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__FILE__

As explained in Modern C, Third Edition

__FILE__ is a predefined C macro that expands to a string literal naming the current source file.

__FILE__ is a predefined C macro that expands to a string literal naming the current source file, or translation unit. It is useful when debugging because diagnostic output can identify where it originated. section 17.3

Source location in diagnostic output

A tracing macro can incorporate __FILE__ into formatted debugging output so that each message carries its originating file name. The related __LINE__ macro supplies the line number, while __func__ supplies the current function name. section 17.3

Related predefined context constructs include:

  • __LINE__ expands to an integer constant representing the actual source line. section 17.3
  • __DATE__ and __TIME__ provide the compilation date and time as string literals. section 17.3
  • __func__ provides the name of the current function through a local static variable. section 17.3

How Modern C, Third Edition covers it

How deep it goes
Defines its string-literal expansion and debugging role, but does not develop its implementation mechanics.
Examples
Uses macro-based trace output that reports source location alongside debugging values.
What it assumes
Assumes familiarity with C macros and formatted output while introducing predefined invocation-context constructs.
What you can do after
Diagnose/debug with it by adding the originating source-file name to tracing or diagnostic output.
How it connects
Connects source-file identification with __LINE__, __DATE__, __TIME__, __func__, and fprintf-based tracing.
Cover of Modern C, Third Edition
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