CODEWITHSUNDEEP
kernel
Jack Chin
Jack Chin

Reputation: 525

In Iinux kernel, what is the meaning of .data and .long?

.data
  ENTRY(sys_call_table)
    .long SYMBOL_NAME(sys_ni_call)           /* 0 */
    .long SYMBOL_NAME(sys_exit)
    .long SYMBOL_NAME(sys_fork)
    ...
    .long SYMBOL_NAME(sys_vfork)             /* 190 */

I read this source code. I can't find .data or .long definition in the source.

Upvotes: 0

Views: 164

Answers (1)

Adam Rosenfield
Adam Rosenfield

Reputation: 400384

They are assembler directives—special directions to the assembler which tell it to do something different, rather than inserting a processor instruction into the compiled machine code.

The .data directive tells the assembler to emit the following instructions onto the end of one of the subsections of the data section of the executable. Normally, machine code is emitted into the so-called text section of executables, whereas non-executable data such as global variables are stored in one of the so-called data sections. The different sections have different memory permissions at runtime, among other features.

The .long directive is equivalent to the .int directive, which just says to insert a literal numeric value into the machine code. So .long SYMBOL_NAME(sys_ni_call) inserts the numeric value of the location of the sys_ni_call symbol.

So putting these together, a .data directive followed by a .long directive results in the assembler putting specific integer values into one of the data sections of the resulting object code. These values will be non-executable, and they may be read-only or read-write, depending on how the permissions of the sys_call_table subsection of the data section are configured.

Upvotes: 3

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