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Following ARG_MAX, maximum length of arguments for a new process it seems like ARG_MAX is wrongly (or at least ambiguously) defined on my Mac Mini 3,1 running Ubuntu 12.04:

$ getconf ARG_MAX # arguments 
2097152
$ locate limits.h | xargs grep -ho 'ARG_MAX[ \t]\+[0-9]\+' | uniq | cut -d ' ' -f 8
131072

The actual limit seems to be somewhere between these:

$ cd "$(mktemp -d)"
$ touch $(seq 1 131072) && find . -mindepth 1 -printf x | wc -c && rm *
131072
$ touch $(seq 1 131073) && find . -mindepth 1 -printf x | wc -c && rm *
131073
$ touch $(seq 1 $(getconf ARG_MAX)) && find . -mindepth 1 -printf x | wc -c && rm *
bash: /usr/bin/touch: Argument list too long

I did a small search:

cd "$(mktemp -d)"
min=131072
max=2097152
while true
do
    search=$((min + (max - min) / 2))
    if touch $(seq 1 $search) 2>/dev/null
    then
        min=$search
    else
        max=$search
    fi
    [[ $((max - min)) -le 1 ]] && echo "ARG_MAX = $min" && break
done

Eventually this resulted in ARG_MAX = 314290, which doesn't seem to have any relation to either of the ARG_MAX values found before. Is this normal? Is there a simpler way to find the actual ARG_MAX?

Did I misunderstand the definition of ARG_MAX? It seems it's actually the byte (or possibly character) length of the arguments with or without (?) the separating spaces. If it's really the byte length, are there also other restrictions?

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2 Answers 2

up vote 4 down vote accepted

Yes, it's the length in bytes, including the environment.

Very roughly:

$ { seq 1 314290; env; } | wc -c
2091391

linux sysconf

The maximum length of the arguments to the exec(3) family of functions. Must not be less than _POSIX_ARG_MAX (4096).

POSIX 2004 limits.h

Maximum length of argument to the exec functions including environment data. Minimum Acceptable Value: {_POSIX_ARG_MAX}

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The page that you linked to about ARG_MAX states that as of kernel version 2.6.23 it is 1/4th the stack size. It even links to the git commit responsible.

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