C Preprocessor output [duplicate] - c

Question

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cpp preprocessor output not able to understand? [duplicate]
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Closed 5 years ago.
For gcc -E sample.c -o sample.i with the following input C program,
#include <stdio.h>
int main() {
printf("hello world\n");
return 0;
}
the sample.i have the following output preceded by # symbols and I wonder what the line with # exactly means.
# 1 "sample.c"
# 1 "<built-in>"
# 1 "<command-line>"
# 1 "/usr/include/stdc-predef.h" 1 3 4
# 1 "<command-line>" 2
# 1 "sample.c"
# 1 "/usr/include/stdio.h" 1 3 4
# 27 "/usr/include/stdio.h" 3 4
...

Answer 1

There are comments that help a person identify how the preprocessor expanded the various #include <...> macros and other items.
Reading these lines provides the equivalent of reading the logging messages of the preprocessor as it encounters the macros and expands them.
# 1 "sample.c"
Start on line one of the input "sample.c"
# 1 "<built-in>"
Process the built-in c pre-procssor directive (must be an implementation detail), but is presented as a fake "file".
# 1 "<command-line>"
Process the command line directive (again implementation detail), presented as a fake "file".
# 1 "/usr/include/stdc-predef.h" 1 3 4
Include (at line 1) the stdc-predef.h file, it's the start of the file, suppress warnings permitted for system header files, assure that the symbols are treated like C symbols.
# 1 "<command-line>" 2
Return from the command line "fake" file.
# 1 "sample.c"
Back in sample.c.
# 1 "/usr/include/stdio.h" 1 3 4
Now starting with file "stdio.h", suppress permitted system warnings, treat symbols in the file a C symbols.
# 27 "/usr/include/stdio.h" 3 4
And so on...
The documentation is here.