Parse command line arguments string into array for posix_spawn/execve - c++

Question

Given single string cmd representing program command line arguments, how to get array of strings argv, that can be passed to posix_spawn or execve.
Various forms of quoting (and escaping quotes) should be processed appropriately (resulting invocation should be same as in POSIX-compatible shell). Support for other escape characters would be desirable. Examples: #1, #2, #3.

Answer 1

As Shawn commented, in Linux and other POSIXy systems, you can use wordexp(), which is provided as part of the standard C library on such systems. For example, run.h:
#ifdef __cplusplus
extern "C" {
#endif
/* Execute binary 'bin' with arguments from string 'args';
'args' must not be NULL or empty.
Command substitution (`...` or $(...)$) is NOT performed.
If 'bin' is NULL or empty, the first token in 'args' is used.
Only returns if fails. Return value:
-1: error in execv()/execvp(); see errno.
-2: out of memory. errno==ENOMEM.
-3: NULL or empty args.
-4: args contains a command substitution. errno==EINVAL.
-5: args has an illegal newline or | & ; < > ( ) { }. errno==EINVAL.
-6: shell syntax error. errno==EINVAL.
In all cases, you can use strerror(errno) for a descriptive string.
*/
int run(const char *bin, const char *args);
#ifdef __cplusplus
}
#endif
and compile the following C source to an object file you link into your C or C++ program or library:
#define _XOPEN_SOURCE
#include <stdlib.h>
#include <unistd.h>
#include <wordexp.h>
#include <string.h>
#include <errno.h>
int run(const char *bin, const char *args)
{
/* Empty or NULL args is an invalid parameter. */
if (!args || !*args) {
errno = EINVAL;
return -3;
}
wordexp_t w;
switch (wordexp(args, &w, WRDE_NOCMD)) {
case 0: break; /* No error */
case WRDE_NOSPACE: errno = ENOMEM; return -2;
case WRDE_CMDSUB: errno = EINVAL; return -4;
case WRDE_BADCHAR: errno = EINVAL; return -5;
default: errno = EINVAL; return -6;
}
if (w.we_wordc < 1) {
errno = EINVAL;
return -3;
}
if (!bin || !*bin)
bin = w.we_wordv[0];
if (!bin || !*bin) {
errno = ENOENT;
return -1;
}
/* Note: w.ve_wordv[w.we_wordc] == NULL, per POSIX. */
if (strchr(bin, '/'))
execv(bin, w.we_wordv);
else
execvp(bin, w.we_wordv);
return -1;
}
For example, run(NULL, "ls -laF $HOME"); will list the contents of the current user's home directory. Environment variables will be expanded.
run("bash", "sh -c 'date && echo'"); executes bash, with argv[0]=="sh", argv[1]=="-c", and argv[2]=="date && echo". This lets you control what binary will be executed.