Why is the subprocess.Popen argument length limit smaller than what the OS reports?

Question

I am running Python 3.4.3 on Linux 3.16.0. I want to use subprocess.Popen to run a command with a long single argument (a complex Bash invocation), roughly 200K

Answer 1

The maximum size for a single string argument is limited to 131072. It has nothing to do with python:

~$ /bin/echo "$(printf "%*s" 131071 "a")">/dev/null
~$ /bin/echo "$(printf "%*s" 131072 "a")">/dev/null
bash: /bin/echo: Argument list too long

It is actually the MAX_ARG_STRLEN that decides the max size for a single string:

And as additional limit since 2.6.23, one argument must not be longer than MAX_ARG_STRLEN (131072). This might become relevant if you generate a long call like "sh -c 'generated with long arguments'". (pointed out by Xan Lopez and Ralf Wildenhues)

See this discussion of ARG_MAX, under "Number of arguments and maximum length of one argument", and this question on unix.stackexchange.

You can see it in binfmts.h:

/*
 * These are the maximum length and maximum number of strings passed to the
 * execve() system call.  MAX_ARG_STRLEN is essentially random but serves to
 * prevent the kernel from being unduly impacted by misaddressed pointers.
 * MAX_ARG_STRINGS is chosen to fit in a signed 32-bit integer.
 */
#define MAX_ARG_STRLEN (PAGE_SIZE * 32)
#define MAX_ARG_STRINGS 0x7FFFFFFF

~$ echo $(( $(getconf PAGE_SIZE)*32 )) 
131072

You can pass multiple strings of length 131071:

subprocess.check_call(['echo', "a"*131071,"b"*131071], executable='/bin/bash',stdout=open("/dev/null","w"))

But a single string arg cannot be longer than 131071 bytes.