[GNUMake] 'shell' function

[Makefile]

$(shell echo hello) # <--- (*A)

all:

    echo all

above make file gives following results.

     

Makefile:1: *** missing separator.  Stop.

error is issued.

But, we can resolve this error by changing line (*A) into below line.

dummy := $(shell echo hello)

or

$(shell echo)

What is root cause of this?

Let's see source code of 'func_shell' function in GNUMake-v4.0.

func_shell_base(...)

{

    ...

      child_execute_job (FD_STDIN, pipedes[1], errfd, command_argv, envp); <= stdout을 pipe로 redirecdtion

    ...

          EINTRLOOP (cc, read (pipedes[0], &buffer[i], maxlen - i)); <= pipe를 읽음

    ...

          /* The child finished normally.  Replace all newlines in its output

             with spaces, and put that in the variable output buffer.  */

          fold_newlines (buffer, &i, trim_newlines);

          o = variable_buffer_output (o, buffer, i);    <= buffer값(pipe값 -> child의 stdout값)을 variable_buffer에 저장. 

    ...

}

That is, 'shell' function stores it's result(stdout) into variable_buffer.

variable_buffer is buffer that is used to store result of variable-expandsion.

What does this mean?

See below samples.

[Makefile]

$(shell echo hello) <= (*1)

[Makefile]

temp:=hello

$(temp) <= (*2)

Above analysis means, (*1) and (*2) is same expression in gnumake's point of view.

Actually, above rules are same on all other functions in gnumake.

But, most function gives empty string - that is, it doesn't write string value to stdout.

So, it doesn't write anything into variable_buffer. That's reason why above issues are not shown at other functions.

And, in gnumake's point of view, 'function' and 'variable' is same in concept.

So, we can say that 'function' is just special variable that can get arguments - a kind of 'predefined variable'.

Therefore, we should use same concept for 'variable' and 'function' to understand gnumake.