Question

在编译时提供定义时如何检查是否定义了宏。

我有Makefile，它在编译示例C文件时解析并传递此字符串（“ -DMACRO_TEST=${DEFINED}”）。变量DEFINED可以具有或可以不具有值（0或非零）。我想知道当变量DEFINED本身没有任何值时，如何检查MACRO_TEST是否已定义。这类似于在编译C文件时传递“ -DMACRO_TEST =“。

#include <stdio.h>

#ifdef MACRO_TEST
char msg[] = "MACRO_TEST is defined";
#else
char msg[] = "MACRO_TEST is NOT-defined";
#endif

int main ()
{
    printf("msg = %s\n", msg);
    return 0;
}

命令行输出

$ gcc tmp.c -DMACRO_TEST=0
$ ./a.out
msg = MACRO_TEST is defined
$ gcc tmp.c -DMACRO_TEST=
$ ./a.out
msg = MACRO_TEST is defined

在第二种情况下进行编译时，我期望它会返回

msg = MACRO_TEST is NOT-defined

Answer 1

如果省略任何选项tensorboard = TensorBoard(log_dir="logs/{}".format(NAME)) model.compile(loss='binary_crossentropy', optimizer='adam', metrics=['accuracy'], ) model.fit(X, y, batch_size=32, epochs=10, validation_split=0.3, callbacks=[tensorboard])或在public void guess(View view) { if (e.getText().toString().length()==0) { Toast.makeText ( this , "YOUR MESSAGE FOR EMPTY" Toast.LENGTH_SHORT ).show(); return; } }命令行中显式添加-DMACRO_TEST（=未定义宏），则不会定义该宏。在这种情况下，预处理程序指令-UMACRO_TEST将为FALSE，您将获得

gcc

在#ifdef MACRO_TEST分支中。

（假设您没有在代码中定义此宏。）

Answer 2

一些测试宏的常见情况：

#ifdef MACRO_TEST
  // MACRO_TEST is defined, whatever its content, even empty
  // for example:
  //   -DMACRO_TEST
  //   -DMACRO_TEST=
  //   -DMACRO_TEST=0
  //   -DMACRO_TEST=1
  //   -DMACRO_TEST=abcd
#else
  // MACRO_TEST is not defined, not even with an empty content
#endif

#if MACRO_TEST // it must not be defined as empty (-DMACRO_TEST= --> error)
  // MACRO_TEST has a non-zero integer value
  // for example:
  //   -DMACRO_TEST
  //   -DMACRO_TEST=1    -- equivalent to -DMACRO_TEST
  //   -DMACRO_TEST=23
#else
  // MACRO_TEST has a zero integer value
  // for example:
  //   (not defined at all, then considered as zero)
  //   -DMACRO_TEST=0
  //   -DMACRO_TEST=what
#endif

Answer 3

只需使用#if xxx。预处理器会将未定义的xxx视为零。

#include <stdio.h>

#if MACRO_TEST
char msg[] = "MACRO_TEST is defined";
#else
char msg[] = "MACRO_TEST is NOT-defined";
#endif

int main ()
{
    printf("msg = %s\n", msg);
    return 0;
}

而且，您不必在命令行上提供值，只要-Dxxx就足够了，编译器/预处理器将假定为1：

cc -Wall -DMACRO_TEST defined.c

更新。所有四种情况：

#include <stdio.h>

#if MACRO_TEST
char msg[] = "MACRO_TEST is defined and non-zero";
#else
char msg[] = "MACRO_TEST is NOT-defined or zero";
#endif

int main (int argc, char**argv)
{
    printf("%s: msg = %s\n", argv[0], msg);
    return 0;
}

#!bin/sh

cc -Wall defined.c -o x
cc -Wall -DMACRO_TEST defined.c -o xd
cc -Wall -DMACRO_TEST=0 defined.c -o xd0
cc -Wall -DMACRO_TEST=1 defined.c -o xd1

./x
./xd
./xd0
./xd1

rm x xd xd0 xd1

结果：

./x: msg = MACRO_TEST is NOT-defined or zero
./xd: msg = MACRO_TEST is defined and non-zero
./xd0: msg = MACRO_TEST is NOT-defined or zero
./xd1: msg = MACRO_TEST is defined and non-zero

如何检查是否定义了CLI定义的MACRO（-DMACRO =）

3 个答案: