Making Arguments

Sometimes you have a string and would want to run a system call like the execv() family calls. The exec() family of functions replaces the current process image with a new process image.
The initial argument for these functions is the pathname of a file which is to be executed. The second argument points to array of pointers to null-terminated strings that represent the argument list available to the new program.

The function below helps you to prepare the second argument for execv() or execvp() from a string. If you use the function and you are using fork(), you should in the parent, free the memory allocated by using FreeMakeArgs().

/*
* $Log: MakeArgs.c,v $
* Revision 2.4 2005-03-30 22:52:47+05:30 Cprogrammer
* BUG - Incorrect free
*
* Revision 2.3 2004-07-12 22:47:58+05:30 Cprogrammer
* bug fix. Free all allocated members
*
* Revision 2.2 2002-12-21 18:21:09+05:30 Cprogrammer
* added functionality of escaping text via quotes
*
* Revision 2.1 2002-08-13 20:35:44+05:30 Cprogrammer
* addition spaces were not getting skipped
*
* Revision 1.2 2002-03-03 17:23:05+05:30 Cprogrammer
* replaced strcpy with scopy
*
* Revision 1.1 2001-12-13 01:46:09+05:30 Cprogrammer
* Initial revision
*
*/
#include "stdio.h"
#include "string.h"
#include "stdlib.h"
#include "ctype.h"

#ifndef lint
static char sccsid[] = "$Id: MakeArgs.c,v 2.4 2005-03-30 22:52:47+05:30 Cprogrammer Stab root $";
#endif

#define isEscape(ch) ((ch) == '"' || (ch) == '\'')

/*
* function to expand a string into command line
* arguments. To free memory allocated by this
* function the following should be done
*
* free(argv); free(argv[0]);
*
*/
char **
MakeArgs(char *cmmd)
{
char *ptr, *sptr, *marker;
char **argv;
int argc, idx;

for(ptr = cmmd;*ptr && isspace((int) *ptr);ptr++);
idx = strlen(ptr);
if(!(sptr = (char *) malloc((idx + 1) * sizeof(char))))
return((char **) 0);
strncpy(sptr, ptr, idx + 1);
/*-
* Get the number of arguments by counting
* white spaces. Allow escape via the double
* quotes character at the first word
*/
for (argc = 0, ptr = sptr;*ptr;)
{
for(;*ptr && isspace((int) *ptr);ptr++);
if(!*ptr)
break;
argc++;
marker = ptr;
for(;*ptr && !isspace((int) *ptr);ptr++)
{
if(ptr == marker && isEscape(*ptr))
{
for (ptr++;*ptr && !isEscape(*ptr);ptr++);
if(!*ptr)
ptr = marker;
}
}
}
#ifdef DEBUG
printf("argc = %d\n", argc);
#endif
/*
* Allocate memory to store the arguments
* Do not bother extra bytes occupied by
* white space characters.
*/
if (! (argv = (char **) malloc((argc + 1) * sizeof(char *))))
return ((char **) NULL);
for (idx = 0, ptr = sptr;*ptr;)
{
for(;*ptr && isspace((int) *ptr);ptr++)
*ptr = 0;
if(!*ptr)
break;
argv[idx++] = ptr;
marker = ptr;
for(;*ptr && !isspace((int) *ptr);ptr++)
{
if(ptr == marker && isEscape(*ptr))
{
for (ptr++;*ptr && !isEscape(*ptr);ptr++);
if(!*ptr)
ptr = marker;
else /*- Remove the quotes */
{
argv[idx - 1] += 1;
*ptr = 0;
}
}
}
}
argv[idx++] = (char *) 0;
#ifdef DEBUG
for(idx = 0;idx <= argc;idx++)
printf("argv[%d] = [%s]\n", idx, argv[idx]);
#endif
return (argv);
}

void
FreeMakeArgs(char **argv)
{
free(argv[0]);
free(argv);
return;
}