How does #define carries the function name in c?-CodePudding

After working for more than 10 years, today a code caught my eye, I am unable to understand the function name defined inside a function gets printed in the output/log without being passed as an argument in macro or being defined as a global variable. Please help me understanding the internal. Please see the screenshot for the reference.

/*...*/
#include <stdio.h>
#define x printf("%s", f);
int main() {
    char *f = "MAIN";
    printf("Hello World");
    x;
    
    return 0;
}

Output:

Hello WorldMAIN

CodePudding user response：

C preprocessor macros simply do text replacement. They have no semantic awareness of your program.

This:

#include <stdio.h>
#define x printf("%s", f);

int main()
{ 
    char* f = "MAIN";  
    printf ("Hello World");
    x;
    return 0;
}

Becomes:

#include <stdio.h>

int main()
{ 
    char* f = "MAIN";  
    printf ("Hello World");
    printf("%s", f);;
    return 0;
}

Please note that if there is no f declared when this macro is used, you will see a compiler error. If f is declared, but is not a char *, you should see compiler warnings.

Some preprocessor macro best practices include (but are not limited to) using capitalized names, as x by convention looks like a variable or function name; and being careful about what syntactically significant symbols (in this case ;) you include in your macro text.

Hopefully this example was done for the sake of learning, because it is wholly unnecessary. Preprocessor macros wouldn't exist if they didn't serve a purpose, but beware they can easily obfuscate code.

CodePudding user response：

Preprocessor macros are just text replacements. All #include statements are replaced with the content of the specified files. All occurrences of #define'd symbols are replaced with their specified text. All comments are omitted. Etc...

So, in your example:

/*...*/
#include <stdio.h>
#define x printf("%s", f);
int main() {
    char *f = "MAIN";
    printf("Hello World");
    x;
    
    return 0;
}

The preprocessor replaces all instances of x with the text printf("%s", f); before the processed code is then sent to the compiler. So, this is the code that the compiler actually sees:

// contents of <stdio.h> here...

int main() {
    char *f = "MAIN";
    printf("Hello World");
    printf("%s", f);;
    
    return 0;
}