GCC Warnings

Many projects enable additional diagnostics by using the -Wall and -Wextra command-line options. Some projects even turn them into errors via -Werror as their first line of defense.

Front-end Warnings

• Preprocessor warnings
  • -Wunused-macros
• Lexical warnings
  • -Wempty-body detects if or for statements with no body
  • -Wsizeof-array-argument detects applying the sizeof operator to a function parameter declared using the array
  • -Wmissing-parameter-type detects declaring function parameters without specifying their type
• Type-safety warnings
  • -Wint-to-pointer-cast and -Wpointer-to-int-cast
  • -Wsign-compare detects comparisons between expressions with different signedness.
  • -Wcast-qual
    • warns whenever a pointer is cast so as to remove a type qualifier from the target type. For example, warn if a const char * is cast to an ordinary char *.
    • warns when making a cast that introduces a type qualifier in an unsafe way. For example, casting char ** to const char ** is unsafe
  • -Wcast-align warns whenever a pointer is cast such that the required alignment of the target is increased. For example, warn if a char * is cast to an int * on machines where integers can only be accessed at two- or four-byte boundaries.
  • -Wcast-function-type warns when a function pointer is cast to an incompatible function pointer. The function type void (*) (void) is special and matches everything, which can be used to suppress this warning.
• Others
  • -Wunused-value, -Wunused-result, and -Wunused-function
  • -Wshift-count-negative, -Wshift-count-overflow, -Wshift-negative-value, and -Wshift-overflow

Middle-end Warnings

Also called flow-based warnings.

• -Warray-bounds detects out-of-bounds array indices
• -Wformat-overflow looks for buffer overflows in calls to sprintf and related functions
• -Wnonnull detects passing null pointers to functions that don’t expect it
• -Wstringop-overflow helps find buffer overflow in calls to string and raw memory functions
• -Wuninitialized finds uses of uninitialized variables

Reference

• Understanding GCC warnings
• Understanding GCC warnings, Part 2
• GCC Online Doc Sec. 3.8 Options to Request or Suppress Warnings
• GCC Online Doc Sec. 14.9 Warning Messages and Error Messages