perror()+invalid argument

Which operating system are you using?
Could you post some code?

I am using windows 98.Here you are some code:

      
if ((stream1 = fopen("infile2.txt","w+"))== NULL) {
      perror("the reason why");
      printf("Error cannot open file 'infile2.txt' for writing...\n");
      exit(0);
}


m = s->m;    
s = global_s;
f = s->f;
   
  for (y=0;y<m->ny;y++)
  {
       for (x=0;x<m->nx;x++)
       {
      for (z=0;z<m->nz;z++)
        {
                                   

mx = f->mx[y][pos];
fprintf(stream1, "mx=%11.3e\n", mx);
            
my = f->my[y][pos];
fprintf(stream1, "my=%11.3e\n", my);
            
mz = f->mz[y][pos];
fprintf(stream1, "mz=%11.3e\n", mz);
}
}
}      
      
//Although It starts writing to the infile2.txt about 800kb i have the error invalid argument

Is it embedded within a loop .... You said, it writes about 800Kb to the file before throwing up error ... That means file has been successfully opened. Now unless you have it embedded in a loop and are opening this file again and again, there is no reason why fopen should show an error ...

Either you have a misplaced loop or you have a misplaced suspicion :-)

well the situation is the following:

I have plenty of files like
a.c
a.h
b.c
b.h
d.cpp
d.for
.
.
.
e.t.c
So the fopen function is not in a loop(for,while...) but in a function which is called many times by the main.c program.What I really don't understand is that even if it is in a loop what's with the message about invalid argument?When does the perror generates this kind of message?