gcc - What does the "aw" flag in the section attribute mean?

Question

In the following line of code (which declares a global variable),

unsigned int __attribute__((section(".myVarSection,"aw",@nobits#"))) myVar;

what does the "aw" flag mean?

My understanding is that the nobits flag will prevent the variable from being initialised to zero, but I am struggling to find info about the "aw" flag.

Also, what meaning do the @ and # have around the nobits flag?

Answer 1

The section("section-name") attribute places a variable in a specific section by producing the following assembler line:

.section    section-name,"aw",@progbits

When you set section-name to ".myVarSection,"aw",@nobits#" you exploit a kind of "code injection" in GCC to produce:

.section    .myVarSection,"aw",@nobits#,"aw",@progbits

Note that # sign starts a one-line comment.

See GNU Assembler manual for the full description of .section directive. A general syntax is

.section name [, "flags"[, @type[,flag_specific_arguments]]]

so "aw" are flags:

• a: section is allocatable
• w: section is writable

and @nobits is a type:

• @nobits: section does not contain data (i.e., section only occupies space)

All the above is also applicable to functions, not just variables.