strftime

akshay_satish · 11-26-2012, 12:57 PM

This is might be really silly, I;ve been doing some trial and error testing and I am kinda clueless here..
I perform a stat() on a file that is not present. As a result errno is set to 2.
After that i call a function to perform logging and suddenly in the leg i see the errno set to 21 (Is a directory). By using printfs at various points in the code, I finally tracked down to strftime causing the change in errno. I dont know much about this function but does this have any potential of setting the errno to 21?

Code:

   if (clock_gettime(CLOCK_REALTIME, &time) == -1) {                   \
        sprintf(timestamp, "%s", <something>);           \
    //errno is 2
    } else {                                                            \
       //errno is 2
        ms_part = (time.tv_nsec/1000000L) % 1000L;                      \
           //errno is 2                                                             \
        (void)strftime(timestamp,                                       \
                       sizeof("Mon dd HH:MM:SS.msec : "),                \
                       "%b %d %H:%M:%S",                                \
                       localtime(&time.tv_sec));                        \
        //errno is 21!!        
    }

millgates · 11-26-2012, 01:27 PM

man errno:

Quote:

Its [errno] value is significant only when the return value of the
call indicated an error (i.e., -1 from most system calls; -1 or NULL from most
library functions); a function that succeeds is allowed to change errno.

Quote:

A common mistake is to do

Code:

           if (somecall() == -1) {
               printf("somecall() failed\n");
               if (errno == ...) { ... }
           }

where errno no longer needs to have the value it had upon return from
somecall() (i.e., it may have been changed by the printf(3)). If the value of
errno should be preserved across a library call, it must be saved:

Code:

           if (somecall() == -1) {
               int errsv = errno;
               printf("somecall() failed\n");
               if (errsv == ...) { ... }
           }