Why does the C preprocessor interpret the word "linux" as the constant "1"?

Question

Why does the C preprocessor in GCC interpret the word `linux` (small letters) as the constant `1`?test.c: #include <stdio.h> int main(void) { int linux = 5; return 0; }Result of `$ gcc -E test.c` (stop after the preprocessing stage): .... int main(void) { int 1 = 5; return 0; }Which -of course- yields an error.(BTW: There is no `#define linux` in the stdio.h file.)

Answer 1

Cannot reproduce this error on macOS,

It is true that beginning variable names with digits is illegal in C (and C++, and Java, ...), but no C compiler should be incorrectly identifying the small letter l as the number 1.

What happens when you use other variables that start with lower case l, such as:

int lower = 10; // OR

int length = 5;

Do you see the same type of C compiler errors you reported?

//-----------------------------------------------------------------------------------

// Below is the code you showed, being compiled and run, and also being shown with the gcc -E test.c option.

// Compiling and running your code produced the following output (as expected)

//------------------------------------------------------------------

#include <stdio.h>

int main(void) {

int linux = 5;

printf("linux variable has value: %d\n", linux);

  return 0;

}

//------------------------------------------------------------------

// which produced the following output:

//------------------------------------------------------------------

linux variable has value: 5

Program ended with exit code: 0

//------------------------------------------------------------------

// and running it with gcc -E test.c produced the following (again, as expected)

//------------------------------------------------------------------

extern int __vsnprintf_chk (char * restrict, size_t, int, size_t,

    const char * restrict, va_list);

# 408 "/Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX.sdk/usr/include/stdio.h" 2 3 4

# 90 "test.c" 2

int main(void) {

int linux = 5;

printf("linux variable has value: %d\n", linux);

  return 0;

}

//------------------------------------------------------------------