Question

We use strace a lot. We would like to output some text into strace to mark places the code has reached. The way i see people have done it so far is to stat an non-existent file. The filename is just the text they want to see in the strace. Its pretty fast, but I’m sure there is a better way. I worry that there might be a lot of code and maybe kernel locks being hit even though the mount point is bogus. Any ideas?

Answer 1

write() to an out-of-range file descriptor shows up well in strace output, and should be much faster – the range check happens early, and it doesn’t need to look at the data at all. (You need to pass the length of the data to be written, rather than just a 0-terminated string, but gcc will normally optimise strlen() of a constant string to a constant.)

$ cat hw.c
#include <stdio.h>
#include <string.h>
#include <unistd.h>

#define STRACE_TRACE(str) write(-1, str, strlen(str))

int main(void)
{
  STRACE_TRACE("before");
  printf("Hello world\n");
  STRACE_TRACE("after");
  return 0;
}
$ gcc -Wall -o hw hw.c
$ strace ./hw
...
write(-1, "before"..., 6)               = -1 EBADF (Bad file descriptor)
fstat64(1, {st_mode=S_IFCHR|0620, st_rdev=makedev(136, 150), ...}) = 0
mmap2(NULL, 4096, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_ANONYMOUS, -1, 0) = 0xb77da000
write(1, "Hello world\n"..., 12Hello world
)        = 12
write(-1, "after"..., 5)                = -1 EBADF (Bad file descriptor)
exit_group(0)                           = ?
$