为什么我的 getline() 没有读取 C 中标准输入的第一行？答案

【问题标题】：Why is my getline() not reading the first line of my standard input in C?为什么我的 getline() 没有读取 C 中标准输入的第一行？
【发布时间】：2020-04-18 21:39:52
【问题描述】：

我对 C 比较陌生，我正在使用 getline() 函数来读取作为标准输入提供的文件的内容。

但是，while 循环不会读取文件的第一行，我不知道为什么！

对于上下文：文件读取-

a b c -d e F

输出正确地读取和拆分-d、e和f，但仅在while循环之外打印a b c。

int main (){

//utilising data provided at http://www.linux.die.net/man/3/getline
//char*linePointer is initialized to NULL, for getline() to allocate a buffer that stores the line
//buffer gets resized dynamically 

char *linePointer = NULL; 
size_t len = 0;

//checks if the file is empty and prints -, if it is
if ((getline(&linePointer, &len, stdin) == -1)){
    printf("-\n");
}

//as long as the stream is valid, and the file can be read 
//if either of the conditions are not satisfied then while loop condition is not satisfied 
//prints the contents of the line 

Clauses cs = createNewArrayList();
printf("%s\n", linePointer);
while ((getline(&linePointer, &len, stdin) != -1)){
        printf("%s\n", linePointer);

    Clause c = createNewArrayList();
    char *token; 
    char *delim = " ";
    token = strtok(linePointer, delim);
    while (token != NULL){      
        char *duplicate = strdup(token);     
        add(c, duplicate);
        printf("%s\n",duplicate);
        token = strtok(NULL, delim);
    }
    add(cs, c);
}

free(linePointer);
exit(EXIT_SUCCESS);

【问题讨论】：

您正在泄漏正在读取的第一行。您应该使用do..while 循环而不是while 循环。您还泄漏了每个 duplicate 令牌。
@RemyLebeau It's a standard POSIX function.
@nicksheen 不，它没有。阅读getline 的文档。
@JosephSible-ReinstateMonica 你是对的，对不起。我刚看了第一段

标签： c getline strtok

【解决方案1】：

因为您的第一个 getline 正在使用第一行：

//checks if the file is empty and prints -, if it is
if ((getline(&linePointer, &len, stdin) == -1)){
    printf("-\n");
}

while 循环再次运行 getline 并忽略第一次运行的结果。

【讨论】：

【解决方案2】：

在进入 while 循环之前，您正在读取并丢弃第一行，这就是循环看不到该行的原因。

试试这个：

int main (){

    //utilising data provided at http://www.linux.die.net/man/3/getline
    //char*linePointer is initialized to NULL, for getline() to allocate a buffer that stores the line
    //buffer gets resized dynamically 

    char *linePointer = NULL; 
    size_t len = 0;

    //checks if the file is empty and prints -, if it is
    if ((getline(&linePointer, &len, stdin) == -1)){
        printf("-\n");
    }
    else{
        //as long as the stream is valid, and the file can be read 
        //if either of the conditions are not satisfied then while loop condition is not satisfied 
        //prints the contents of the line 

        Clauses cs = createNewArrayList();
        do{
            printf("%s\n", linePointer);

            Clause c = createNewArrayList();
            char *delim = " ";
            char *token = strtok(linePointer, delim);
            while (token != NULL){      
                char *duplicate = strdup(token);     
                add(c, duplicate);
                printf("%s\n",duplicate);
                //if add() makes its own copy, then uncomment this,
                //or simply don't use strdup() above to begin with:
                //free(duplicate);
                token = strtok(NULL, delim);
            }
            add(cs, c);

            free(linePointer);
            linePointer = NULL; 
            len = 0;
        }
        while (getline(&linePointer, &len, stdin) != -1);
    }

    free(linePointer);
    exit(EXIT_SUCCESS);
}

【讨论】：