How can you copy a string to non-accessible memory? - c

Question

char* argv[MAXARGS];
char* buf2=malloc(MAXLINE * sizeof(char));
strcpy(buf2, buf); //buf is string with some words
char* ptr = strtok(buf2, " ");
argv[0]=ptr;
strcpy(argv[0], ptr);
free(buf2);
Like above, I want to copy value of ptr to argv[0] but I can't use strcpy(argv[0],ptr) directly because accessing argv[0] without argv[0]=ptr cause segmentation fault. So I made code like above but then, after I free buf2, argv[0] becomes null. How can I copy ptr to argv without using =ptr in advance?
Code:
#define MAXARGS 128
#define MAXLINE 256
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <unistd.h>
#include <sys/types.h>
#include <sys/wait.h>
#include <fcntl.h>
void eval(char* cmdline);
int parseline(char* buf, char** argv);
int main()
{
char cmdline[MAXLINE]; /* Command line */
char* ret;
while (1) {
/* Read */
printf("mini> ");
ret = fgets(cmdline, MAXLINE, stdin);
if (feof(stdin) || ret == NULL)
exit(0);
/* Evaluate */
eval(cmdline);
}
}
void eval(char* cmdline)
{
char** argv=malloc(MAXARGS*sizeof(char)); /* Argument list execve() */
char buf[MAXLINE]; /* Holds modified command line */
int bg; /* Should the job run in bg or fg? */
pid_t pid; /* Process id */
strcpy(buf, cmdline);
bg = parseline(buf, argv);
free(argv);
}
int parseline(char* buf, char** argv)
{
int argc; /* Number of args */
int bg; /* Background job? */
char* buf2=malloc(MAXLINE * sizeof(char));
while (*buf && (*buf == ' '))
buf++;
buf[strlen(buf) - 1] = ' ';/* Replace trailing '\n' with space */
strcpy(buf2, buf);
/* Build the argv list */
argc = 0;
char* ptr = strtok(buf2, " ");
printf("ptr: %s\n", ptr);
while (ptr != NULL) {
//argv[argc]=ptr;
strcpy(argv[argc++], ptr);
ptr = strtok(NULL, " ");
}
argv[argc] = NULL;
printf("0: %s\n", argv[0]);
/* Ignore blank line */
if (argc == 0)
return 1;
/* Should the job run in the background? */
if ((bg = (*argv[argc - 1] == '&')) != 0)
argv[--argc] = NULL;
free(buf2);
printf("0: %s\n", argv[0]);
if(argv[1]!=NULL)
printf("1: %s\n", argv[1]);
return bg;
}

Answer 1

Many errors in your code - I will not check everything only your issue.
Wrong allocation:
char** argv=malloc(MAXARGS*sizeof(char));
You need to allocate space for char pointers - you allocate for char. It is better to use objects instead of types.
char **argv=malloc(MAXARGS * sizeof(*argv));
Now you have allocated the memory for pointers - but not for char arrays to accommodate the strings. To directly copy to argv[n] you need to allocate this memory:
argv[n] = malloc(sizeof(**argv) * (strlen(ptr)+1));
if(argv[n]) strcpy(argv[n], ptr);
In your code you never check the result of malloc - you need to do it after every allocation/reallocation