crash when using FILE descriptor with popen across unix file system - c++

Question

We have a piece of legacy code that uses Flex with C-style FILE* descriptors. To support reading compressed files into Flex, we extended the "open" semantics to open gzip'ed files using
FILE* file = popen("gzip -cd <filename>");
rather than fopen.
We've encountered some problems recently where attempting this across a unix filesystem (probably another filesystem mounted using NFS on a NetApp) causes this entire code stream to crash (segfault), the first message we see is
gzip: stdout: Broken Pipe
and our own crash frame.
If we take the file and move it to the local filesystem where the process is running, there is no segfault and everything works as normal.
What have we tried to replicate or fix?
read files compressed using gzip/ bzip2 etc from internal test NFS filesystems
verify that the target file can be opened
"open" the file and read a few bytes to make sure it can be opened by this process
All of this succeeds and yet we still encounter the crash.
We are out of ideas and could use some suggestions.
Sam Appleton

Answer 1

When you test it, SIGPIPE has its default action (kill the gzip process). When the process runs on your client's side, SIGPIPE is masked. Here's a minimal program that reproduces the error:
#include <signal.h>
#include <stdlib.h>
#include <stdio.h>
#define SZ 4096
void mask()
{
struct sigaction sa;
sa.sa_handler = SIG_IGN;
sa.sa_flags = 0;
if (-1 == sigaction(SIGPIPE, &sa, 0))
{
perror("sigaction");
exit(1);
}
}
int main(int argc, char ** argv)
{
char buf[SZ];
#ifdef MASK
mask();
#endif
FILE * f = popen("gzip -dc foo.gz", "r");
if (0 != fread(buf, SZ, 1, f))
{
fwrite(buf, SZ, 1, stdout);
}
fprintf(stderr, "%d\n", pclose(f));
}
And here's the output with and without MASK:
$ gcc -o foo foo.c
$ gcc -DMASK -o foomask foo.c
$ /foo > /dev/null
13
$ /foomask > /dev/null
gzip: stdout: Broken pipe
256
$
In short, it has nothing to do with NFS. That's a red herring.