2.201 Pragma Warnings ¶

Syntax:

pragma Warnings ([TOOL_NAME,] DETAILS [, REASON]);

DETAILS ::= On | Off
DETAILS ::= On | Off, local_NAME
DETAILS ::= static_string_EXPRESSION
DETAILS ::= On | Off, static_string_EXPRESSION

TOOL_NAME ::= GNAT | GNATprove

REASON ::= Reason => STRING_LITERAL {& STRING_LITERAL}

Note: in Ada 83 mode, a string literal may be used in place of a static string expression (which does not exist in Ada 83).

Note if the second argument of DETAILS is a local_NAME then the second form is always understood. If the intention is to use the fourth form, then you can write NAME & "" to force the intepretation as a `static_string_EXPRESSION'.

Note: if the first argument is a valid TOOL_NAME, it will be interpreted that way. The use of the TOOL_NAME argument is relevant only to users of SPARK and GNATprove, see last part of this section for details.

Normally warnings are enabled, with the output being controlled by the command line switch. Warnings (Off) turns off generation of warnings until a Warnings (On) is encountered or the end of the current unit. If generation of warnings is turned off using this pragma, then some or all of the warning messages are suppressed, regardless of the setting of the command line switches.

The Reason parameter may optionally appear as the last argument in any of the forms of this pragma. It is intended purely for the purposes of documenting the reason for the Warnings pragma. The compiler will check that the argument is a static string but otherwise ignore this argument. Other tools may provide specialized processing for this string.

The form with a single argument (or two arguments if Reason present), where the first argument is ON or OFF may be used as a configuration pragma.

If the LOCAL_NAME parameter is present, warnings are suppressed for the specified entity. This suppression is effective from the point where it occurs till the end of the extended scope of the variable (similar to the scope of Suppress). This form cannot be used as a configuration pragma.

In the case where the first argument is other than ON or OFF, the third form with a single static_string_EXPRESSION argument (and possible reason) provides more precise control over which warnings are active. The string is a list of letters specifying which warnings are to be activated and which deactivated. The code for these letters is the same as the string used in the command line switch controlling warnings. For a brief summary, use the gnatmake command with no arguments, which will generate usage information containing the list of warnings switches supported. For full details see the section on Warning Message Control in the GNAT User’s Guide. This form can also be used as a configuration pragma.

The warnings controlled by the -gnatw switch are generated by the front end of the compiler. The GCC back end can provide additional warnings and they are controlled by the -W switch. Such warnings can be identified by the appearance of a string of the form [-W{xxx}] in the message which designates the -W`xxx' switch that controls the message. The form with a single `static_string_EXPRESSION' argument also works for these warnings, but the string must be a single full -W`xxx' switch in this case. The above reference lists a few examples of these additional warnings.

The specified warnings will be in effect until the end of the program or another pragma Warnings is encountered. The effect of the pragma is cumulative. Initially the set of warnings is the standard default set as possibly modified by compiler switches. Then each pragma Warning modifies this set of warnings as specified. This form of the pragma may also be used as a configuration pragma.

The fourth form, with an On|Off parameter and a string, is used to control individual messages, based on their text. The string argument is a pattern that is used to match against the text of individual warning messages (not including the initial "warning: " tag).

The pattern may contain asterisks, which match zero or more characters in the message. For example, you can use pragma Warnings (Off, "bits of*unused") to suppress the warning message warning: 960 bits of "a" unused. No other regular expression notations are permitted. All characters other than asterisk in these three specific cases are treated as literal characters in the match. The match is case insensitive, for example XYZ matches xyz.

Note that the pattern matches if it occurs anywhere within the warning message string (it is not necessary to put an asterisk at the start and the end of the message, since this is implied).

The above use of patterns to match the message applies only to warning messages generated by the front end. This form of the pragma with a string argument can also be used to control warnings provided by the back end and mentioned above. By using a single full -W`xxx' switch in the pragma, such warnings can be turned on and off.

There are two ways to use the pragma in this form. The OFF form can be used as a configuration pragma. The effect is to suppress all warnings (if any) that match the pattern string throughout the compilation (or match the -W switch in the back end case).

The second usage is to suppress a warning locally, and in this case, two pragmas must appear in sequence:

pragma Warnings (Off, Pattern);
... code where given warning is to be suppressed
pragma Warnings (On, Pattern);

In this usage, the pattern string must match in the Off and On pragmas, and (if `-gnatw.w' is given) at least one matching warning must be suppressed.

Note: if the ON form is not found, then the effect of the OFF form extends until the end of the file (pragma Warnings is purely textual, so its effect does not stop at the end of the enclosing scope).

Note: to write a string that will match any warning, use the string "***". It will not work to use a single asterisk or two asterisks since this looks like an operator name. This form with three asterisks is similar in effect to specifying pragma Warnings (Off) except (if -gnatw.w is given) that a matching pragma Warnings (On, "***") will be required. This can be helpful in avoiding forgetting to turn warnings back on.

Note: the debug flag -gnatd.i can be used to cause the compiler to entirely ignore all WARNINGS pragmas. This can be useful in checking whether obsolete pragmas in existing programs are hiding real problems.

Note: pragma Warnings does not affect the processing of style messages. See separate entry for pragma Style_Checks for control of style messages.

Users of the formal verification tool GNATprove for the SPARK subset of Ada may use the version of the pragma with a TOOL_NAME parameter.

If present, TOOL_NAME is the name of a tool, currently either GNAT for the compiler or GNATprove for the formal verification tool. A given tool only takes into account pragma Warnings that do not specify a tool name, or that specify the matching tool name. This makes it possible to disable warnings selectively for each tool, and as a consequence to detect useless pragma Warnings with switch -gnatw.w.