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execs is (was) missing

what

execve(2) system call has a number of helper functions giving users many options to specify the command line args. (e.g. execl, execlp, execle, execv, execvp, execvpe...). A way to specify the args as one string (as args are given when typing a command using a shell) is missing. More precisely, it was missing, because the library s2argv-execs has filled the lack.

why

  • because otherwise programmers use the unsafe system(3)
  • or (even worse) use fork/exec of /bin/ssh -c "..."
  • wise programmers must survive code wrestling using strtok

how

the syntax is clear and follows the standard naming scheme of exec* functions:

int execs(const char *path, const char *args);
int execse(const char *path, const char *args, char *const envp[]);
int execsp(const char *args);
int execspe(const char *args, char *const envp[]);

Notes:

  • Command arguments in args are delimited by space characters (blank, tabs or new lines). Single or double quotes can be used to delimitate command arguments including spaces and a non quoted back‐ slash (\) is the escape character to protect the next char.
  • execsp does not need any pathname, it uses argv[0] as parsed from args.
  • args is const, i.e. exec* functions do not modify it.
  • execs* functions do not use dynamic allocation (allocate memory on the stack)
  • execs* functions are thread safe
  • the library provides also eexecs functions (using less memory, but modifying args)
  • for lazy programmers, the library includes replacements for system(3) and popen(3) (named system_nosh and popen_nosh respectively) using execs instead of starting a shell.
  • system_safe is a safe version of system(3). It solves the security problems of system: attacks using environment variables (e.g. PATH), command separators (like ; || &&) or redirections.

where

Virtualsquare uses execs in many places. An incomplete list includes:

  • cado/scado.c: to start the command
  • libvdestack/vdestack.c: uses a more advanced interface provided by the library s2multiargv (mutiple commands, semicolon ; separated.)

references:

on GITHUB