Question

In the past whenever I came across #define it was used like

#define MOD 1000000007

In the case above all instances of MOD in the code was replaced by 1000000007.
I am new to open source development and was looking at several video filters of VLC media player. It has several uses of #define as-

//example1
#define MSG_LONGTEXT N_( \
  "Marquee text to display. " \
  "(Available format strings: " \
  "Time related: %Y = year, %m = month, %d = day, %H = hour, " \
  "%M = minute, %S = second, ... " \
  "Meta data related: $a = artist, $b = album, $c = copyright, " \
  "$d = description, $e = encoded by, $g = genre, " \
  "$l = language, $n = track num, $p = now playing, " \
  "$r = rating, $s = subtitles language, $t = title, "\
  "$u = url, $A = date, " \
  "$B = audio bitrate (in kb/s), $C = chapter," \
  "$D = duration, $F = full name with path, $I = title, "\
 "$L = time left, " \
 "$N = name, $O = audio language, $P = position (in %), $R = rate, " \
 "$S = audio sample rate (in kHz), " \
 "$T = time, $U = publisher, $V = volume, $_ = new line) ")

 //example 2
 #define POSY_TEXT N_("Y offset")

 //example 3
 #define TIMEOUT_LONGTEXT N_("Number of milliseconds the marquee must remain " \
                         "displayed. Default value is " \
                         "0 (remains forever).")

can somebody explain these examples with respect to

#define

and software development both or provide some resources?

Answer 1

It’s exactly the same, the only addition is that \ marks the continuation of the current line in the next one. It’s there for readability reasons.

For example:

 #define TIMEOUT_LONGTEXT N_("Number of milliseconds the marquee must remain " \
                         "displayed. Default value is " \
                         "0 (remains forever).")

is equivalent to

 #define TIMEOUT_LONGTEXT N_("Number of milliseconds the marquee must remain " "displayed. default value is " "0 (remains forever).")

So whenever TIMEOUT_LONGTEXT appears in the code, the preprocessor will replace it with N_("whatever").