如何检查文件是否写入成功？

Question

我已经写了函数，不知道它是否正确但是我如何使用if条件返回true和false？

函数 bool save_book(Book, char)** 输入参数： 指向 Book 的指针和表示文件名的字符串。 返回值：如果图书的内容成功存储到文件中，则返回 true。否则为假。 注意此函数应根据文件名创建或打开一个文件，并在其中存储书籍的每一行。

下面是我的代码

bool save_book(Book *b, char* fileName){

FILE  *filePointer = fopen(fileName,"w");
for (int i = 0; i < pBook->lineCount; i++)
{
    fprintf(filePointer, "%s", b->lines[i]);
}
fclose(filePointer);

return true;
}

这是我正在使用的结构：

typedef struct _Book
{
   int characterCount;
   int lineCount;
   int maxLineCount;
   char **lines;
}Book;
 

Answer 1

您必须检查每个单独的 I/O 操作。

为了让它变得有趣，它们都会在出错时返回不同的东西。 fopen 将返回 NULL。 fprintf 将返回一个负数。 fclose 将返回 EOF。

这里附注了每个步骤可能出错的简短列表。

bool save_book(Book *b, char* fileName) {
    // Maybe the directory doesn't exist.
    // Maybe you don't have permission.
    // Maybe there's a disallowed character.
    // Maybe the disk is full.
    // Maybe it's a network drive and there's a network error.
    // Maybe the drive got unmounted.
    FILE  *filePointer = fopen(fileName,"w");
    if( filePointer == NULL ) {
        return false;
    }

    for (int i = 0; i < b->lineCount; i++)
    {
        // Maybe the disk is full.
        // Maybe it's a network drive and there's a network error.
        // Maybe the drive got unmounted.
        if( fprintf(filePointer, "%s", b->lines[i]) < 0 ) {
            // Even though the filePointer variable will be automatically freed
            // the underlying file handle will not be automatically closed.
            // There's a limit to how many open file handles one can have open.
            // No need to check for error, we know something has already gone wrong.
            fclose(filePointer);
            return false;
        }
    }
    // Maybe the disk is full.
    // Maybe it's a network drive and there's a network error.
    // Maybe the drive got unmounted.
    if( fclose(filePointer) != 0 ) {
        return false;
    }

    return true;
}

实际上您可能不需要检查fprintf，检查fclose 应该会发现相同的错误。但是，如果您正在编写一个非常大且成本高昂的文件，您可能想知道您是否迟早会耗尽磁盘空间。

您还可以选择打印错误。这些功能中的每一个都将设置全局errno 失败。您可以使用 strerror 将其转换为人类可读的字符串。

    if( filePointer == NULL ) {
        fprintf(stderr, "Error while opening '%s' for writing: %s", fileName, strerror(errno));
        return false;
    }

请注意，我倾向于检查不是成功的代码，而不是检查确切的错误代码。而不是if( fclose(filePointer) == EOF )，我检查了if( fclose(filePointer) != 0 )，缺少成功代码。这是一种防御性编程实践，以防错误严重到甚至无法返回正确的错误代码（在标准库代码中极不可能）或者我没有完全正确地阅读规范。