C语言 使用信号量的未定义引用问题

Undefined Reference issues using Semaphores

提问by TheFatness

I am playing around with using Semaphores, but I keep encountering Undefined Reference warnings, thus causing my code not to work. I pulled example code from a text, but was having issues with some of their syntax, so I went to POSIX's semaphore tutorial and changed things around to their syntax and as a result am now getting these reference errors.

我正在尝试使用信号量，但我不断遇到未定义的引用警告，从而导致我的代码无法工作。我从文本中提取了示例代码，但是他们的一些语法有问题，所以我去了 POSIX 的信号量教程，并改变了他们的语法，结果我现在得到了这些参考错误。

I may simply be overlooking something, but I cannot find it.

我可能只是忽略了一些东西，但我找不到它。

Errors:

错误：

Producers_Consumers.c:52: warning: return type of ‘main' is not ‘int'
/tmp/cceeOM6F.o: In function `producer':
Producers_Consumers.c:(.text+0x1e): undefined reference to `sem_init'
Producers_Consumers.c:(.text+0x3a): undefined reference to `sem_init'
Producers_Consumers.c:(.text+0x46): undefined reference to `sem_wait'
Producers_Consumers.c:(.text+0x52): undefined reference to `sem_wait'
Producers_Consumers.c:(.text+0x5e): undefined reference to `sem_post'
Producers_Consumers.c:(.text+0x6a): undefined reference to `sem_post'
/tmp/cceeOM6F.o: In function `consumer':
Producers_Consumers.c:(.text+0x7e): undefined reference to `sem_wait'
Producers_Consumers.c:(.text+0x8a): undefined reference to `sem_wait'
Producers_Consumers.c:(.text+0x96): undefined reference to `sem_post'
Producers_Consumers.c:(.text+0xa2): undefined reference to `sem_post'
collect2: ld returned 1 exit status

What I have (It may look a bit ugly due to the way I commented things out from my old method) I also know my adding method won't work, but I'll get to that when I fix my syntax issues:

我所拥有的（由于我从旧方法中注释出来的方式，它可能看起来有点难看）我也知道我的添加方法不起作用，但是当我修复语法问题时我会解决这个问题：

#include <stdio.h>
#include <semaphore.h>
#include <string.h>
#include <pthread.h>
#include <unistd.h>
#include <stdlib.h>
#include <sys/types.h>
#include <sys/wait.h>
#include <errno.h>

#define N 10     //Number of slots in buffer
typedef int semaphore;  //Semaphores ae a special kind of int
sem_t mutex; //Controls access to critical region 1
sem_t empty;  //Counts empty buffer slots N
sem_t  full;  //Counts full buffer slots 0
int count = 0; //What we're putting in 
//int buffer[N];

void producer(void) {
    sem_init(&mutex, 0, 1);
    //sem_init(&empty, 0, N);
    sem_init(&full, 0, 0);

    while(1) { 
        sem_wait(&empty);
        sem_wait(&mutex);
        //printf("Empy: %d\n",empty);
        //printf("Mutex: %d\n",mutex);
        //printf("Both Downs Ran\n");
        //buffer = buffer + 1;
        sem_post(&mutex);
        sem_post(&full);
        //printf("Producer produced: %d\n",buffer);
    }
}

void consumer(void) {
    while(1) { 
        sem_wait(&full);
        sem_wait(&mutex);
        //item = buffer;
        sem_post(&mutex);
        sem_post(&empty);
        //printf("Consumer consumed: %d/n",item);
    }
}

void main() {

}

回答by Ryan Haining

If you are on a linux system, you'll need to compile and link with the -pthreadflag to link the pthreads library.

如果您使用的是 linux 系统，则需要编译和链接-pthread标志以链接 pthreads 库。

gcc -pthread Producers_Consumers.c

As Paul Griffiths has pointed out, you can also use -lrt, which is more portable, and links the POSIX Realtime Extensions library

正如 Paul Griffiths 指出的那样，您还可以使用-lrt更便携的 ，并链接 POSIX Realtime Extensions 库

gcc Producers_Consumers.c -lrt

---

---

• int main(void)not void main()
• typedef int semaphoreis wrong, sem_tshould be treated as an opaque type, you never use this typedef in your code anyway.
• A problem I foresee is that your consumercode uses the semaphores before they are initialized in producer. You should initialize them in your main

• int main(void)不是 void main()
• typedef int semaphore是错误的，sem_t应该被视为不透明类型，无论如何你永远不会在你的代码中使用这个 typedef。
• 我预见的一个问题是您的consumer代码在producer. 你应该在你的main

回答by Alexey

Got same error in ubuntu qt. After adding

在 ubuntu qt 中遇到同样的错误。添加后

LIBS += -lpthread -lrt

LIBS += -lpthread -lrt

to project.pro file all compiled fine.

到 project.pro 文件都编译得很好。