Advanced Programming in UNIX Environment——Chapter Three

3.2 File descriptors

1. UNIX shells associate file descriptor 0 with the process' standard input, file descriptor 1 with standard output
   , and file descriptor 2 with standard error. This is the convention used by UNIX shells and many applications, regardless of the kernel
2. ** In POSIX .1 applications, the magic numbers 0, 1, and 2 should be replaced by the symbolic constants STDIN_FILENO, STDOUT_FILENO, and STDERR_FILENO. These constants are defined in the header file <unistd.h>. **
3. The range of the file descriptor is 0 ~ OPEN _ MAX (see Table 2-7). The upper limit used by early UNIX versions is 19 (allowing each process to open 20 files), and now many systems increase it to 63

3.3 open function

#include <sys/types.h>
#include <sys/stat.h>
#include <fnctl.h>

int open(const char *pathname, int oflag, ...);

3.4 creat function

This function is equivalent to:

open(pathname, O_WRONLY | O_CREAT | O_TRUNC, mode);

3.6 lseek function

1. By default, when opening a file, unless the O_APPEND option is specified, the offset is set to 0.
2. An open file can be explicitly located by calling the lseek function.

3. #include <sys/types.h>
   #include <unistd.h>
   off_t lseek(int filedes, off_t offset, int whence);
   

   • If whence is SEEK_SET, set the offset of the file to offset bytes from the beginning of the file;
   • If whence is SEEK_CUR, set the offset of the file to its current value plus offset, offset can be positive or negative;
   • If whence is SEEK_END, set the offset of the file to the file length plus offset, offset can be positive or negative;
   • If lseek is successfully executed, the new file displacement will be returned

3.7 read function

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3.10 File Sharing

3.12 dup and dup2 functions

did not understand

3.13 fcntl function

#include <sys/types.h>
#include <uinistd.h>
#include <fcntl.h>

int fcntl(int filedes, int cmd, ...);

• Duplicate a sink village's descriptor (cmd=F_DUPFD)
• get/set file descriptor flags (cmd = F_GETFD or F_SETFD)
• Get/set file status flags (cmd = F_GETFL or F_SETFL)
• Get/set asynchronous I/O authority (cmd = F_GETOWN or F_SETOWN)
• Get/set record lock (cmd = F_GETLK, F_SETLK or F_SETLKW)
• O_ACCMODE is used to take out the lower two bits in the flag, which are used to describe the write read in the file status flag