redirecting output of execvp into a file in C

Question

I don't know what I am doing wrong... but here is the snippet of code that is being executed:

if (fork() == 0)
    {       
             // child
        int fd = open(fileName, O_RDWR | O_CREAT, S_IRUSR | S_IWUSR);

        dup2(fd, 1);   // make stdout go to file

        execvp("ls","ls");
        close(fd);
            exit(0);
    }
if(wait(&status) == -1)
    {
        printf("ERROR REDIRECT\n");
    }

fileName gets created but there is nothing inside.What am I doing wrong?

Did you try the exact same code without the `dup2`? It should print to standard output. This test could help focus on the problem. — ugoren, Jan 30 '12 at 20:56
@dbeer: That won't work, `exec...` functions expect executable files, not shell commands. You could use `system()` though... — dreamlax, Jan 30 '12 at 22:14

cnicutar · Accepted Answer · 2012-01-30T20:23:54.103

6

My guess is that the execvp doesn't work but since you don't handler errors you don't see it.

Try this:

char *const args[] = {"ls", NULL};
execvp(args[0], args);

/* If this is reached execvp failed. */

perror("execvp");

Alternatively you can use compound literals:

execvp("ls", (char *[]){"ls", NULL});

Second idea: try to run things normally, without redirect and see how it works.

edited Jan 30 '12 at 20:23

answered Jan 30 '12 at 20:18

cnicutar

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I tried that and nothing gets printed... it seems that `execvp` gets executed succesfully. – user1162954 Jan 30 '12 at 20:23
@user1162954 - There is not a chance in the world your execvp is working. Take cnicutar fix and your stuff will work. – Duck Jan 31 '12 at 01:17

score 1 · Answer 2 · answered Nov 15 '16 at 06:47

1

close fd before execvp. because the code after execvp never runs unless execvp fails.

answered Nov 15 '16 at 06:47

Sandeep Raju

11
3

redirecting output of execvp into a file in C

2 Answers2

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