The way that programs talk to the operating system is via ``system calls.'' A system call looks like a procedure call (see below), but it's different -- it is a request to the operating system to perform some activity.
1. int open(char *path, int flags [ , int mode ] );
2. int close(int fd);
3. int read(int fd, char *buf, int size);
4. int write(int fd, char *buf, int size);
5. off_t lseek(int fd, off_t offset, int whence);
You'll note that they look like regular procedure calls. This is how
you program with them -- like regular procedure calls. However, you
should know that they are different: A system call makes a request to
the operating system. A procedure call just jumps to a procedure
defined elsewhere in your program. That procedure call may itself
make a system call (for example, fopen() calls open()),
but it is a different call.
The reason the operating system controls I/O is for safety -- the computer must ensure that if my program has a bug in it, then it doesn't crash the system, and it doesn't mess up other people's programs that may be running at the same time or later. Thus, whenever you do disk or screen or network I/O, you must go through the operating system and use system calls.
These five system calls are defined fully in their man pages (do 'man -s 2 open', 'man -s 2 close', etc).
All actions that you will perform on files will be done through the operating system. Whenever you want to do file I/O, you specify the file by its file descriptor. Thus, whenever you want to do file I/O on a specific file, you must first open that file to get a file descriptor.
Example: o1.c opens the file in1 for reading, and prints the value of the file descriptor. If you haven't copied over the file in1, then it will print -1, since in1 does not exist. If in1 does exist, then it will print 3, meaning that the open() request has been granted (i.e. a non-negative integer was returned). 3 is the file descriptor you obtained. Why 3? For each process, three I/O streams are opened by default, they are stdin, stdout and stderr. These three takes the file descriptors 0, 1 and 2.
There is a limit on how many files a program can open, because it takes resources to store all information needed to correctly handle an opened file. So, close all files you don't currently need to save some resources. As to the question of how many files can be opened, the answer varies from system to system and user to user. Some system allows as many as 2000 files, and sometimes a user's resource is limited. (Try running limit and find out). On the Sun machines we have, I found the limit to be 256 files, while a newer linux machine tells me that I can open 1024 files at the same time. Note: technically, the set of system constants that are assumed, i.e. the limits, can be found by tracing /usr/include/limits.h. However, great care must be taken since sometimes the necessary documentation is not available.
Note the value of 'flags' -- the man page for open() (or chapter 3 of the book) will give you a description of the flags and how they work. They are described in fcntl.h, which can be found in the directory /usr/include. (Note that fcntl.h merely includes /usr/include/sys/fcntl.h, so you'll have to look at that file to see what O_RDONLY and all really mean).
Example: o2.c tries to open the file "out1" for writing. That fails because out1 does not exist already. In order to open a new file for writing, you should open it with (O_WRONLY | O_CREAT | O_TRUNC) as the flags argument. See o3.c for an example of that. Notice that it creates the file out2, which is of zero length when the program terminates. Note also how o2.c and o3.c use perror() to flag errors.
UNIX> o2 o2: No such file or directory UNIX> o3 UNIX> ls -l out* -rw-r--r-- 1 plank 0 Sep 11 08:50 out2 UNIX>Finally, the 'mode' argument should only be used if you are creating a new file. It specifies the protection mode of the new file. 0644 is the most typical value -- it says "I can read and write it; everyone else can only read it"
You can open the same file more than once. You will get a different fd each time. If you open the same file for writing more than once at a time, you may get bizarre results.
UNIX> cat in1 Jim Plank Claxton 221 UNIX> r1 called read(3, c, 10). returned that 10 bytes were read. Those bytes are as follows: Jim Plank called read(3, c, 99). returned that 12 bytes were read. Those bytes are as follows: Claxton 221 UNIX>There are a few things to note about this program. First, buf should point to valid memory. In r1.c, this is achieved by calloc()-ing space for c (read the man page on calloc() if you have not seen it before. Alternatively, I could have declared c to be a static array with 100 characters:
char c[100];Second, I null terminate c after the read() calls to ensure that printf() will understand it.
Third, when read() returns 0, then the end of file has been reached. When you are reading from a file, if read() returns fewer bytes than you requested, then you have reached the end of the file as well. This is what happens in the second read() in r1.c.
Finally, note that the 10th character in the first read() call and the 12th character in the second are both newline characters. That is why you get two newlines in the printf() statement. One is in c, and the other is in the printf() statement.
w1.c writes the string "cs360\n" to the file out3.
UNIX> w1 called write(3, "cs360\n", 6). it returned 6 UNIX> cat out3 cs360 UNIX>
Thus, we can write a very simple cat program (one that copies standard input to standard output) with one line: (this is in simpcat.c):
main()
{
char c;
while (read(0, &c, 1) == 1) write(1, &c, 1);
}
UNIX> simpcat < in1 Jim Plank Claxton 221 UNIX>
UNIX> cat tfile hope not
The following code, opens tfile twice, write once and read once via two different file descriptors.
#include < string.h >
#include < unistd.h >
#include < fcntl.h >
int main (void)
{
int fd[2];
char buf1[12] = "just a test";
char buf2[12];
fd[0] = open("tfile",O_RDWR);
fd[1] = open("tfile",O_RDWR);
write(fd[0],buf1,strlen(buf1));
write(1, buf2, read(fd[1],buf2,12));
close(fd[0]);
close(fd[1]);
return 0;
}
The output is then:
UNIX> a.out just a testUNIX>
In this case, the content of tfile is consistent, regardless of using fd[0] or fd[1].
#include < stdio.h >
#include < stdlib.h >
#include < unistd.h >
int main (void)
{
int i, r, w;
char msg[12];
char buf[2] = {0, 0};
for (i = 0; i < 3; i ++) {
if ((r = read(i,buf,1))<0) {
sprintf(msg,"read f%d:%s",i,buf);
perror(msg);
}
if ((w = write(i,buf,1))<0) {
sprintf(msg,"write f%d:%s",i,buf);
perror(msg);
}
fprintf(stderr,"%d, r = %d, w = %d, char = %d\n",i,r,w,(int)(buf[0]));
}
return 0;
}
This piece of code is interesting because running it differently, we get very different results. In the following, bold letters signify keyboard inputs, while plain characters are the output. The file in1 is a plain text file:
UNIX> cat in1 ABCDEFGHIJK
UNIX> a.out |UNIX> a.out < in1 |UNIX> a.out < in1 > out1
A |write f0: Bad file descriptor |write f0: Bad file descriptor
A0, r = 1, w = 1, char = 65 |0, r = 1, w = -1, char = 65 |0, r = 1, w = -1, char = 65
|B |read f1: Bad file descriptor
1, r = 1, w = 1, char = 10 |B1, r = 1, w = 1, char = 66 |1, r = -1, w = 1, char = 65
C | |B
C2, r = 1, w = 1, char = 67 |2, r = 1, w = 1, char = 10 |B2, r = 1, w = 1, char = 66
Who can tell me what is going on?
Finally, let's take a peek at a concept called data driven programming. In the above program, outputting f0, f1, f2, etc. makes our life easy, since there is a consistent pattern. However, wouldn't outputing names like stdin, stdout and stderr be more clear? But then we cannot leverage a neat for loop any more. In fact, in many applications, when the number of possibilities for outputs goes way beyond 3, the code would look like some big mess like below:
....
for (i = 0; i < 3; i++) {
if (read(...) < 0) {
switch i {
case 0: sprintf(msg, "read stdin ...
case 1:
case 2:
....
}
perror....
}
if (write(...) < 0) {
switch i {
case 0: sprintf(msg, "write stdin ...
case 1:
case 2:
....
}
perror....
}
....
}
In light of this, take a look at the following instead:
#include < stdio.h >
#include < stdlib.h >
#include < unistd.h >
char msg[6][15] = {"read stdin",
"write stdin",
"read stdout",
"write stdout",
"read stderr",
"write stderr"};
int main (void)
{
int i, r, w;
char buf[2] = {0, 0};
for (i = 0; i < 3; i ++) {
if ((r = read(i,buf,1))<0)
perror(msg[i*2]);
if ((w = write(i,buf,1))<0)
perror(msg[i*2+1]);
fprintf(stderr,"%d, r = %d, w = %d, char = %d\n",i,r,w,(int)(buf[0]));
}
return 0;
}
The output is then:
UNIX> a.out < in1 > out1 write stdin: Bad file descriptor 0, r = 1, w = -1, char = 65 read stdout: Bad file descriptor 1, r = -1, w = 1, char = 65 B B2, r = 1, w = 1, char = 66 UNIX>
We have already talked about distinguishing mechanism and policy. In this program, the output mechanism employed is simply a table lookup. The exact content of each output message should be a "policy" that gets defined separately. The actual code is this program is shortened. With this, we gain simplicity, robustness, and an ability to re-configure (by storing the output messages into a separate file). Hopefully, now you know that CS folks appreciate a very different kind of "simple and stupid", as opposed to how the general public interpret this term. Indeed, elegance in designs is what we strive for.