Makefile variable assignment

The ifeq directive begins the conditional, and specifies the condition. For each of those files, it issues the recipes recorded in the data base. But the echo command may additionally interpret strings with a leading dash or those containing backslashes. This is called a conditional variable assignment operator, because it only has an effect if the variable is not yet defined.

The value of a simply expanded variable is scanned once and for all, expanding any references to other variables and functions, when the variable is defined.

Some of the modifiers split the string into words and then operate on the words, others operate on the string as a whole. Every conditional must end with an endif.

There are several types of variables that should be handled differently. Since trailing space characters are not stripped from variable values, just a space at the end of the line would have the same effect but be rather hard to read. Echoing a string exactly as-is Echoing a string containing special characters needs special work.

If a header file has changed, each C source file that includes the header file must be recompiled to be safe. To see a more complex example of a makefile, see Complex Makefile.

The lines of the makefile following the ifeq are obeyed if the two arguments match; otherwise they are ignored. Once you have a precise problem you can report it in one of two ways.

Their elements are separated by whitespace. To trim leading and trailing whitespace, use the: The else directive causes the following lines to be obeyed if the previous conditional failed. Any other characters are allowed.

Using the make 1 system as a programming language for a big system like pkgsrc requires some discipline to keep the code correct and understandable.

Handling possibly empty variables When a possibly empty variable is used in a shell program, it may lead to a syntax error. In a program, typically, the executable file is updated from object files, which are in turn made by compiling source files.

Naming conventions All variable names starting with an underscore are reserved for use by the pkgsrc infrastructure.

Finally, be sure to explain what you expected to occur; this will help us decide whether the problem was really in the documentation. Variable substitution is performed on both arguments and then they are compared. This does not happen when one of the commands fails like false 1 above.

Q modifier would result in an empty string too, which would then be skipped by the shell. You can provide command line arguments to make to control which files should be recompiled, or how. Before reporting a bug or trying to fix it yourself, try to isolate it to the smallest possible makefile that reproduces the problem.

Handling possibly empty variables Pkgsrc consists of many Makefile fragments, each of which forms a well-defined part of the pkgsrc system. Among these shell commands may even be more complex ones like awk 1 programs.

See How to Run make. Example 2 quotes the string so that the shell interprets it correctly.Variable Assignment. Variable definitions are parsed as follows: (see How make Reads a Makefile).

GNU make also has the ability to enable a second expansion of the prerequisites (only) for some or all targets defined in the makefile.

In order for this second expansion to occur. Pattern-specific Variable Values. The variable-assignment can be any valid form of assignment. Any command line variable setting will take precedence, unless override is specified.

-specific variables which result in the same stem length are considered in the order in which they were defined in the makefile. The Two Flavors of Variables. Simply expanded variables generally make complicated makefile programming more predictable because they work like variables in most programming languages.

This is called a conditional variable assignment operator, because it only has an effect if the variable is not yet defined. This statement. But then all assignments to FOO variable within the makefile will be ignored unless you use the override directive in assignment.

(The effect is the same as with -e option for environment variables). Conditional Parts of Makefiles. A conditional causes part of a makefile to be obeyed or ignored depending on the values of variables.

Conditionals can compare the value of one variable to another, or the value of a variable to a constant string. How to Use Variables A variable is a name defined in a makefile to represent a string of text, called the variable's value. These values are substituted by explicit request into targets, prerequisites, commands, and other parts of the makefile.

Makefile variable assignment
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