execvp() function 一直返回-1

Question

In my code, I'm trying to use the function execvp() to execute a command that I get in my shell but the function always returns -1 that indicates unsuccess, when I replace the function first argument by (for example) "ps" it works fine but when it is (command) it doesn't work, I've checked that command is fine by printing it after getting it from the input line and it is a fine string with no problems, but the function keeps returning me an error!!

#include <sys/types.h>
#include <sys/wait.h>
#include <stdio.h>
#include <unistd.h>
#include <string.h>
#include <stdlib.h>

#define BUFFER_SIZE 100

int main(void)
{
    close(2);
    dup(1);
    char command[BUFFER_SIZE];

    while (1)
    {
        char *arg[3];

        fprintf(stdout, "my-shell> ");
        memset(command, 0, BUFFER_SIZE);
        fgets(command, BUFFER_SIZE, stdin);
        if(strncmp(command, "exit", 4) == 0)
        {
            break;
        }
        arg[0] = command;
        arg[1] = "\0";
        arg[2] = "\0";
        i = execvp(command,arg);
        printf("%d",i);

    }

    return 0;
}

I expect that the problem is in the way that command is passed in the function but after trying so much edites to the code, I still can't figure out what the problem really is!

Answer 1

There are 3 major problems and 1 minor one that can be picked out of the code shown (plus what I take to be an artefact of reducing your full code to the code in the question, plus some oddities):

1. The fgets() function includes the newline in the returned string unless the line is too long (a separate problem). You need to zap that newline:

    command[strcspn(command, "\n")] = '\0';

2. The code does not parse the line that's entered, so only single word commands can sensibly be entered. To fix that, you'd have to be prepared to split the line into words using an appropriate algorithm, removing quotes where appropriate, expanding variables and so on. That will be part of the later stages of developing your shell.

3. The second argument to execvp() needs to be a NULL-terminated list of strings. You only provide the command name and two empty strings without the null terminator, which gives undefined behaviour.

The minor problem is that using "\0" instead of just "" is pointless.

The artefact is that there is no fork() in the code, so if the command is executed successfully, the 'shell' is replaced by the command and exits when the replacement exits.

The close(2); dup(1); close(2); dup(1); sequence is weird — it means standard error refers to the same file descriptor as standard output. Those lines really aren't needed (or desirable). Leave the errors separate from standard output.

The memset() is superfluous too. Using fprintf(stdout, "my-shell> "); is a funny way of writing printf("my-shell> "); . Using strncmp(command, "exit", 4) means that if the user types exit-or-continue , you'll treat it the same as exit , which is far from ideal.

Putting most of those numerous changes into effect (omitting parsing the command line into separate arguments) leaves:

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <sys/wait.h>
#include <unistd.h>

#define BUFFER_SIZE 100

int main(void)
{
    char command[BUFFER_SIZE];

    while (1)
    {
        printf("my-shell> ");
        fflush(stdout);
        if (fgets(command, BUFFER_SIZE, stdin) != command)
            break;
        command[strcspn(command, "\n")] = '\0';
        if(strcmp(command, "exit") == 0)
        {
            break;
        }
        int pid = fork();
        if (pid < 0)
        {
            fprintf(stderr, "failed to fork()\n");
            exit(EXIT_FAILURE);
        }
        if (pid == 0)
        {
            /* Child - execute command */
            /* Should break line into command plus arguments */
            char *arg[2];
            arg[0] = command;
            arg[1] = NULL;
            execvp(command, arg);
            fprintf(stderr, "failed to execute command '%s'\n", command);
            exit(EXIT_FAILURE);
        }
        /* Parent - wait for child to finish */
        int corpse;
        int status;
        while ((corpse = wait(&status)) > 0)
        {
            if (corpse == pid)
                break;
            printf("PID %d exited with status 0x%.4X\n", corpse, status);
        }
    }

    return 0;
}