How to handle ls | "pipe" command in C

Question

I have a shell program written in C, and when I try to run the command "ls |" (Just ls and a pipe with nothing after it), my program loops forever. You can ignore the modePtr... What this parser does is take a string of characters from user input, so it is parsing the command "ls |" I'm guessing the issue is with the fact there isn't a "case" that an empty pipe command will fall into, causing an infinite loop? Any suggestions?

int parse(char *inputString, char *cmdArgv[], char **supplementPtr, int *modePtr)
{
int cmdArgc = 0, terminate = 0;
char *srcPtr = inputString;
//printf("parse fun%sends", inputString);
while(*srcPtr != '\0' && terminate == 0)
{
    *cmdArgv = srcPtr;
    cmdArgc++;
    //printf("parse fun2%sends", *cmdArgv);
    while(*srcPtr != ' ' && *srcPtr != '\t' && *srcPtr != '\0' && *srcPtr != '\n' && terminate == 0)
    {
        // Handles redirection/pipes
        switch(*srcPtr)
        {
            // Background mode
            case '&':
                *modePtr = BACKGROUND;
                break;
            // Output mode
            case '>':
                *modePtr = OUTPUT_REDIRECTION;
                *cmdArgv = '\0';
                srcPtr++;
                if(*srcPtr == '>')
                {
                    *modePtr = OUTPUT_APP;
                    srcPtr++;
                }
                while(*srcPtr == ' ' || *srcPtr == '\t' || *srcPtr == '\0')
                    srcPtr++;
                *supplementPtr = srcPtr;
                chop(*supplementPtr);
                terminate = 1;
                break;
            // Input mode
            case '<':
                *modePtr = INPUT_REDIRECTION;
                *cmdArgv = '\0';
                srcPtr++;
                while(*srcPtr == ' ' || *srcPtr == '\t' || *srcPtr == '\0')
                    srcPtr++;
                *supplementPtr = srcPtr;
                chop(*supplementPtr);
                terminate = 1;
                break;
            // Pipe mode
            case '|':
                *modePtr = PIPELINE;
                *cmdArgv = '\0';
                srcPtr++;
                while(*srcPtr == ' ' || *srcPtr == '\t' || *srcPtr == '\0')
                    srcPtr++;
                *supplementPtr = srcPtr;
                //chop(*supplementPtr);
                terminate = 1;
                break;
        }
        srcPtr++;
    }
    // Process commands when these occur
    while((*srcPtr == ' ' || *srcPtr == '\t' || *srcPtr == '\n') && terminate == 0)
    {
        *srcPtr = '\0';
        srcPtr++;
    }
    cmdArgv++;
}
/*srcPtr++;
*srcPtr = '\0';
destPtr--;*/
*cmdArgv = '\0';
return cmdArgc;
}

Answer 1

You may have overcomplicated the problem. If you want a program to read a list items piped to it...

#define MAXLINELEN 1000
#include <stdio.h>
#include <stdlib.h>

int main (int argc, char *argv[]) {

    char line[MAXLINELEN];
    FILE *fpin;

    fpin=stdin;

    while(fgets(line,MAXLINELEN,fpin)!=NULL) printf("%s",line);

}

... you can then parse the items on each line using strtok(), or if you use "ls -1", each list item is on a unique line anyway.

Hope this helps.

Answer 2

If you try ls | in an ordinary shell, it will prompt you for the rest of the pipeline (another command) before trying to launch anything. A pipe with no command to read the output is nonsense.

This idiom (repeated several times in the code) is broken:

srcPtr++;
while (*srcPtr == ' ' || *srcPtr == '\t' || *srcPtr == '\0')
    srcPtr++;

You can't afford to skip over the null '\0' at the end of the string! You immediately fall into 'undefined behaviour'. You need to review the loop logic, perhaps to:

srcPtr++;
while (*srcPtr == ' ' || *srcPtr == '\t')
    srcPtr++;
if (*srcPtr == '\0')
    ...no more data in string...