Reading from a file into the *argv[] array - c

Question

I try to convert a Linux program in C to DOS. Since the DOS command line allows only 128 bytes I try to read in the arguments from a file. This file has one line for each argument.
So I read in the arguments from the file into a string array and want to override the *argv[] address with the address of my string array.
But I get confused with pointers to pointers. What shall I do to correctly override the *argv[] address in my function?
This program is called with "argvtest.exe commands.dat"
Here is my test code:
#include <stdio.h>
#include <string.h>
void read_cmd_file(int *argc,char *argv[]){
if (*argc != 2 ) return;
char buf[256];
char arguments[30][256];
int line_nr=0,i;
FILE *fp=(FILE *) NULL;
/* open the file */
fp=fopen(argv[1],"r");
if (fp == (FILE *) NULL)
{
printf("Could not open command file: %s",argv[1]);
return;
}
while (fgets(buf,sizeof(buf)-1,fp) && !feof(fp))
{
line_nr++;
strcpy(arguments[line_nr],buf);
//printf("Argument read: %s\n",buf);
}
if (fp != (FILE *) NULL)
(void) fclose(fp);
for (i=1;i<=line_nr;i++) printf("%d,%s\n",i,arguments[i]);
*argv=&arguments[0][0];
*argc=line_nr;
return;
}
int main(int argc,char **argv)
{
int i;
read_cmd_file(&argc,argv);
printf("argc=%d\n",argc);
for (i=0;i<argc;i++) printf("%d,%s\n",i,argv[i]);
}

Answer 1

There are two things you should do. First, you will need a pointer to pointer to pointer:
void read_cmd_file(int *argc,char ***argv)
{
...
fp=fopen((*argv)[1],"r");
...
*argv = result;
}
...
read_cmd_file(&argc,&argv);
Then, you shall never try to return a reference to a local variable from a function: in your code
*argv=&arguments[0][0];
is a sign of evil and shall not be used...
Instead, you will have to allocate memory in the function, and then release it after use. Something like:
void read_cmd_file(int *argc,char ***argv)
{
char **arguments;
...
arguments = calloc(30, sizeof(*arguments)); // 30 args
...
while (fgets(buf,sizeof(buf)-1,fp) && !feof(fp))
{
line_nr++;
arguments[line_nr] = strdup(buf); // allocates memory on heap
}
...
*argv=arguments;
}
...
int main(int argc,char **argv)
{
int i;
read_cmd_file(&argc,&argv);
for (i=0;i<argc;i++) printf("%d,%s\n",i,argv[i]);
...
for (i = 0; i <argc; i++)
{
free(argv[i]);
}
free(argv);
}