fopen takes more time with “w” than “a” mode

Question

Does fopen and fclose take more time when opened in "w" mode than "a" mode for the following conditions 1. file already present 2. No file present something on the lines of the following logFile = fopen(log,"a"); and logFile = fopen(log,"w");

I have the following code which I did a quick strace profiling on to get the clock cycles on each system call with no file present at the start of test.

code

int main(int argc, char **argv)
{
    const char* log = "log.txt";
    FILE* logFile = NULL;
    char timeBuf[100];
    time_t now;
    struct tm *logtime1;

    time(&now);
    logtime1 = localtime(&now);
    strftime(timeBuf,sizeof(timeBuf),"[%Y-%m-%d %H:%M:%S]",logtime1);
    int i;
    char *inMessage = "The Quick Brown Fox Jumps Over The Lazy Dog";
    for(i=0;i<50000;i++)
    {
        logFile = fopen(log,"a"); //or **logFile = fopen(log,"w");**
        if(logFile != NULL)
        {
           fflush( 0 );
           int error = 0;

           fprintf(logFile, "%s\t%s %d at (%s:%d)\n",timeBuf,
           inMessage, error,__FILE__, __LINE__);
           fclose(logFile);
        }
   }
   return 0;

}

---

When I open the file in "w" mode strace profiling is showing that it takes way more time in open system call. Is there a reason behind that ?

Below are the two profiles

1. ---------- strace on code with "w" mode

   strace -c ./test.out -o report

   % time seconds usecs/call calls errors syscal

   ---

   76.17 0.522555 10 50006 open

   12.13 0.083197 2 50006 close

   4.61 0.031626 1 50000 write

   3.96 0.027151 1 50002 munmap

   1.57 0.010737 0 50017 mmap

   1.55 0.010663 0 50007 fstat

   ---

   100.00 0.686068 300060 1 total

   2. strace on code with "a" mode

   ---------- strace -c ./test.out -o report

   % time seconds usecs/call calls errors syscall

   ---

   22.08 0.020467 0 50002 munmap

   20.24 0.018763 0 50000 write

   16.76 0.015542 0 100007 fstat

   13.43 0.012450 0 50006 open

   9.90 0.009177 0 50006 close

   9.44 0.008756 0 50017 mmap

   8.15 0.007558 0 50001 lseek

   ---

   100.00 0.092713 400060 1 total

Answer 1

With mode="w", the file is emptied before the write, so the system must :

1. Reset the size of the file to zero (open)
2. Mark the blocks used on disk as unused (open)
3. Allocate a new block (write)
4. Update file size (write)

With mode="a", the system only has :

3. Allocate a new block (sometimes, not always, if what you write goes into a new block)
4. Update file size (write)

Obviously, mode="w" doing more work will take more time...