Question

I am looking at how to write pipes and am trying to understand how the standard c library exposes them in some detail. I wrote the following code and would expect that the output would be

2 3
MAJOR ERROR
BYE

However the second line does not show up. Could someone explain why that is happening?

int main()
{
    int rc;
    int p[2];
    char buffer[40];

    close(2);

    rc = pipe(p);
    printf("%d %d\n", p[0], p[1]);
    FILE* pipeWrite = fdopen(p[1], "w");
    fprintf(pipeWrite, "MAJOR ERROR\n");
    close(p[1]);

    rc = read(p[0], buffer, 40);
    buffer[rc] = '\0';
    printf("%s\n", buffer);
    printf("BYE\n");

    return 0;
}

Thanks.

Answer 1

You are writing into the pipe using standard IO buffered operations (fprintf(3)), but then close the filedescriptor before flushing output. Add fflush(pipeWrite); immediately before the close(p[1]); call and see if your output is what you expect. (You could also use fclose(pipeWrite);, as closing the standard IO stream will also flush the output.)

See the setvbuf(3) manpage for more details on the standard IO stream buffering options.