Question

I am now read some c code. And is not very clear about the “#define someting” expression.

For example, I saw this code:

typedef enum cairo_path_op {
    CAIRO_PATH_OP_MOVE_TO = 0,
    CAIRO_PATH_OP_LINE_TO = 1,
    CAIRO_PATH_OP_CURVE_TO = 2,
    CAIRO_PATH_OP_CLOSE_PATH = 3
} __attribute__ ((packed)) cairo_path_op_t; /* Don't want 32 bits if we can avoid it. */


#ifndef __GNUC__
#define __attribute__(x)
#endif

I take attention to the “__attribute__(x)”. since in other header file , the “__attribute__(x)” is defined with no value, but how it take effect in the enum “cairo_path_op” define?

Answer 1

This is for portability reasons.

__attribute__() is a GCC extension for modifying various properties and behavior of functions, variables, types, etc.

If a non-GCC-compatible compiler tries to compile code that uses this extension, it won’t able to do so and will throw a syntax error.

In order to avoid this, the author of the code makes the preprocessor replace this keyword with nothing if __GNUC__ is not defined (i. e. if the compiler is not a GCC-compatible one), so that the code builds on a bigger variety of platforms.