C System Calls Input Output

programming

Two examples

char a[] = "Hello world!";
a[3] = 'z';

It’s okay.

char *a = "Hello world!";
a[3] = 'z';

It will drop. Tried to change string constant.

System Calls Input/Output

fscanf, fread and etc. uses integrated buffer

read and write has no buffer

Standard IO Streams

Use redirection

• Utils:
  • cat - file content
  • head - file header
  • wc - count of lines, words, letters
• Generating IO streams:
  • ls -l > "dir.txt"
  • > - redirection parameter of stdout
  • < - redirection parameter of stdin
  • 2> - redirection parameter of stderr

my_prog.out > "out.txt" 2> "err.txt" < "in.txt"

How do we do it in C program?

• int dup(int fd);
  • Returns first free descriptor. Its content equal to fd content
  • Example:

fd = open();
close(0);
dup(fd);
 
read(0, ...);
close(fd);

• int dup2(int old_fd, int new_fd);
  • Returns first free descriptor.
  • old_fd line in FDT := new_fd line. old_fd close, if necessary
  • Example:

stdout_save = dup(1);
dup2(1, fd);
printf("i'm in fd!");
 
dup2(1, stdout_save);

File Descriptor Table

• fd = open();

0	stdin
1	stdout
2	stderr
3
…

Open

• int open(const char *path, int flags, mode_t mode)
  • flags - access modes and etc.
  • Access modes:
    • O_RDONLY - r
    • O_WRONLY - w
    • O_APPEND - a
    • O_RDWR - r+
    • O_CREAT - create if not exists
    • O_TRUNC - truncate file content
    • O_EXCLUDE - returns -1 if file already exists (only with O_CREAT)
  • mode - access permissions for file
  • Returns file descriptor number or -1 in error case.
  • Example:

fd = open("my_file.dat", O_TRUNC|O_CREAT, 0664);

fopen -> FILE*. open -> int (file descriptor).

Read / Write

About regular files, but also we have:

• sockets
• channels
• devices files

• int read(int fd, const void *buf, size_t size)
  • Returns count of read symbols or -1 in error case.
  • Example:

read(0, buf, 256);

• int write(int fd, void *buf, size_t size)
  • Returns count of written symbols or -1 in error case.

Seek

• off_t lseek(int fd, off_f offset, int whence)
  • whence is start position. It has amount of constants:
    • SEEK_SET
    • SEEK_CUR
    • SEEK_END
  • Examples:

lseek(fd, 0, SEEK_END);

returns length of file

lseek(fd, 0, SEEK_CUR);

Close

Terminal

int main() {
	if() return 0;
	else return 1;
}

• Execution code:
  • gcc my_prog.c -o my_prog
  • `./my_prog
  • `echo Hello
  • echo $? -> execution code
• Multiple commands:
  • && - execute second if first was success
  • || - execute all
  • mkdir NewFolder && touch ./NewFolder/tst.txt
  • vim ./NewFolder/tst.txt