stdin into execvp() while using fork() and pipe()

Question

So I am trying to read from standard input and then get the input ready so that later on it can be used inside execvp().

What I am implementing here is basically a pipe for some terminal commands.

Here is how an example of my code goes.

input:

ls -s1

sort -n

output:

commands[0]="ls"

commands[1]="-s1"

commands2[0]="��""

commands2[1]="��""

sort: cannot read: t: No such file or directory

Here is my code

# include <stdlib.h>
# include <stdio.h>
# include <unistd.h>
# include <string.h>
# include <sys/wait.h>
# define BUF_SIZE 256
    

int main()
{
    char buffer[BUF_SIZE];
    char *commands[5];
    char *commands2[5];
    int argc = 0;
    int argc2 = 0;

    fgets(buffer, BUF_SIZE, stdin);
        
        for ( commands[argc] = strtok(buffer, " \t\n"); 
              commands[argc] != NULL; 
              commands[++argc] = strtok(NULL, " \t\n") ) {
            printf("commands[%d]=\"%s\"\n", argc, commands[argc]);
        }
        commands[argc] = NULL;

    fgets(buffer, BUF_SIZE, stdin);
        
        for ( commands2[argc2] = strtok(buffer, " \t\n"); 
              commands2[argc2] != NULL; 
              commands2[++argc2] = strtok(NULL, " \t\n") ) {
            printf("commands2[%d]=\"%s\"\n", argc2, commands2[argc]);
        }
        commands2[argc2] = NULL;

    int my_pipe[2];

    if (pipe(my_pipe) == -1)
    {
        perror("cannot create pipe\n");
    }

    pid_t my_pid;

    my_pid = fork();

    if (my_pid < 0)
    {
        perror("Failed fork\n");
    }

    if (my_pid > 0)
    {
        close(my_pipe[1]);
        dup2(my_pipe[0], 0);
        close(my_pipe[0]);

        wait(NULL);
        execvp(commands2[0],commands2); 
    }
    else
    {
        close(my_pipe[0]);   
        dup2(my_pipe[1], 1);
        close(my_pipe[1]);

        execvp(commands[0],commands);
    }
}

Answer 1

One major problem is that you read the second line over the first in buffer, and the commands[] array contains pointers into buffer too. That's not a recipe for happiness. The simplest fix is to define char buffer2[BUF_SIZE]; and use that in the second fgets() call and for loop.

Using argc in printf("commands2[%d]=\"%s\"\n", argc2, commands2[argc]); is a copy'n'paste bug — it should reference argc2 twice. This helped hide the previous problem.

Note that perror() does not exit; your code blunders on if pipe() fails, or if fork() fails.

The wait() in if (my_pid > 0) is bad; remove it.

If execvp() fails, you should report an error and exit with a non-zero status.

Putting those changes together yields code such as:

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

#define BUF_SIZE 256

int main(void)
{
    char buffer[BUF_SIZE];
    char buffer2[BUF_SIZE];
    char *commands[5];
    char *commands2[5];
    int argc = 0;
    int argc2 = 0;

    fgets(buffer, BUF_SIZE, stdin);

    for (commands[argc] = strtok(buffer, " \t\n");
         commands[argc] != NULL;
         commands[++argc] = strtok(NULL, " \t\n"))
    {
        printf("commands[%d]=\"%s\"\n", argc, commands[argc]);
    }
    commands[argc] = NULL;

    fgets(buffer2, BUF_SIZE, stdin);

    for (commands2[argc2] = strtok(buffer2, " \t\n");
         commands2[argc2] != NULL;
         commands2[++argc2] = strtok(NULL, " \t\n"))
    {
        printf("commands2[%d]=\"%s\"\n", argc2, commands2[argc2]);
    }
    commands2[argc2] = NULL;

    int my_pipe[2];

    if (pipe(my_pipe) == -1)
    {
        perror("cannot create pipe\n");
        exit(EXIT_FAILURE);
    }

    pid_t my_pid = fork();

    if (my_pid < 0)
    {
        perror("Failed fork\n");
        exit(EXIT_FAILURE);
    }

    if (my_pid > 0)
    {
        close(my_pipe[1]);
        dup2(my_pipe[0], 0);
        close(my_pipe[0]);

        execvp(commands2[0], commands2);
        perror(commands2[0]);
        exit(EXIT_FAILURE);
    }
    else
    {
        close(my_pipe[0]);
        dup2(my_pipe[1], 1);
        close(my_pipe[1]);

        execvp(commands[0], commands);
        perror(commands[0]);
        exit(EXIT_FAILURE);
    }
    return EXIT_FAILURE;
}

When I run the program, it produces the appropriate output. Note that the return at the end of main() is actually never reached.