Unable to pass a user input array to execvp

Question

This is my input function:

    int i,j,n,len,c;
    char *buffer = 0;
    size_t bufsize = 0;
    ssize_t characters;

    characters = getline(&buffer, &bufsize, stdin);

    len = strlen(buffer);
    buffer[len-1]='\0';

    if (characters > 0)
    {
        char *end_str1;
        char *token1 = strtok_r(buffer, ";", &end_str1);
        int count = 0, wordcnt;
        while (token1 != NULL)
        {
            char cmd[10][101];
            memset(cmd,0,sizeof(cmd));
            wordcnt = 0;
            char *end_str2;
            count++;
            char *token2 = strtok_r(token1, " ", &end_str2);
            while (token2 != NULL)
            {
                n = strlen(token2);
                strncpy(cmd[wordcnt],token2,n);
                wordcnt++;
                token2 = strtok_r(NULL, " ", &end_str2);
            }
            cmd[wordcnt+1][0]='\0';
            execvp(cmd[0],cmd);
            token1 = strtok_r(NULL, ";", &end_str1);
        }
    }
    free(buffer);

The only warning is incompatible pointer type, but my cmd array is a **ptr, so I don't understand what is wrong.

I tried equating it to **k, and then passing that into execvp. Didn't work. I tried changing cmd into a **cmd, I think I did something wrong there, because it should have worked but it didn't.

whatever commands I enter into the program like cd ..; pwd are stored in buffer. — Rosie Red
– Rosie Red, Commented Sep 24, 2016 at 9:36

alk · Accepted Answer · 2016-09-24 13:06:13Z

1

my cmd array is a [char] **ptr

No, it isn't.

It's a char[10][100] and when being passed to execvp() it decays to a pointer to its first element, which is char(*)[101].

Also you have an "off-by-one" error here:

cmd[wordcnt+1][0]='\0';

wordcnt had already been incremented inside the tokenising loop.

What you want is:

#define MAX_NO_OF_CMD_ELEMENTS (10)

...

        char * cmd[MAX_NO_OF_CMD_ELEMENTS + 1]; /* 1+ for the NULL-terminator */
        size_t wordcnt = 0;
        char *end_str2;
        count++;
        token2 = strtok_r(token1, " ", &end_str2);
        while ((NULL != token2)
               && (MAX_NO_OF_CMD_ELEMENTS > wordcnt)) /* Prevent writing
                                                         out of `cmd`'s bounds. */
        {
            cmd[wordcnt] = token2;
            wordcnt++;
            token2 = strtok_r(NULL, " ", &end_str2)
        }
        cmd[wordcnt] = NULL;

        execvp(cmd[0], cmd);

Also^2 getline() returns a 0-terminated char-array (a "string") already. No need for this

    len = strlen(buffer);
    buffer[len-1]='\0';

edited Sep 24, 2016 at 13:06

answered Sep 24, 2016 at 10:11

alk

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9 Comments

Rosie Red Over a year ago

What about doing the token2 = strtok_r(NULL, " ", &end_str2);? Is that included in the while loop?

Rosie Red Over a year ago

When I use this loop for input, the program only reads the first word into all the places in the array. Did I make a mistake? ideone.com/fRxv5z (relevant code at the end)

Rosie Red Over a year ago

No problem. However I think I have integrated your code into mine wrong, because pwd works, but echo doesn't. ideone.com/fRxv5z [relevant code at end]

Rosie Red Over a year ago

Oh no... That means the progam will end there. I need to fork there, don't I?

alk Over a year ago

@RosieRed: Make sure you call token1 = strtok_r(NULL, ";", &end_str1); after fork/exec inside the parent.'s outer while-loop.

|

Cyclotron3x3 · Accepted Answer · 2016-09-24 09:43:23Z

0

Its because you are passing wrong parameters to function execvp.

execvp(cmd[0],cmd); //this line

Its declaration is like this:

extern int execvp (const char *__file, char *const __argv[])

and you are passing argument of type char (*)[101] where it is expecting char *, const* hence throwing incompatible error.

answered Sep 24, 2016 at 9:43

Cyclotron3x3

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1 Comment

Rosie Red Over a year ago

How do I fix it? Shall I create a **cmd array and then malloc to it and write to it and then pass it to exec??

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