This topic describes compiler remarks, warnings, and errors. The compiler sends these messages, along with the erroneous source line, to stderr.
Warning messages report legal but questionable use of C or C++. The compiler displays warnings by default. You can suppress warning messages by specifying an appropriate compiler option. Warnings do not stop translation or linking. Warnings do not interfere with any output files.
The following are some representative warning messages:
declaration does not declare anything.
pointless comparison of unsigned integer with zero.
possible use of = where == was intended.
Some warnings that start with -W can be disabled using the negative form of the option -Wno-; for example, option -Wno-unknown-pragmas disables option -Wunknown-pragmas.
Additional Warnings
The following Linux* and macOS* options produce additional warnings:
Option |
Result |
|---|---|
-W[no-]missing-prototypes |
Warn for missing prototypes. |
| -W[no-]missing-declarations | Warn for missing declarations. |
| -W[no-]unused-variable | Warn for unused variable. |
-W[no-]pointer-arith |
Warn for questionable pointer arithmetic. |
-W[no-]uninitialized |
Warn if a variable is used before being initialized. |
-W[no-]deprecated |
Display warnings related to deprecated features. |
-W[no-]abi |
Warn if generated code is not C++ ABI compliant. |
-W[no-]unused-function |
Warn if declared function is not used. |
-W[no-]unknown-pragmas |
Warn if an unknown #pragma directive is used. |
-W[no-]main |
Warn if return type of main is not expected. |
-W[no-]comment[s] |
Warn when /* appears in the middle of a /* */ comment. |
-W[no-]return-type |
Warn when a function uses the default
int return type |
These messages report syntactic or semantic misuse of C or C++. The compiler always displays error messages. Errors suppress object code for the module containing the error and prevent linking, but they allow parsing to continue to detect other possible errors.
The following are some representative error messages:
missing closing quote.
expression must have arithmetic type.
expected a ";".
Remark messages report common but sometimes unconventional use of C or C++. Remarks do not stop translation or linking. Remarks do not interfere with any output files.
The following are some representative remark messages:
function declared implicitly.
type qualifiers are meaningless in this declaration.
controlling expression is constant.
Some remarks, warnings, and errors are numbered and can be disabled using option -diag-disable=list or /Qdiag-disable:list.
Example |
|---|
// Windows* /Qdiag-disable:117,230,450 |
// Linux* and macOS* -diag-disable=117,230,450 |
You can use the following compiler options to control remarks, warnings, and errors (Note: options with a '-' prefix supported on Linux* and macOS* and options with a '/' prefix supported on Windows*):
Option |
Result |
|---|---|
-w, /W0 |
Enables diagnostics for errors; disables diagnostics for warnings. |
-w1, /W1, /W2 |
Enables diagnostics for warnings and errors. |
-w2 |
Enables diagnostics for verbose warnings, warnings, and errors. |
-w3, /W3 |
Enables diagnostics for remarks, warnings, and errors. Additional warnings are also enabled above level 2 warnings. |
/W4, /Wall |
Enables diagnostics for all level 3 warnings, plus informational warnings and remarks, which, in most cases, can be safely ignored. |
/W5 |
Enables diagnostics for all remarks, warnings, and errors. This option produces the most diagnostic messages. |
-Wremarks |
Display remarks and comments. |
-Wbrief, /WL |
Display brief one-line diagnostics. |
-Wcheck |
Enable more strict diagnostics. |
-Werror-all |
Change all warnings and remarks to errors. |
-Werror, /WX |
Change all warnings to errors. |
/Wp64 |
Display diagnostics for 64-bit porting. |
You can also control the display of diagnostic information with variations of the [Q]diag compiler option. This compiler option accepts numerous arguments and values, allowing you wide control over displayed diagnostic messages and reports.
Some of the most common variations include the following:
Option |
Result |
|---|---|
[Q]diag-enable<:|=>list |
Enables a diagnostic message or a group of messages. |
[Q]diag-disable<:|=>list |
Disables a diagnostic message or a group of messages. |
[Q]diag-warning<:|=>list |
Tells the compiler to change diagnostics to warnings. |
[Q]diag-error<:|=>list |
Tells the compiler to change diagnostics to errors. |
[Q]diag-remark<:|=>list |
Tells the compiler to change diagnostics to remarks (comments). |
The list items can be specific diagnostic IDs, one of the keywords warn, remark, or error, or a keyword specifying a certain group (par, vec, driver, thread, port-linux (available on Windows* systems), port-win (available on Linux* and macOS* systems), or openmp). For more information, see [Q]diag.
Other diagnostic-related options include the following:
Option |
Result |
|---|---|
[Q]diag-dump |
Tells the compiler to print all enabled diagnostic messages and stop compilation. |
[Q]diag-file[<:|=>file] |
Causes the results of diagnostic analysis to be output to a file. |
[Q]diag-file-append[<:|=>file] |
Causes the results of diagnostic analysis to be appended to a file. |
| [Q]diag-error-limit<:|=>n | Specifies the maximum number of errors allowed before compilation stops. |