使用execvp将md5sum重定向到文件？

Question

i need to get the checksum of a file using md5sum, inside a C project.

I can't use the openssl library because it isn´t installed, and i can't install it, because it's the university server that i'm working on.

Also i have some requirements, and i can not use system(), which would be very simple to just: system("md5sum fileName > testFile");

They also don't allow me to use popen();

Im trying to make it work using execvp, but it's not actually working, and i don't know if i can actually work.

The test file that im actually using is this:

  int main(){

      char *const args[] = {"md5sum","file"," > ","test", NULL};                                                                                                                                             

      execvp(args[0],args);  

      return 0;
  }

When i open the file "test" nothing is writed there,

Any clue on how to do it, or why it isn't working??

Thanks in advance.

Answer 1

> is processed by the shell, it's not an argument to the program that you run with execvp .

Redirect your process's stdout before calling execvp .

int main(){
    char *const args[] = {"md5sum", "file", NULL};  

    int fd = open("test", O_WRONLY, 0777);
    dup2(fd, STDOUT_FILENO);
    close(fd);
    execvp(args[0], args);

    return 0;
}

Answer 2

You are passing > as an argument to the command. Usually writing command > file works because the shell that you are using parses > as redirection symbol and redirects the standard output of your program to the file ( > is never passed to the command itself).

What you are trying to is

int main()
{
    const char* args[]={"md5sum","file",0};
    int fd=open("test",O_CREAT|O_WRONLY,S_IRWXU);
    pid_t pid=fork();

    if(!pid)
    {
         dup2(fd,STDOUT_FILENO);
         close(fd);
         execvp(agrs[0],args);
    }
 // ...

 }