while循环中连续运行，但在我看来，它们不是必需的。
关于如何解决此问题的任何想法？
1 个解决方案
解决方案1
1 2019-04-17 15:50:37
使用gcc添加-lpthread选项以与pthread库链接，我也鼓励您添加-pedantic -Wextra -Wall选项
您的程序中有几个问题，有些是编译器指出的
在：
printf("thread %i - ", pthread_self()); printf("error in process data - %i\\n", pthread_self());
在POSIX标准中，不需要pthread_t是算术类型，因此它可以是struct等，并且不能将其写为int 。 在您的情况下，它可能是一个int （很可能是unsigned long ），但这不是可移植的，因此最好自己管理与每个线程关联的标识符。
在reader_thread ：
return;
但是函数返回一个void* ，用return NULL;代替return NULL; 例如
在writer_thread ：
printf("thread %d wrote - %s", buffer);
有三个问题：
- 类型为int的参数丢失（可能是您想要
pthread_self()） - 缓冲区是由
get_external_data使用strncpy设置的，因此不存在空结束符，但printf％s必需（当缺少的参数将被添加或％d被删除时） - 缓冲区可以通过释放
reader_thread，也许你认为它是由信号量保护，但这情况并非如此
在get_external_data 状态未使用
在主要
usleep(100);
这是给线程工作的非常短的时间，实际上您只想被阻塞，因此可以将其替换为pthread_join(dummy, NULL);
有线程标识符的建议：在堆中分配一个int ，为它设置一个唯一的数字，然后将in参数提供给创建的线程：
int main() {
...
for(i = 0; i < N; i++) {
int * m = malloc(sizeof(int));
*m = i;
pthread_create(&dummy, NULL, reader_thread, m);
}
for(i = 0; i < M; i++) {
int * m = malloc(sizeof(int));
*m = 100 + i;
pthread_create(&dummy, NULL, writer_thread, m);
}
...
}
和
void *writer_thread(void *arg) {
int id = *((int*) arg);
...
free((int*) arg);
...
printf("thread %d wrote - %s", id, buffer);
...
}
和
void *reader_thread(void *arg) {
int id = *((int*) arg);
...
free((int*) arg);
...
process_data(removed_node->data, removed_node->length, id);
...
}
和
void process_data(char *buffer, int bufferSizeInBytes, int id){
...
printf("thread %i - ", id);
...
printf("error in process data - %i\n", id);
...
}
在上次更改之后，由于其他问题，在valgrind下执行的操作会出现错误：
pi@raspberrypi:/tmp $ gcc -g -pedantic -Wextra -Wall t.c -lpthread
pi@raspberrypi:/tmp $ valgrind ./a.out
==3847== Memcheck, a memory error detector
==3847== Copyright (C) 2002-2017, and GNU GPL'd, by Julian Seward et al.
==3847== Using Valgrind-3.13.0 and LibVEX; rerun with -h for copyright info
==3847== Command: ./a.out
==3847==
==3847== Thread 22:
==3847== Conditional jump or move depends on uninitialised value(s)
==3847== at 0x484B20C: strlen (vg_replace_strmem.c:458)
==3847== by 0x48FD68F: vfprintf (vfprintf.c:1637)
==3847== by 0x4902ADF: printf (printf.c:33)
==3847== by 0x10B83: writer_thread (t.c:144)
==3847== by 0x4898FC3: start_thread (pthread_create.c:458)
==3847== by 0x498D037: ??? (clone.S:76)
==3847==
thread 100 wrote - 0123456789abcdefghijklmnopqrstuvwxyxABCDEthread 103 wrote - 0123456789abcdefghijklmnopqrstuvwxyxthread 1 - 0123456789abcdefghijklmnopqrstuvwxyxABCDE
thread 0 - 0123456789abcdefghijklmnopqrstuvwxyx
thread 102 wrote - 0123456789abcdefghijklmnopqrstuvwxyxABthread 103 wrote - 0123456thread 2 - 012
thread 101 wrote - 0123456789abcdefghithread 105 wrote - 0123456789abcdefghijklmnopqrstuvwxyxABCDEFGHIJKLMthread 3 - 0123456
thread 109 wrote - 0123456789abcdefghijklmnopqrstuvwxyxAthread 3 - 0123456789abcdefghi
==3847== Invalid read of size 1
==3847== at 0x484B1EC: strlen (vg_replace_strmem.c:458)
==3847== by 0x48FD68F: vfprintf (vfprintf.c:1637)
==3847== by 0x4902ADF: printf (printf.c:33)
==3847== by 0x10B83: writer_thread (t.c:144)
==3847== by 0x4898FC3: start_thread (pthread_create.c:458)
==3847== by 0x498D037: ??? (clone.S:76)
==3847== Address 0x49fcd18 is 0 bytes inside a block of size 50 free'd
==3847== at 0x4848B8C: free (vg_replace_malloc.c:530)
==3847== Block was alloc'd at
==3847== at 0x4847568: malloc (vg_replace_malloc.c:299)
==3847==
thread 107 wrote - 0123456789abcdefgthread 100 wrote - thread 100 wrote - 0123456789athread 101 wrote - 0123456789abcthread 7 - 0123456789abcdefghijklmnopqrstuvwxyxABCDEFGHIJKLM
thread 7 - 0123456789abcdefghijklmnopqrstuvwxyxAB
thread 9 - 0123456789abc
thread 9 - 0123456789abcdefghijklmnopqrstuvwxyxA
thread 108 wrote - 0123456789abcdefghijklmnopqrstuvwxyxABthread 105 wrote - 01thread 8 - 0123456789ab
thread 108 wrote - 0123456789abcdefghithread 12 - 0123
thread 103 wrote - 0123456789abcdefghijklmnopqrstuvwxyxABCDEFGthread 101 wrote - 0123456789abcdefghijklmnopqrstuvthread 103 wrote - 0123456789abcdefghijklmnopqrstuvwxyxABCDEFGthread 101 wrote - 0123456789abcdefghijklmnopqrstuvthread 14 - 0123456789a
thread 101 wrote - 0123456789abcdefghijklthread 101 wrote - 0123456789abcdefghijklmnopqrstuvwxyxthread 101 wrote - 0123456789abcdefghijklmnopqrstuvwxyxABCDEFGHthread 101 wrote - 0123456789abcdefghijklmnopqthread 101 wrote - 0123456789abcdefghijklmnopqrstuthread 101 wrote - 0123thread 102 wrote - thread 2 - 0123456789abcdefghijklmnopqrstuv
thread 2 - 0123456789abcdefghijklmnopqr
thread 2 - 0123456789abcdefghi
thread 2 - 0123456789abcdefghijklmnopqrstuvwxyxABCDEFG
thread 100 wrote - 0123456789abcthread 100 wrote - 0123456789abcdefghijklmnopqrstuvwxyxABCDEFGHIJKthread 4 - 0123456789abcdefghijkl
thread 109 wrote - thread 13 - 0123456789abcdefghijklmnopqrstuvwxyxAB
thread 108 wrote - 0123456789abcdefghijklmnopqrstuvwxyxABCDEFGHIthread 19 - 0123456789abc
thread 101 wrote - 0thread 18 - 01
thread 108 wrote - 0123456thread 108 wrote - 01thread 108 wrote - 0123456789abcdefghijthread 10 - 0123456789abcdefghijklmnopq
thread 8 - 0123456789abcdefghijklmnopqrstu
==3847== Thread 30:
==3847== Conditional jump or move depends on uninitialised value(s)
==3847== at 0x484B1F4: strlen (vg_replace_strmem.c:458)
==3847== by 0x48FD68F: vfprintf (vfprintf.c:1637)
==3847== by 0x4902ADF: printf (printf.c:33)
==3847== by 0x10B83: writer_thread (t.c:144)
==3847== by 0x4898FC3: start_thread (pthread_create.c:458)
==3847== by 0x498D037: ??? (clone.S:76)
==3847==
==3847== Conditional jump or move depends on uninitialised value(s)
==3847== at 0x48FBEEC: vfprintf (vfprintf.c:1637)
==3847== by 0x4902ADF: printf (printf.c:33)
==3847== by 0x10B83: writer_thread (t.c:144)
==3847== by 0x4898FC3: start_thread (pthread_create.c:458)
==3847== by 0x498D037: ??? (clone.S:76)
==3847==
==3847== Conditional jump or move depends on uninitialised value(s)
==3847== at 0x48FBF0C: vfprintf (vfprintf.c:1637)
==3847== by 0x4902ADF: printf (printf.c:33)
==3847== by 0x10B83: writer_thread (t.c:144)
==3847== by 0x4898FC3: start_thread (pthread_create.c:458)
==3847== by 0x498D037: ??? (clone.S:76)
==3847==
==3847== Conditional jump or move depends on uninitialised value(s)
==3847== at 0x49245B8: _IO_file_xsputn@@GLIBC_2.4 (fileops.c:1294)
==3847== by 0x48FBF7B: vfprintf (vfprintf.c:1637)
==3847== by 0x4902ADF: printf (printf.c:33)
==3847== by 0x10B83: writer_thread (t.c:144)
==3847== by 0x4898FC3: start_thread (pthread_create.c:458)
==3847== by 0x498D037: ??? (clone.S:76)
==3847==
==3847== Conditional jump or move depends on uninitialised value(s)
==3847== at 0x48FBF80: vfprintf (vfprintf.c:1637)
==3847== by 0x4902ADF: printf (printf.c:33)
==3847== by 0x10B83: writer_thread (t.c:144)
==3847== by 0x4898FC3: start_thread (pthread_create.c:458)
==3847== by 0x498D037: ??? (clone.S:76)
==3847==
==3847== Conditional jump or move depends on uninitialised value(s)
==3847== at 0x48FBF90: vfprintf (vfprintf.c:1637)
==3847== by 0x4902ADF: printf (printf.c:33)
==3847== by 0x10B83: writer_thread (t.c:144)
==3847== by 0x4898FC3: start_thread (pthread_create.c:458)
==3847== by 0x498D037: ??? (clone.S:76)
==3847==
==3847== Conditional jump or move depends on uninitialised value(s)
==3847== at 0x48FBE24: vfprintf (vfprintf.c:1668)
==3847== by 0x4902ADF: printf (printf.c:33)
==3847== by 0x10B83: writer_thread (t.c:144)
==3847== by 0x4898FC3: start_thread (pthread_create.c:458)
==3847== by 0x498D037: ??? (clone.S:76)
==3847==
==3847== Conditional jump or move depends on uninitialised value(s)
==3847== at 0x48FBE6C: vfprintf (vfprintf.c:1668)
==3847== by 0x4902ADF: printf (printf.c:33)
==3847== by 0x10B83: writer_thread (t.c:144)
==3847== by 0x4898FC3: start_thread (pthread_create.c:458)
==3847== by 0x498D037: ??? (clone.S:76)
==3847==
thread 108 wrote - thread 108 wrote - 0123456789abcdefghijkthread 12 - 0123456789abcdefghijklmnopqrstuvwxyxABCDEFGHIJKL
thread 108 wrote - 0123456789abcdefghijthread 106 wrote - thread 5 - 0123456789abcdefg
thread 106 wrote - thread 106 wrote - 0123456789abcdefghijklmnopqrstuvwxyxABCDthread 16 - 0123456789abcdefghijklmnopqrstuvwx
thread 16 - 0123456789abcdefghijklmnopqrstuv
==3847==
==3847== HEAP SUMMARY:
==3847== in use at exit: 5,444 bytes in 74 blocks
==3847== total heap usage: 155 allocs, 81 frees, 8,138 bytes allocated
==3847==
==3847== LEAK SUMMARY:
==3847== definitely lost: 0 bytes in 0 blocks
==3847== indirectly lost: 0 bytes in 0 blocks
==3847== possibly lost: 4,080 bytes in 30 blocks
==3847== still reachable: 1,364 bytes in 44 blocks
==3847== suppressed: 0 bytes in 0 blocks
==3847== Rerun with --leak-check=full to see details of leaked memory
==3847==
==3847== For counts of detected and suppressed errors, rerun with: -v
==3847== Use --track-origins=yes to see where uninitialised values come from
==3847== ERROR SUMMARY: 51 errors from 10 contexts (suppressed: 6 from 3)
使用C中posix信号量的读写器应用中的分段错误
[英]Segmentation fault in reader-writer appilication using posix semaphore in c
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