Question

I have a small example program which simply fopens a file and uses fgets to read it. Using strace, I notice that the first call to fgets runs a mmap system call, and then read system calls are used to actually read the contents of the file. on fclose, the file is munmaped. If I instead open read the file with open/read directly, this obviously does not occur. I’m curious as to what is the purpose of this mmap is, and what it is accomplishing.

On my Linux 2.6.31 based system, when under heavy virtual memory demand these mmaps will sometimes hang for several seconds, and appear to me to be unnecessary.

The example code:

#include <stdlib.h>
#include <stdio.h>
int main ()
{
   FILE *f;
   if ( NULL == ( f=fopen( "foo.txt","r" )))
   {
     printf ("Fail to open\n");
   }
   char buf[256];
   fgets(buf,256,f);
   fclose(f);
}

And here is the relevant strace output when the above code is run:

open("foo.txt", O_RDONLY)               = 3
fstat64(3, {st_mode=S_IFREG|0644, st_size=9, ...}) = 0
mmap2(NULL, 4096, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_ANONYMOUS, -1, 0) = 0xb8039000
read(3, "foo\nbar\n\n"..., 4096)        = 9
close(3)                                = 0
munmap(0xb8039000, 4096)                = 0

Answer 1

It’s not the file that is mmap‘ed – in this case mmap is used anonymously (not on a file), probably to allocate memory for the buffer that the consequent reads will use.

malloc in fact results in such a call to mmap. Similarly, the munmap corresponds to a call to free.