How to fill array of character pointers with arguments taken from scanf? - c

Question

I'm trying to write a VERY basic shell program in C. The problem I am facing is trying to fill my argv array of character pointers with the words taken from input. When I attempt to print out the contents of the argv array after attempting to fill it using the parse() function below I get a segmentation fault. I know this means that I am probably trying to access part of the argv array that is out of bounds. However, even when supplying only one argument to fill the array, I still get the segfault. The printf call used to print argc returns the correct value for argc based on input, but the second printf call with *argv[0] is the one causing the segfault. I am wondering if my error is in the way I am attempting to print the contents of argv, or if the error is because I am attempting to fill argv incorrectly.
EDIT: I should add that the getword() function takes in a line of text and returns the first word delimited by spaces, and a number of other delimiters. I can post all the delimiters it breaks the words up by if necessary, but I do not think the problem is because of getword().
EDIT 2: Added in the header file and included the #include statement in main.
EDIT 3: Added the getword function under main(), and getword.h below p2.h
Here is p2.h, the header file included in main:
#include <stdio.h>
#include <unistd.h>
#include <sys/types.h>
#include <sys/stat.h>
#include <fcntl.h>
#include "getword.h"
#include <signal.h>
#define MAXITEM 100
getword.h:
#include <stdio.h>
#include <string.h>
#include <strings.h>
#define STORAGE 255
int getword(char *w);
int parse(char *, char *[]);
Here is the main function :
#include "p2.h"
int main() {
pid_t pid, child_pid;
int argc, inputRedirect;
char *devNull;
devNull = (char *) malloc(10);
strcpy(devNull, "/dev/null");
char *argv[MAXITEM];
char commandLine[STORAGE];
for (;;) {
printf("p2: ");
scanf("%s", commandLine);
argc = parse(commandLine, argv);
printf("argc = %d\n", argc);
if(argc == 0)
continue;
printf("*argv = %s\n", *argv[0]);
child_pid = fork();
if (child_pid < 0) {
printf("Cannot fork! Terminating...");
exit(1);
} else if (child_pid == 0) {
inputRedirect = open(devNull, O_RDONLY);
dup2(inputRedirect, STDIN_FILENO);
close(inputRedirect);
execvp(*argv, argv);
}
else {
for(;;) {
pid = wait(NULL);
if(pid == child_pid)
break;
}
printf("Child's pid is %d\n", child_pid);
}
}
killpg(getpid(), SIGTERM);
printf("p2 Terminated.\n");
exit(0);
}
int parse(char *commandLine, char *argv[]) {
int i, argc = 0;
char *commandPointer = commandLine;
while (*commandPointer != '\0') {
*argv = commandPointer;
argc++;
getword(commandPointer);
}
*commandPointer = '\0';
*argv = '\0';
return argc;
}
getword.c:
#include "getword.h"
#include <stdlib.h>
/*Function Prototypes*/
int tilde(char *p, int i);
int BSFollowedByMetaCharacter(int c, char *w);
int getword(char *w) {
int c;
int index = 0;
/*This while loop removes all leading blanks and whitespace characters
* The if statement then tests if the first character is a new line or
* semicolon metacharacter*/
while ((c = getchar()) == ' ' || c == '\t' || c == '\n' || c == ';') {
if (c == '\n' || c == ';') {
w[index] = '\0';
return 0;
}
}
/*This if statement calls ungetc() to push whatever character was taken
* from the input stream in the previous while loop back to the input
* stream. If EOF was taken from the input stream, ungetc() will return EOF,
* which will then cause getword() to return -1, signalling that it reached
* the End Of File. */
if (ungetc(c, stdin) == EOF)
return -1;
/*This if statement deals with some of the "non-special" metacharacters.
* If one of these metacharacters is pulled from the input stream by getchar(),
* it is stored in w and null-terminated. getword() then returns the length of
* the current string stored in w. If getchar() pulls anything besides one of the
* specified metacharacters from the input stream, it is then returned using ungetc() after
* the if statement.*/
if ((c = getchar()) == '<' || c == '>' || c == '|' || c == '&') {
w[index++] = c;
int d = getchar();
if (c == '>' && d == '>')
w[index++] = d;
else {
ungetc(d, stdin);
}
w[index] = '\0';
return index;
}
ungetc(c, stdin);
/*This while statement handles plain text from the input stream, as well as a few 'special'
* metacharacters. It also ensures that the word scanned is shorter than STORAGE-1 bytes.*/
while ((c = getchar()) != ' ' && c != '<' && c != '>' && c != '|'
&& c != ';' && c != '&' && c != '\t' && c != '\n' && c != '\0'
&& index <= STORAGE - 1) {
if (c == '~') {
int *ip = &index;
index = tilde(&w[index], *ip);
continue;
}/*END IF*/
else if (c == '\\') {
int d = c;
c = getchar();
if (BSFollowedByMetaCharacter(c, w)) {
w[index++] = c;
continue;
} else {
w[index++] = d;
}
}/*END ELSE IF*/
w[index] = c;
index++;
}/*END WHILE*/
ungetc(c, stdin);/*This final ungetc() call is used to push any meta characters*/
w[index] = '\0'; /*used as delimiters back to the input stream, to be retrieved*/
return index; /*at the next call of getword(). */
}/*END getword()*/
int tilde(char *cp, int i) {
int *ip;
ip = &i;
char *p = cp;
char *o;
o = (strcpy(p, getenv("HOME")));
int offset = strlen(o);
*ip = *ip + offset;
return i;
}
int BSFollowedByMetaCharacter(int c, char *w) {
if (c == '~' || c == '<' || c == '>' || c == '|' || c == ';' || c == '&'
|| c == ' ' || c == '\t' || c == '\\') {
return 1;
} else {
return 0;
}
}

Answer 1

The functions in getword.c seems correct. Your problem is in function parse.
To use execvp, contents of argv should be following (input:"hello world"):
argv[0] -> "hello"
argv[1] -> "world"
argv[2] -> NULL
Here, argv is an array of character pointers. But, in parse function, you are treating argv like simple character pointers in here:
*argv = commandPointer;
and here:
*argv = '\0';
Change your parse function into something like this:
int parse(char *commandLine, char *argv[]) {
int argc = 0;
char *commandPointer;
argv[argc++] = commandLine;
do{
commandPointer = (char*)malloc(sizeof(char) * STORAGE);
argv[argc++] = commandPointer;
getword(commandPointer);
}while(*commandPointer != '\0');
argv[argc] = NULL;
return argc;
}
Now, you should free allocated memory after if-else tree like:
for(int i = 0; i < argc; i++) free(argv[i]);