尝试从文件中获取令牌时出现分段错误。 [英] Segmentation Fault while trying to grab tokens from file.

问题描述

您好，

我正在尝试从C下的文件中获取令牌，进入预先分配的缓冲区。到目前为止看起来像这样：

Hello,
I'm trying get "tokens" from a file under C, into a preallocated buffer. So far it looks like this:

char* token;
....
int main(int argc, const char* argv[])
{
     ....
     token = (char*)malloc(256);
     grab_token();
     ....
}
void grab_token()
{
	char a;
	int i = 0;
	while(a == ' ' || a == '\t' || a == '\r' || a == '\n' || a != EOF) { a = fgetc(source_FILE); }
	while(a != ' ' || a != '\t' || a != '\r' || a != '\n' || a != EOF)
	{
		a = fgetc(source_FILE);
		token[i] = a;
		i++;
	}
        // NULL terminate token
	token[i+1] = '\0'; 
}

现在这很简单，只需检查收到的字符是否为空格，制表符，换行符....或者如果是EOF则退出它从文件中获取words或tokens并将其放入已分配的缓冲区中。然而，grab_token（）特别失败了臭名昭着的分段错误（核心转储），SIGEGV意味着无效的内存访问，但由于某种原因我不明白为什么我应该访问我没有分配的内存。是的，fgetc（）完美运行。

It's quite simple for now, just checks if the character received is a whitespace, tab, newline.... or exits if it's an EOF or it takes "words" or "tokens" from the file and puts it in an allocated buffer. However the "grab_token()" specifically fails with the infamous "Segmentation Fault (Core Dumped)", with SIGEGV which means invalid memory access, however for some reason I don't see why I should be accessing memory I haven't allocated. And yes, fgetc() works perfectly.

推荐答案

你正在使用OR（ || ）你应该在哪里改为使用AND（&& ）。

您没有检查令牌（实际上你甚至不检查 malloc 是否成功）。

You are using OR (||) where you should use instead AND (&&).
You are not checking for buffer overrun on token (actually you are not checking even if malloc succeeded).

CPallini是对的。但要更清楚，请参阅以下代码。它包括一些评论和建议。你可以根据自己的需要改变它。我没有测试过代码，但我认为不会有任何核心转储（如果它成功编译:)）。

CPallini is right. But to be more clear, see the following piece of code. It includes some comments and advices. you can change it according to your needs. I haven't tested the code, but I think there won't be any core dump (if it compiles successfully :) ).

// define size as constant
const int BUFF_SIZE = 256;

// 1- I would return some value to check if successful or not
// 2- pass 'token' pointer as parameter
// 3- dont use global variables if you dont have to do so.
void grab_token()
{
    char a;
    int i = 0;

    while((a = fgetc(source_FILE))!=EOF) {
        if(a == ' ' || a == '\t' || a == '\r' || a == '\n') // this part can be converted to macro
            continue;
        break;
    }
    if(a==EOF) {
        token[0]=0;
        // maybe some error handling
        return;
    }

    while((a = fgetc(source_FILE))!=EOF) {
        if(a == ' ' || a == '\t' || a == '\r' || a == '\n')
            break;
        if(i<BUFF_SIZE-2) // BUFF_SIZE-1 is reserved for termination
            token[i++]=a;
        else {
            // some error handling
            // break;
            // or
            // return;
        }
    }
    token[i]=0;
}

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