how to redirect content of file to stdin, why the exec function doesn't work?

Question

I want to redirect a content of input.txt file to stdin.

I tried:

int RunCompiledFile(char *path, char * input, char *thirdLine, char* name){

    int fdFile = open(input, O_RDONLY), exitStatus, check,
    fdFileOutput = open("newOutput.txt", O_RDWR|O_CREAT, 0777), status;
    char* args[] = {"comp.out", "newOutput.txt", thirdLine};
    char buffer[100];
    pid_t  pid;


    dup2(0, fdFile);
    dup2(fdFileOutput , 1);
    getcwd(buffer, 95);
    i = strlen(buffer);
    buffer[i] = '/';
    buffer[++i] = '\0';
    check = execv(buffer,"a.out");
    close(fdFileOutput);

    //if ((pid = fork()) == 0){
        check = execvp("comp.out", args);
      //  return 5;}

    wait(&status);
    exitStatus = WEXITSTATUS(status);

    switch (exitStatus){
        case 1:
            result("0", name, BADOUTPUT);
            unlink("newOutput.txt");
            return 1;
        case 2:
            if (depth){
                char buffer[5];
                sprintf(buffer, "%d", 100 - (10 * depth));
                result(buffer, name, WRONGDIRECTORY);}
            else
                result("100", name, GREATJOB);
                unlink("newOutput.txt");
            return 1;
        case 3:
            result("80", name, SIMILLAROUTPUT);
            unlink("newOutput.txt");
            return 1;
    }

    return fdFileOutput;
}

But becuase of all the forks it's hard to debbug. the execv - after the "getcwd" returns -1. a.out is a program the gets input via scanf(), and then does something and printf the answer.

I am supposed to get a file, turn it to stdin for the fgets of a.out, and then open a new file for the output of a.out.

it's still doesn't work, I tried your fixes:

    dup2(fdFile, STDIN_FILENO);
    dup2(fdFileOutput, STDOUT_FILENO);
    getcwd(buffer, 95);
    i = strlen(buffer);
    buffer[i] = '/';
    buffer[++i] = 'a';
    buffer[++i] = '.';
    buffer[++i] = 'o';
    buffer[++i] = 'u';
    buffer[++i] = 't';
    buffer[++i] = '\0';
    close(fdFileOutput);
    close(fdFile);
    check = execv(buffer,"a.out");

but the return from execv is still -1. the a.out is working on different users (my friends tried it before).

Answer 1

From In C how do you redirect stdin/stdout/stderr to files when making an execvp() or similar call?:

The right way to do it is to replace the file descriptors STDIN_FILENO, STDOUT_FILENO and STDERR_FILENO with the opened files using dup2(). You should also then close the original files in the child process:

else if (pid == 0)
{
    dup2(fileno(someopenfile), STDIN_FILENO);
    dup2(fileno(someotherfile), STDOUT_FILENO);
    dup2(fileno(somethirdopenfile), STDERR_FILENO);
    fclose(someopenfile);
    fclose(someotheropenfile);
    fclose(somethirdopenfile);
    execvp(args[0], args);
    // handle error ...
}

So open input and output files, create the childprocess (fork()) and then dup2() and rest of the code above

Beside this you can use freopen() (Rerouting stdin and stdout from C)

freopen("newin", "r", stdin); (without assignment to file handle [!])

These links are also interesting because they give information about the system functions like fork(), dup2(),...

How to execute a shell script from C in Linux?

Bash: read stdin from file and write stdout to file