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In linux, executing ls causes read and exec system calls; executing any shell command calls fork and exec, but checking through strace that the system call caused by ls does not have a fork,ls command to list the files in the directory, so call read.

The operating environment of this tutorial: linux7.3 system, Dell G3 computer.

What system calls will be caused when linux executes ls? the answer is read, exec series.

The shell command execution mechanism is fork+exec, fork is the split, execve is the transformation. The ls command lists the files in the directory, so read is also called.

Shell accesses the Linux kernel through the fork and exec commands, and fork command creation can be done with the same thread.

To check the system calls caused by ls through strace, it is true that there is no fork, but because any shell command will call fork

The transformation of execve is to create a new process and replace the original process with a new one.

First, let's discuss what is a system call (system calls)?

Users can access and control files and devices with a small number of functions directly provided by UNIX/linux, which are system calls [1].

Using the strace ls command, we can view the system call [2] used by the ls command, and some of the output is as follows:

Open (".", O_RDONLY | O_NONBLOCK | O_LARGEFILE | O_DIRECTORY | O_CLOEXEC) = 3getdents64 (3, / * 68 entries * /, 32768) = 2240getdents64 (3, / * 0 entries * /, 32768) = 0close (3) = 0

The open system call opens the current directory file and returns the obtained file descriptor. You can see that the file is opened with the O_RDONLY flag.

As long as the file is opened with the O_RDONLY or O_RDWR flag, you can use the read () system call to read bytes [3] from the file.

So ls uses the read system call. In addition, any shell command creates a process and uses the exec system call.

Let's go back and sort out the doubts we may have about these concepts:

How does a program run, including ls?

The open system call opens the current directory file and returns the obtained file descriptor. So what is a file descriptor?

1 how does the process run

Each running program is called a process [1]

Unix separates process creation from loading a new process image. The advantage is that there is more room to manage both operations. When we create a process, we usually replace the child process with a new process image. So any shell command creates a process and uses the exec system call.

For example, when you execute the ps command on the shell command line, the shell process actually calls fork to copy a new child process and uses the exec system call to completely replace the newly generated child process with the ps process.

The current process can be replaced with a new process with the exec function, and the new process has the same PID as the original process. Exec is a complete series of correlation functions [4]

After calling fork to create a new process, the parent process is almost exactly the same as the child process [1p398].

Fork is a UNIX term, when fork a process (a running program), it is basically copied, and the two processes after fork continue to run from the current execution point, and each process has its own memory copy.

The original process is the parent process and the new process is the child process. It can be distinguished by the return value of fork ().

The fork call in the parent process returns the pid (process id) of the new child process, while the fork call in the child process returns 0

For example:

# include#include#define LEN 10int main () {pid_t id=getpid (); printf ("Main pid:% d\ n", id); int I; pid_t res=fork (); if (res==0) {for (I = 0 for: No such file or directory child pid: 11230 child process return 512 > echoA NULL argv [0] was passed through an exec system call. Child pid: 11231 child process return 134 > ps child pid: 11247 child process return 139 dollars /

First fork, and then the child process calls the program command with the help of exec. The corresponding output is given for error commands, commands that require parameters, and commands that do not require parameters.

2 File descriptor (file descripter,fd)

All devices can be treated as files.

For the kernel, all open files are referenced by the file descriptor [7]. The file descriptor is a non-negative integer in the range of [0recoery open Max-1]. Now the OPEN_MAX is usually 64

But [7] also said that for FreeBSD 8.0 focus Linux 3.2.0, Mac OS X 10.6.8, etc., fd varies almost infinitely, and is only restricted by the number of memory, the word length of int, and the soft and hard limits configured by the system administrator. Why?

When open or create a new file, the kernel returns a file descriptor to the process.

When reading or writing a file, use the file descriptor returned by open or create to identify the file and pass it as a parameter to read / write

By convention, fd is associated with STDIN_FILENO / STDOUT_FILENO / STDERR_FILENO at 0 / 1 / 2, respectively. These constants are also defined in unistd.h.

3 which header files are included in the system call?

Many system calls, including exec, fork, read, write, are contained in the unistd.h header file

POSIX,Portable Operating System Interface . Is a design standard for UNIX systems, and many types of UNIX systems also support compatibility with this standard, such as Linux.

Unistd.h is the header file of unix system definition symbol constant defined by POSIX standard, which contains many function prototypes of UNIX system services [5]. In this header file, there are five underlying functions (system calls) used to access the device driver: open/close/read/write/ioctl [1].

4 File Icano

The five functions used in most of the files mentioned in [7] are: open/read/write/lseek/close

4.1Function read

Call the read function to read the data from the open file.

# includessize_t read (int filedes, void * buf, size_t nbytes)

Return value:

Successful, number of bytes read out

Failed.-1.

Encountered the end of the file, 0

There are several situations in which the number of bytes actually read is less than the number of bytes required to be read:

When reading a normal file, you have reached the end of the file before reading the required number of bytes.

For example, if there are 30 bytes left before reaching the end of the file and 100 bytes are required to be read, read returns 30, and the next time read is called, it will return 0.

When reading from an end device, usually read up to one line at a time

When reading from the network, the buffer mechanism in the network may cause the return value to be less than the number of bytes required to be read.

When reading from a pipe or FIFO, if the pipe contains less than the required number of bytes, read will return only the actual number of bytes available.

When reading from some record-oriented devices, such as disks, return at most one record at a time.

When a signal causes an interruption and part of the data has been read. The read operation starts with the current offset of the file, which increases the actual unique number of bytes before it is returned successfully

The classic prototype definition of read is:

Int read (int fd, char*buf, unsigned nbytes)

Thank you for reading! This is the end of the article on "what system calls will be caused by the implementation of ls by linux". I hope the above content can be of some help to you, so that you can learn more knowledge. if you think the article is good, you can share it out for more people to see!

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