Very Computer

1. max size of arguments can be >, =, < ARG_MAX, a bug?

I am writing an application that I need to know the maximum bytes of
arguments. This ain't hard, cause the man page for exec* says the
conbined maximum of arguments size and environment variable is ARG_MAX,
and ARG_MAX is defined in /usr/include/limits.h, and it is 1048320
for solaris.

By using env -i, I do not inherit the environment. So maximum bytes of
arguments = 1048320. But my tests show that the maximum can be greater
than, equal to, less than 1048320. The following crude code show that
in that particular example, the maximum is 1048415. What is the problem?

Thanks.

[julie:/opt/mwang/c]env -i ./maxargs 1048405
arg0:1048404
arg1:1
arg2:3
arg3:2
arg4:5
======
total:1048415 NOT 1048420!

[julie:/opt/mwang/c]env -i ./maxargs 1048406
exec: Arg list too long

maxargs.c:

#include <stdlib.h>

int
main(int argc, char **argv)
{
    size_t _arg_max = (size_t) atoi(*(argv+1));
    char *arg = malloc(_arg_max);
    memset(arg, ' ', _arg_max);
    arg[_arg_max-1]='\0';
    execlp("./numargs", arg, "1", "123", "21", "12345", '\0');
    perror("exec");

numargs.c:

int
main(int argc, char **argv) {
    int i, n;
    for (i=0; i<argc; i++) {
       n=strlen(*(argv+i));
       printf("arg%d:%d\n", i, n);
    }

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