15.2 Facilities in System V m4 not in GNU m4

The version of m4 from System V contains a few facilities that have not been implemented in GNU m4 yet. Additionally, POSIX requires some behaviors that GNU m4 has not implemented yet. Relying on these behaviors is non-portable, as a future release of GNU m4 may change.

• System V m4 supports multiple arguments to defn, and POSIX requires it. This is not yet implemented in GNU m4. Unfortunately, this means it is not possible to mix builtins and other text into a single macro; a helper macro is required.
• POSIX requires an application to exit with non-zero status if it wrote an error message to stderr. This has not yet been consistently implemented for the various builtins that are required to issue an error (such as include (see Include) when a file is unreadable, eval (see Eval) when an argument cannot be parsed, or using m4exit (see M4exit) with a non-numeric argument).
• Some traditional implementations only allow reading standard input once, but GNU m4 correctly handles multiple instances of `-' on the command line.
• POSIX requires m4wrap (see M4wrap) to act in FIFO (first-in, first-out) order, but GNU m4 currently uses LIFO order. Furthermore, POSIX states that only the first argument to m4wrap is saved for later evaluation, bug GNU m4 saves and processes all arguments, with output separated by spaces.

  However, it is possible to emulate POSIX behavior by including the file examples/wrapfifo.m4 from the distribution:

            undivert(`wrapfifo.m4')dnl
            =>dnl Redefine m4wrap to have FIFO semantics.
            =>define(`_m4wrap_level', `0')dnl
            =>define(`m4wrap',
            =>`ifdef(`m4wrap'_m4wrap_level,
            =>       `define(`m4wrap'_m4wrap_level,
            =>               defn(`m4wrap'_m4wrap_level)`$1')',
            =>       `builtin(`m4wrap', `define(`_m4wrap_level',
            =>                                  incr(_m4wrap_level))dnl
            =>m4wrap'_m4wrap_level)dnl
            =>define(`m4wrap'_m4wrap_level, `$1')')')dnl
            include(`wrapfifo.m4')
            =>
            m4wrap(`a`'m4wrap(`c
            ', `d')')m4wrap(`b')
            =>
            ^D
            =>abc
       

• POSIX requires that all builtins that require arguments, but are called without arguments, behave as though empty strings had been passed. For example, a`'define`'b would expand to ab. But GNU m4 ignores certain builtins if they have missing arguments, giving adefineb for the above example.
• Traditional implementations handle define(`f',`1') (see Define) by undefining the entire stack of previous definitions, and if doing undefine(`f') first. GNU m4 replaces just the top definition on the stack, as if doing popdef(`f') followed by pushdef(`f',`1').
• POSIX requires syscmd (see Syscmd) to evaluate command output for macro expansion, but this appears to be a mistake in POSIX since traditional implementations did not do this. GNU m4 follows traditional behavior in syscmd, and provides the extension esyscmd that provides the POSIX semantics.
• POSIX requires maketemp (see Maketemp) to replace the trailing `X' characters with the m4 process id, giving the same result on identical input, without creating any files, which leaves the door open for a data race in which other processes can create a file by the same name. GNU m4 actually creates a temporary file for each invocation of maketemp, which means that the output of the macro is different even if the input is identical.
• POSIX requires changequote(arg) (see Changequote) to use newline as the close quote, but GNU m4 uses `'' as the close quote. Meanwhile, some traditional implementations use arg as the close quote, making it impossible to nest quotes. For predictable results, never call changequote with just one argument.
• Some implementations of m4 give macros a higher precedence than comments when parsing, meaning that if the start delimiter given to changecom (see Changecom) starts with a macro name, comments are effectively disabled. POSIX does not specify what the precedence is, so the GNU m4 parser recognizes comments, then macros, then quoted strings.
• Traditional implementations allow argument collection, but not string and comment processing, to span file boundaries. Thus, if a.m4 contains `len(', and b.m4 contains `abc)', m4 a.m4 b.m4 outputs `3' with traditional m4, but gives an error message that the end of file was encountered inside a macro with GNU m4. On the other hand, traditional implementations do end of file processing for files included with include or sinclude (see Include), while GNU m4 seamlessly integrates the content of those files. Thus include(`a.m4')include(`b.m4') will output `3' instead of giving an error.
• Traditional m4 treats traceon (see Trace) without arguments as a global variable, independent of named macro tracing. Also, once a macro is undefined, named tracing of that macro is lost. On the other hand, when GNU m4 encounters traceon without arguments, it turns tracing on for all existing definitions at the time, but does not trace future definitions; traceoff without arguments turns tracing off for all definitions regardless of whether they were also traced by name; and tracing by name, such as with -tfoo at the command line or traceon(`foo') in the input, is an attribute that is preserved even if the macro is currently undefined.
• POSIX requires eval (see Eval) to treat all operators with the same precedence as C. However, GNU m4 currently follows the traditional precedence of other m4 implementations, where bitwise and logical negation (`~' and `!') have lower precedence than equality operators, rather than equal precedence with other unary operators. Use explicit parentheses to ensure proper precedence. As extensions to POSIX, GNU m4 treats the shift operators `<<' and `>>' as well-defined on signed integers (even though they are not in C), and adds the exponentiation operator `**'.
• POSIX requires translit (see Translit) to treat each character of the second and third arguments literally, but GNU m4 treats `-' as a range operator.
• POSIX requires m4 to honor the locale environment variables of LANG, LC_ALL, LC_CTYPE, LC_MESSAGES, and NLSPATH, but this has not yet been implemented in GNU m4.