c - C (GCC) 宏扩展期间的语句表达式语法错误

Question

你好，我正在尝试使用语句表达式来替换某些函数调用...有人可以向我解释为什么语句表达式无法编译吗？

    char* malloc_and_copy(char * str){
        int tmp_len = strlen(str);  
        char *tmp = malloc(tmp_len+1); 
        bzero(tmp, tmp_len+1);  
        if (tmp != NULL) strncpy(tmp,str,tmp_len);
        return tmp;
    }

    //statement expression to simplify the code
    #define MALLOC_AND_COPY(str)(                           \
            int tmp_len = strlen(str);                      \
            char *tmp = malloc(tmp_len+1);                  \
            bzero(tmp, tmp_len+1);                          \
            if (tmp != NULL) strncpy(tmp,str,tmp_len);      \
            tmp;)

    void init(){

        char * OK =             malloc_and_copy("OK");
        char * FILE_NOT_FOUND =     MALLOC_AND_COPY("FILE_NOT_FOUND")
    }

我收到以下错误:

client.c:107:7: error: expected ‘)’ before ‘tmp_len’
   int tmp_len = strlen(str);      \
       ^
client.c:116:36: note: in expansion of macro ‘MALLOC_AND_COPY’
 FILE_NOT_FOUND =  MALLOC_AND_COPY("FILE_NOT_FOUND");
                                    ^
client.c:116:69: error: expected expression before ‘;’ token
  FILE_NOT_FOUND =  MALLOC_AND_COPY("FILE_NOT_FOUND");

我的 GCC 版本信息:

Using built-in specs.
COLLECT_GCC=gcc
COLLECT_LTO_WRAPPER=/usr/lib/gcc/x86_64-linux-gnu/5/lto-wrapper
Target: x86_64-linux-gnu
Configured with: ../src/configure -v --with-pkgversion='Ubuntu 5.4.0-6ubuntu1~16.04.6' --with-bugurl=file:///usr/share/doc/gcc-5/README.Bugs --enable-languages=c,ada,c++,java,go,d,fortran,objc,obj-c++ --prefix=/usr --program-suffix=-5 --enable-shared --enable-linker-build-id --libexecdir=/usr/lib --without-included-gettext --enable-threads=posix --libdir=/usr/lib --enable-nls --with-sysroot=/ --enable-clocale=gnu --enable-libstdcxx-debug --enable-libstdcxx-time=yes --with-default-libstdcxx-abi=new --enable-gnu-unique-object --disable-vtable-verify --enable-libmpx --enable-plugin --with-system-zlib --disable-browser-plugin --enable-java-awt=gtk --enable-gtk-cairo --with-java-home=/usr/lib/jvm/java-1.5.0-gcj-5-amd64/jre --enable-java-home --with-jvm-root-dir=/usr/lib/jvm/java-1.5.0-gcj-5-amd64 --with-jvm-jar-dir=/usr/lib/jvm-exports/java-1.5.0-gcj-5-amd64 --with-arch-directory=amd64 --with-ecj-jar=/usr/share/java/eclipse-ecj.jar --enable-objc-gc --enable-multiarch --disable-werror --with-arch-32=i686 --with-abi=m64 --with-multilib-list=m32,m64,mx32 --enable-multilib --with-tune=generic --enable-checking=release --build=x86_64-linux-gnu --host=x86_64-linux-gnu --target=x86_64-linux-gnu
Thread model: posix
gcc version 5.4.0 20160609 (Ubuntu 5.4.0-6ubuntu1~16.04.6)

Answer 1

语句表达式的(非标准)使用需要表达式 block 的大括号(请参阅 https://gcc.gnu.org/onlinedocs/gcc/Statement-Exprs.html )。即

//statement expression to simplify the code
#define MALLOC_AND_COPY(str)(                   \
    {int tmp_len = strlen(str);                 \
      char *tmp = malloc(tmp_len+1);            \
      bzero(tmp, tmp_len+1);                    \
      if (tmp != NULL) strncpy(tmp,str,tmp_len);\
      tmp;})

完成工作。