Is it possible to redirect stdout twice with dup2?

Question

I have a problem when I used dup2 direct stdout first to a temporal file, and after to a common file. This is a portion my code:

FILE* fdArch=fopen("file.txt","w");

    char nameBuff[100];

    memset(nameBuff,0,sizeof(nameBuff));

    strncpy(nameBuff,"/tmp/myTmpFile-XXXXXX",sizeof("/tmp/myTmpFile-XXXXXX"));

    int fdNewFile=mkstemp(nameBuff);

    if(fdNewFile==-1){
        perror("error creating temporal file");
    }

    dup2(fdNewFile,STDOUT_FILENO);

    read(fd[0], readbuffer, sizeof(readbuffer));

    char *argv[]={ "/home/user/workspace/CProgram/Debug/program", readbuffer, NULL};

    execv(argv[0],argv);

    close(fdNewFile);

    dup2(fileno(fdArch),STDOUT_FILENO);

    char *argv1[]={"/usr/bin/sort",nameBuff, NULL};

    execv(argv1[0],argv1);

    fclose(fdArch);

    unlink(nameBuff);

I run an external program called program and the out is redirected with dup2 to a temporal file, well this works fine, but after when I redirect with dup2 the stdout to fdArch and run sort over the temporal file, the out is not redirected to fdArch.

Is there an error of concept or something is missing?

Answer 1

I run an external program called program and the out is redirected with dup2 to a temporal file, well this works fine, but after when I redirect with dup2 the stdout to fdArch and run sort over the temporal file,the out is not redirected to fdArch. Is there an error of concept or something is missing?

Your usage of dup2() is fine, but you do have a major error: the execv() function does not return, except when it fails. It, and all the other functions in its family, replace the program in which it is called with the one specified. Thus, in fact you never do redirect stdout a second time or run /bin/sort. If you want to do those things, then you should fork(), perform the execv() in the child process, and have the parent wait() for it to complete before continuing.