Question

I wanna understand this code specially the parse_args. This code is as simple shell to run the basic linux command as pwd cat and so on. I wanna understand how parse_args works.

#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <string.h>
#include <errno.h>
#include <sys/types.h>
#define BUFFER_SIZE 1<<16
#define ARR_SIZE 1<<16

void parse_args(char *buffer, char** args, 
                size_t args_size, size_t *nargs)
{
    char *buf_args[args_size]; /* You need C99 */
    char **cp;
    char *wbuf;
    size_t i, j;

    wbuf=buffer;
    buf_args[0]=buffer; 
    args[0] =buffer;

    for(cp=buf_args; (*cp=strsep(&wbuf, " \n\t")) != NULL ;){
        if ((*cp != '\0') && (++cp >= &buf_args[args_size]))
            break;
    }

    for (j=i=0; buf_args[i]!=NULL; i++){
        if(strlen(buf_args[i])>0)
            args[j++]=buf_args[i];
    }

    *nargs=j;
    args[j]=NULL;
}


int main(int argc, char *argv[], char *envp[]){
    char buffer[BUFFER_SIZE];
    char *args[ARR_SIZE];

    int *ret_status;
    size_t nargs;
    pid_t pid;

    while(1){
        printf("$ ");
        fgets(buffer, BUFFER_SIZE, stdin);
        parse_args(buffer, args, ARR_SIZE, &nargs); 

        if (nargs==0) continue;
        if (!strcmp(args[0], "exit" )) exit(0);       
        pid = fork();
        if (pid){
            printf("Waiting for child (%d)\n", pid);
            pid = wait(ret_status);
            printf("Child (%d) finished\n", pid);
        } else {
            if( execvp(args[0], args)) {
                puts(strerror(errno));
                exit(127);
            }
        }
    }    
    return 0;
}

Answer 1

in this section of code

for(cp=buf_args; (*cp=strsep(&wbuf, " \n\t")) != NULL ;){
        if ((*cp != '\0') && (++cp >= &buf_args[args_size]))
            break;
 }

check the wbuf array for ” \n\t” delimiters (first is space) and save its positions in buf_args
then in

for (j=i=0; buf_args[i]!=NULL; i++){
        if(strlen(buf_args[i])>0)
            args[j++]=buf_args[i];
    }

section exrtacting the arguments into args array to have the result out of the function