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(nasm.info)Section 2.1.18

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2.1.18. The `-w' Option: Enable or Disable Assembly Warnings
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   NASM can observe many conditions during the course of assembly which
are worth mentioning to the user, but not a sufficiently severe error to
justify NASM refusing to generate an output file. These conditions are
reported like errors, but come up with the word `warning' before the
message. Warnings do not prevent NASM from generating an output file and
returning a success status to the operating system.

   Some conditions are even less severe than that: they are only
sometimes worth mentioning to the user. Therefore NASM supports the `-w'
command-line option, which enables or disables certain classes of
assembly warning. Such warning classes are described by a name, for
example `orphan-labels'; you can enable warnings of this class by the
command- line option `-w+orphan-labels' and disable it by
`-w-orphan-labels'.

   The suppressible warning classes are:

   * `macro-params' covers warnings about multi-line macros being
     invoked with the wrong number of parameters. This warning class is
     enabled by default; see *Note Section 4.3.1:: for an example of
     why you might want to disable it.

   * `macro-selfref' warns if a macro references itself. This warning
     class is enabled by default.

   * `orphan-labels' covers warnings about source lines which contain no
     instruction but define a label without a trailing colon. NASM does
     not warn about this somewhat obscure condition by default; see
     *Note Section 3.1:: for an example of why you might want it to.

   * `number-overflow' covers warnings about numeric constants which
     don't fit in 32 bits (for example, it's easy to type one too many
     Fs and produce `0x7ffffffff' by mistake). This warning class is
     enabled by default.

   * `gnu-elf-extensions' warns if 8-bit or 16-bit relocations are used
     in `-f elf' format. The GNU extensions allow this. This warning
     class is enabled by default.

   * In addition, warning classes may be enabled or disabled across
     sections of source code with `[warning +warning-name]' or
     `[warning -warning-name]'. No "user form" (without the brackets)
     exists.