forum

Home / DeveloperSection / Forums / When can I call a function in C as a system call?

Ask Question
Revati S Misra

Revati S Misra

Follow
Skilled in SEO, content writing, and digital marketing. Completed several years of working in many organizations including multinational companies. I love to learn new things in life that keep me motivated.

Can you answer this question?

Answer

1 Answers

Aryan Kumar

Aryan Kumar

25-Jul-2023

You can call a function in C as a system call when you need to access a resource or perform an action that is only available to the kernel. For example, you might need to call a system call to open a file, read from a file, write to a file, or create a new process.

Here are some of the most common system calls in C:

  • open(): Opens a file for reading, writing, or both.
  • read(): Reads data from a file into memory.
  • write(): Writes data from memory to a file.
  • close(): Closes a file.
  • fork(): Creates a new process.
  • exec(): Executes a new program.

To call a system call in C, you use the syscall() function. The syscall() function takes one argument, which is the number of the system call you want to call. The system call numbers are defined in the header file <unistd.h>.

For example, the following code calls the open() system call to open a file for reading:

C

int fd = open("myfile.txt", O_RDONLY);

The open() system call takes two arguments: the name of the file to open and the mode in which to open the file. The O_RDONLY mode indicates that the file will be opened for reading only.

The syscall() function returns the result of the system call. In the case of the open() system call, the result is a file descriptor. A file descriptor is a number that identifies a file that is open in the current process.

You can use the file descriptor to access the file later. For example, the following code reads data from the file that was opened in the previous example:

C

char buffer[100];
int bytes_read = read(fd, buffer, sizeof(buffer));

The read() system call takes three arguments: the file descriptor, the address of a buffer where the data will be read, and the size of the buffer. The read() system call returns the number of bytes that were read from the file.

System calls can be a powerful tool for accessing resources and performing actions that are only available to the kernel. However, it is important to use system calls carefully, as they can have a significant impact on the performance of your program.

Liked By