Question

Consider the following code:

writer.c

mkfifo("/tmp/myfifo", 0660);

int fd = open("/tmp/myfifo", O_WRONLY);

char *foo, *bar;

...

write(fd, foo, strlen(foo)*sizeof(char));
write(fd, bar, strlen(bar)*sizeof(char));

reader.c

int fd = open("/tmp/myfifo", O_RDONLY);

char buf[100];
read(fd, buf, ??);

My question is:

Since it’s not know before hand how many bytes will foo and bar have, how can I know how many bytes to read from reader.c?
Because if I, for example, read 10 bytes in reader and foo and bar are together less than 10 bytes, I will have them both in the same variable and that I do not want.
Ideally I would have one read function for every variable, but again I don’t know before hand how many bytes will the data have.
I thought about adding another write instruction in writer.c between the write for foo and bar with a separator and then I would have no problem decoding it from reader.c. Is this the way to go about it?

Thanks.

Answer 1

A separator is one way to go about it, and this will work fine, as long as you know the order of your data, and you use the separator as only a separator, and never as part of your data.

Another way is to precede each write to the pipe with the number of bytes to follow, in a fixed width. Thus, you will know how much data is about to come down the pipe. Use a fixed width, so you know exactly how long the width field will be, so you know both when to start and stop reading each chunk of data.