Is it possible to use #define inside a function?

Question

For example, I saw source code like the following. Can we use #define in a function? How does it work? (more information: this code is what I copied from openvswitch source code):

void *
ofputil_put_action(enum ofputil_action_code code, struct ofpbuf *buf)
{
switch (code) {
case OFPUTIL_ACTION_INVALID:

#define OFPAT13_ACTION(ENUM, STRUCT, EXTENSIBLE, NAME) case OFPUTIL_##ENUM:
#include "ofp-util.def"
    OVS_NOT_REACHED();

#define OFPAT10_ACTION(ENUM, STRUCT, NAME)                  \
case OFPUTIL_##ENUM: return ofputil_put_##ENUM(buf);
#define OFPAT11_ACTION(ENUM, STRUCT, EXTENSIBLE, NAME)      \
case OFPUTIL_##ENUM: return ofputil_put_##ENUM(buf);
#define NXAST_ACTION(ENUM, STRUCT, EXTENSIBLE, NAME)        \
case OFPUTIL_##ENUM: return ofputil_put_##ENUM(buf);
#include "ofp-util.def"
}
OVS_NOT_REACHED();
}

#define OFPAT10_ACTION(ENUM, STRUCT, NAME)                        \
void                                                        \
ofputil_init_##ENUM(struct STRUCT *s)                       \
{                                                           \
    memset(s, 0, sizeof *s);                                \
    s->type = htons(ENUM);                                  \
    s->len = htons(sizeof *s);                              \
}                                                           \
                                                            \
struct STRUCT *                                             \
ofputil_put_##ENUM(struct ofpbuf *buf)                      \
{                                                           \
    struct STRUCT *s = ofpbuf_put_uninit(buf, sizeof *s);   \
    ofputil_init_##ENUM(s);                                 \
    return s;                                               \
}
#define OFPAT11_ACTION(ENUM, STRUCT, EXTENSIBLE, NAME) \
OFPAT10_ACTION(ENUM, STRUCT, NAME)
#define OFPAT13_ACTION(ENUM, STRUCT, EXTENSIBLE, NAME) \
OFPAT10_ACTION(ENUM, STRUCT, NAME)
#define NXAST_ACTION(ENUM, STRUCT, EXTENSIBLE, NAME)            \
void                                                        \
ofputil_init_##ENUM(struct STRUCT *s)                       \
{                                                           \
    memset(s, 0, sizeof *s);                                \
    s->type = htons(OFPAT10_VENDOR);                        \
    s->len = htons(sizeof *s);                              \
    s->vendor = htonl(NX_VENDOR_ID);                        \
    s->subtype = htons(ENUM);                               \
}                                                           \
                                                            \
struct STRUCT *                                             \
ofputil_put_##ENUM(struct ofpbuf *buf)                      \
{                                                           \
    struct STRUCT *s = ofpbuf_put_uninit(buf, sizeof *s);   \
    ofputil_init_##ENUM(s);                                 \
    return s;                                               \
}
#include "ofp-util.def"

Answer 1

#define is a preprocessor directive: it is used to generate the eventual C++ code before it is handled to the compiler that will generate an executable. Therefore code like:

for(int i = 0; i < 54; i++) {
  #define BUFFER_SIZE 1024
}

is not executed 54 times (at the preprocessor level): the preprocessor simply runs over the for loop (not knowing what a for loop is), sees a define statement, associates 1024 with BUFFER_SIZE and continues. Until it reaches the bottom of the file.

You can write #define everywhere since the preprocessor is not really aware of the program itself.