C fopen模式参数

Question

为什么 C 中 fopen 的 'mode' 参数由字符串给出？将它作为位掩码或类似的东西会更有意义(以我的思维方式)。字符串所需的开销是低效且不必要的。

Answer 1

C11 §7.21.5.3 The fopen function

The argument mode points to a string. If the string is one of the following, the file is open in the indicated mode. Otherwise, the behavior is undefined.271)

在脚注中:

271) If the string begins with one of the above sequences, the implementation might choose to ignore the remaining characters, or it might use them to select different kinds of a file (some of which might not conform to the properties in 7.21.2

根据 C99 基本原理，委员会认为实现可以选择使用 mode 而不是标志:

Rationale for International Standard — Programming Languages — C §7.19.5.3 The fopen function

An implementation may choose to allow additional file specifications as part of the mode string argument. For instance,

file1 = fopen(file1name, "wb,reclen=80");

might be a reasonable extension on a system that provides record-oriented binary files and allows a programmer to specify record length.

GNU libc 有一个扩展，允许 mode 包含 ccs=STRING，参见 glibc manual