c - 使用 ftell 读取文本文件中数据的偏移量

Question

所以我有一个文本文件，其中存储了数据。现在我想计算每个数据在其中的位置，所以我使用了下面的方法。

        while (fscanf(fp, "%d", &id) != EOF) {
            offset = ftell(fp);
        }

现在的问题是，通过这段代码我得到了数据结尾的位置，所以下一个数据的开始。我怎样才能始终获得数据的开头，所以从 0 开始？

Answer 1

使用这个

while (fscanf(fp, "%d%n", &id, &length) != EOF) {
    offset = ftell(fp) - length;
}

来自 scanf() linux 手册

n      Nothing is expected; instead, the number of characters consumed thus far from the input is stored through the next pointer, which must be a pointer  to  int.
       This is not a conversion, although it can be suppressed with the * assignment-suppression character. The C standard says: "Execution of a %n directive does
       not increment the assignment count returned at the completion of execution" but the Corrigendum seems to contradict this. Probably it is wise not to make any
       assumptions on the effect of %n conversions on the return value.