Why is ARG_MAX not defined via limits.h?

Question

On Fedora Core 7, I'm writing some code that relies on ARG_MAX. However, even if I #include <limits.h>, the constant is still not defined. My investigations show that it's present in <sys/linux/limits.h>, but this is supposed to be portable across Win32/Mac/Linux, so directly including it isn't an option. What's going on here?

Answer 1

The reason it's not in limits.h is that it's not a quantity giving the limits of the value range of an integral type based on bit width on the current architecture. That's the role assigned to limits.h by the ISO standard.

The value in which you're interested is not hardware-bound in practice and can vary from platform to platform and perhaps system build to system build.

The correct thing to do is to call sysconf and ask it for "ARG_MAX" or "_POSIX_ARG_MAX". I think that's the POSIX-compliant solution anyway.

Acc. to my documentation, you include one or both of unistd.h or limits.h based on what values you're requesting.

One other point: many implementations of the exec family of functions return E2BIG or a similar value if you try to call them with an oversized environment. This is one of the defined conditions under which exec can actually return.

Answer 2

ARG_MAX is defined in /usr/include/linux/limits.h. My linux kernel version is 3.2.0-38.

Answer 3

For the edification of future people like myself who find themselves here after a web search for "arg_max posix", here is a demonstration of the POSIXly-correct method for discerning ARG_MAX on your system that Thomas Kammeyer refers to in his answer:

cc -x c <(echo '
  #include <unistd.h>
  #include <stdio.h>
  int main() { printf("%li\n", sysconf(_SC_ARG_MAX)); }
')

This uses the process substitution feature of Bash; put the same lines in a file and run cc thefile.c if you are using some other shell.

Here's the output for macOS 10.14:

$ ./a.out
262144

Here's the output for a RHEL 7.x system configured for use in an HPC environment:

$ ./a.out
4611686018427387903

$ ./a.out | numfmt --to=iec-i  # 'numfmt' from GNU coreutils
4.0Ei

For contrast, here is the method prescribed by https://porkmail.org/era/unix/arg-max.html, which uses the C preprocessor:

cpp <<HERE | tail -1
#include <limits.h>
ARG_MAX
HERE

This does not work on Linux for reasons still not entirely clear to me—I am not a systems programmer and not conversant in the POSIX or ISO specs—but probably explained above.