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Error messages from compilers should look like this:
source-file-name:lineno: message
If you want to mention the column number, use one of these formats:
source-file-name:lineno:column: message
source-file-name:lineno.column: message
Line numbers should start from 1 at the beginning of the file, and column numbers should start from 1 at the beginning of the line. (Both of these conventions are chosen for compatibility.) Calculate column numbers assuming that space and all ASCII printing characters have equal width, and assuming tab stops every 8 columns.
The error message can also give both the starting and ending positions of the erroneous text. There are several formats so that you can avoid redundant information such as a duplicate line number. Here are the possible formats:
source-file-name:lineno-1.column-1-lineno-2.column-2: message
source-file-name:lineno-1.column-1-column-2: message
source-file-name:lineno-1-lineno-2: message
When an error is spread over several files, you can use this format:
file-1:lineno-1.column-1-file-2:lineno-2.column-2: message
Error messages from other noninteractive programs should look like this:
program:source-file-name:lineno: message
when there is an appropriate source file, or like this:
program: message
when there is no relevant source file.
If you want to mention the column number, use this format:
program:source-file-name:lineno:column: message
In an interactive program (one that is reading commands from a terminal), it is better not to include the program name in an error message. The place to indicate which program is running is in the prompt or with the screen layout. (When the same program runs with input from a source other than a terminal, it is not interactive and would do best to print error messages using the noninteractive style.)
The string message should not begin with a capital letter when it follows a program name and/or file name, because that isn't the beginning of a sentence. (The sentence conceptually starts at the beginning of the line.) Also, it should not end with a period.
Error messages from interactive programs, and other messages such as usage messages, should start with a capital letter. But they should not end with a period.