写入共享内存核心转储分段故障

I need to write to shared memory, thus I have

我需要写共享内存，因此我有

#define FLAGS IPC_CREAT | 0644
int main() {
key = ftok("ex31.c", 'k');
shmid = shmget(key, 3, FLAGS);
shmaddr = shmat(shmid,0,0);    // THOSE LINES WORK AS EXPECTED

char* userInput = malloc(5);
read(0, userInput, 3);  // I want to read "b34" for example, WORKS GOOD
strcpy(shmaddr,userInput);   // THROWS EXCEPTION!
}

it throws me exception in strcat, and if I delete it the exception is thrown in the next line of strcpy. I need to write to the memory "b34\0" (4 chars) and then read it.

它在strcat中抛出异常，如果我删除它，则会在strcpy的下一行抛出异常。我需要写入内存“b34 \ 0”（4个字符）然后读取它。

2 个解决方案

#1

This:

这个：

strcat(userInput, '\0');   //THROWS EXCEPTION!!

Is not valid C, and it doesn't "throw an exception" because C doesn't have exceptions. Maybe it crashes, but anyway that should be expected since you're not even writing valid code. Use a compiler that rejects obvious invalid code like this.

是无效的C，它不会“抛出异常”，因为C没有异常。也许它会崩溃，但无论如何都应该预料到，因为你甚至没有编写有效的代码。使用拒绝明显无效代码的编译器。

Edit: and this:

编辑：和：

char* userInput = malloc(5);
read(0, userInput, 3);
strcpy(shmaddr,userInput);

Is invalid because you read three characters, leaving the last two characters in userInput uninitialized, then you call strcpy() which reads a null terminated string from userInput, but you have not null-terminated the string, so it is undefined behavior and anything could happen--including a crash. So try this:

是无效的，因为你读了三个字符，留下userInput中的最后两个字符未初始化，然后你调用strcpy（）从userInput读取一个空终止的字符串，但你没有空终止字符串，所以它是未定义的行为，任何东西都可以发生 - 包括崩溃。所以试试这个：

const size_t INPUT_MAX_SIZE = 3;
char userInput[INPUT_MAX_SIZE + 1];
read(STDIN_FILENO, userInput, INPUT_MAX_SIZE);
userInput[INPUT_MAX_SIZE] = '\0'; // add null terminator
strcpy(shmaddr,userInput);

Or better yet:

或者更好的是：

read(STDIN_FILENO, shmaddr, INPUT_MAX_SIZE);

That is, just read directly to the destination, not a temporary buffer.

也就是说，只需直接读取目标，而不是临时缓冲区。

#2

Both the function strcat and strcpy expect the arguments to be null-terminated strings, in you case, neither userInput or shmaddr satisfies this condition, that's why you see the program crashes. Try this:

函数strcat和strcpy都希望参数是以null结尾的字符串，在你的情况下，userInput或shmaddr都不满足这个条件，这就是你看到程序崩溃的原因。尝试这个：

#define FLAGS IPC_CREAT | 0644
int main(void) {
    key = ftok("ex31.c", 'k');
    shmid = shmget(key, 4, FLAGS);  // the buffer needs at least size 4 to hold the 3 char string and the null terminator
    shmaddr = shmat(shmid, 0, 0);

    char* userInput = malloc(5);
    read(0, userInput, 3);
    userInput[3] = '\0';
    strcpy(shmaddr, userInput);
}

#1