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How long can be a string passed to system()?

I know the POSIX minimum is 4096, but I would like to know the actual size I can use. Is there any macro defined in any header for that, similar to FILENAME_MAX?

char cmd[SOME_MACRO];

...
system(cmd);

system exec's a shell with arguments "sh","-c", YourAgumentToSystem, (char*)0 (guaranteed by POSIX), so
the maximum length (not counting the '' terminator) is ARG_MAX -1 -3 -3 - size_of_your_environment.

ARG_MAX is defined in limits.h as

"Maximum length of argument to the exec functions including
environment data."

To measure the size of your environment, you can run:

extern char **environ;
size_t envsz = 0; for(char **e=environ; *e; e++) envsz += strlen(*e)+1;

The limit is highly system dependent. It may even depend on the command shell that will be used. You should test the return value of system() to see if the system call was successful: -1 means failure and errno should give you more information. The behavior should be defined for any proper C string.

POSIX documents that system(command) is equivalent to:

execl(<shell path>, "sh", "-c", command, (char *)0);

And also documents ARG_MAX defined in <limits.h> as the limit for the combined lengths of the arguments to exec and the environment variables.

Note however that command may contain wildcards and/or other shell words whose expansion may exceed some other limit. Always check the return value for failure.

man 3 system

gives us

DESCRIPTION

The system() library function uses fork(2) to create a child process that executes the shell command specified in command
using execl(3) as follows:

 execl("/bin/sh", "sh", "-c", command, (char *) 0);

 system() returns after the command has been completed.

so system() is a wrapper for execl()

From the same page we also see that this call conforms to some standards.

CONFORMING TO

POSIX.1-2001, POSIX.1-2008, C89, C99.

Looking up POSIX.1-2008 produces the following online reference

https://pubs.opengroup.org/onlinepubs/9699919799/

Where we can search for info on the execl function which system takes us to

https://pubs.opengroup.org/onlinepubs/9699919799/functions/exec.html

Which offers up the following

The number of bytes available for the new process' combined argument and environment lists is ARG_MAX. It is implementation-defined whether null terminators, pointers, and/or any alignment bytes are included in this total.

And finally ...

ERRORS

The exec functions shall fail if:

[E2BIG] The number of bytes used by the new process image's argument
list and environment list is greater than the system-imposed limit of
ARG_MAX bytes.

So the final check to carry out here is the actual exec implementation rather than relying on the standard just in case the implementation deviated from the standard.

So, man 3 execl reports that the errors returned are the same as documented for execve(2) and man 2 execvw reports the following:

ERRORS

E2BIG The total number of bytes in the environment (envp) and argument list (argv) is too large.

Not as precise as the POSIX standard? Best check the code or see the (now) accepted answer :)