我正在尝试学习C语言的读/写程序，我在网上找到了一个我认为应该开始学习的例子。然而，当我尝试编译代码时，我收到了一些警告和一个类似的错误：
read.c:21:8: error: conflicting types for ‘write’
void * write(void *temp) {
^
In file included from read.c:11:0:
/usr/include/unistd.h:369:16: note: previous declaration of ‘write’ was here
extern ssize_t write (int __fd, const void *__buf, size_t __n) __wur;
^
read.c: In function ‘write’:
read.c:25:4: warning: assignment makes pointer from integer without a cast [-Wint-conversion]
ret=pthread_rwlock_wrlock(&rwlock);
^
read.c: In function ‘write_2’:
read.c:45:4: warning: assignment makes pointer from integer without a cast [-Wint-conversion]
ret=pthread_rwlock_wrlock(&rwlock);
^
read.c: At top level:
read.c:106:1: warning: return type defaults to ‘int’ [-Wimplicit-int]
main() {
我还是C编程的新手，我不知道如何解决这些警告和错误，因为我只是在不太了解语言的情况下跳到这个主题。这是我试图编译的代码。
#include <stdio.h>
#include <pthread.h>
#include <stdlib.h>
#include <unistd.h>
/*
From the output we can see that the two writes were executed one after the other. But in case of reads, even though read_1 had not unlocked the rwlock, read_2 was allowed into the critical section and read the file. That shows us that multiple readers are allowed but only one writer is allowed into the critical section.
*/
pthread_rwlock_t rwlock; // allows multiple readers to access the resource, but only one reader at any given time.
void * write(void *temp) {
char *ret;
FILE *file1;
char *str;
ret=pthread_rwlock_wrlock(&rwlock);
printf("nFile locked, please enter the message n");
str=(char *)malloc(10*sizeof(char));
file1=fopen("temp","w");
scanf("%s",str);
fprintf(file1,"%s",str);
fclose(file1);
pthread_rwlock_unlock(&rwlock);
printf("nUnlocked the file you can read it now n");
return ret;
}
void * write_2(void *temp) {
char *ret;
FILE *file1;
char *str;
sleep(3);
ret=pthread_rwlock_wrlock(&rwlock);
printf("nFile locked, please enter the message n");
str=(char *)malloc(10*sizeof(char));
file1=fopen("temp","a");
scanf("%s",str);
fprintf(file1,"%s",str);
fclose(file1);
pthread_rwlock_unlock(&rwlock);
printf("nUnlocked the file you can read it now n");
return ret;
}
void * read_1(void *temp) {
char *ret;
FILE *file1;
char *str;
sleep(5);
pthread_rwlock_rdlock(&rwlock);
printf("n1 Opening file for readingn");
file1=fopen("temp","r");
str=(char *)malloc(10*sizeof(char));
fscanf(file1,"%s",str);
printf("nMessage from file is %s n",str);
sleep(3);
fclose(file1);
printf("nUnlocking rwlockn");
pthread_rwlock_unlock(&rwlock);
return ret;
}
void * read_2(void *temp) {
char *ret;
FILE *file1;
char *str;
sleep(6);
pthread_rwlock_rdlock(&rwlock);
printf("n2 Opening file for readingn");
file1=fopen("temp","r");
str=(char *)malloc(10*sizeof(char));
fscanf(file1,"%s",str);
printf("nMessage from file is %s n",str);
fclose(file1);
pthread_rwlock_rdlock(&rwlock);
return ret;
}
main() {
pthread_t thread_id,thread_id1,thread_id3,thread_id4;
pthread_attr_t attr;
int ret;
void *res;
pthread_rwlock_init(&rwlock,NULL);
ret=pthread_create(&thread_id,NULL,&write,NULL);
ret=pthread_create(&thread_id1,NULL,&read_1,NULL);
ret=pthread_create(&thread_id3,NULL,&read_2,NULL);
ret=pthread_create(&thread_id4,NULL,&write_2,NULL);
printf("n Created thread");
pthread_join(thread_id,&res);
pthread_join(thread_id1,&res);
pthread_join(thread_id3,&res);
pthread_join(thread_id4,&res);
pthread_rwlock_destroy(&rwlock);
}
问题是为什么这些警告和这个错误会出现在这个代码中？
附言：不知道为什么，但当我在收到这些警告和错误后尝试运行它时，它仍然按照我想象的方式运行。这很奇怪。感谢您的阅读。
write是系统调用的名称。函数的声明存在于<unistd.h>中，您已将其包含在C程序中。
您的C程序继续工作，因为您没有使用实际的write系统调用；C标准库使用它，但它静态地链接到其他CCD_ 4函数。
至于其他警告，
main()
根本不是合适的C。它需要有int main(void)或int main(int argc, char *argv[])的原型。
对于pthread_*函数，必须#include <pthread.h>。
pthread_rwlock_wrlock的返回值是不是指针，而是int，因此您必须将其分配给int类型的对象，但您将其分配到char *类型的ret
总之，您应该在C编译器(-Wall(中打开所有通常有用的诊断，并将每个警告视为错误(-Werror(。
最后，所有编译和链接命令行上都应该有-pthread标志。