How to prevent fgets blocks when file stream has no new data

c popen fgets
SinisterDex picture SinisterDex · Sep 29, 2008 · Viewed 23.3k times · Source

I have a popen() function which executes tail -f sometextfile. Aslong as there is data in the filestream obviously I can get the data through fgets(). Now, if no new data comes from tail, fgets() hangs. I tried ferror() and feof() to no avail. How can I make sure fgets() doesn't try to read data when nothing new is in the file stream?

One of the suggestion was select(). Since this is for Windows Platform select doesn't seem to work as anonymous pipes do not seem to work for it (see this post).

Answer

DGentry picture DGentry · Sep 29, 2008

In Linux (or any Unix-y OS), you can mark the underlying file descriptor used by popen() to be non-blocking. 

#include <fcntl.h>

FILE *proc = popen("tail -f /tmp/test.txt", "r");
int fd = fileno(proc);

int flags;
flags = fcntl(fd, F_GETFL, 0);
flags |= O_NONBLOCK;
fcntl(fd, F_SETFL, flags);

If there is no input available, fgets will return NULL with errno set to EWOULDBLOCK.

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