Understanding the Term \\\“$$PWD/\\\” in qmake project file

Question

I found this line in a qmake project file:

 DEFINES += SRCDIR=\\\\\\"$$PWD/\\\\\\" 

I know that the PWD -part stands for print working directory and I let give me the value of SRCDIR :

 std::cout << "SRCDIR :" << SRCDIR << std::end; 

I also changed the variable to SRCDIR=$$PWD but it will not work.

But why we need a slash / , backslahses \\ and quotation marks " to get the PWD ?

Answer 1

qmake will handle quote characters and backslashes specially.

Your end goal is to have a string literal defined to be the expansion of SRCDIR . String literals in C++ are contained within double quotes, so you need those quotes to make it through to the compiler as part of the definition.

In other words, you want the equivalent of:

#define SRCDIR "somedir/"

where somedir is the current working directory, in this example. To achieve this, you want to escape the quote (so it isn't handled specially by qmake ) using \\" . This stands at both ends of the string.

Now, what about the escaped backslash, \\\\ ? Well, this further escapes the quote from shell processing. When the command

cc -DSRCDIR="somedir/"

is passed to the shell, the quotes will be stripped as part of the shell's processing. To make sure those quotes remain, and define a string literal, you need to escape them with a backslash at this level, too. The shell will convert \\" into " . So the full escape sequence for a double quote character in this case is:

\\\"

This token appears at both ends of the defined string. The forward-slash just makes it easier to use the path inside the code; it eliminates the need to add a / everywhere you use the path.

The command that the shell sees will look like

cc -DSRCDIR=\"somedir/\"

and the definition of SRCDIR inside the compiler will be a string literal, equivalent to the following definition in-source:

#define SRCDIR "somedir/"