---
title: "When can I call a function in C as a system call?"  
description: "When can I call a function in C as a system call?"  
author: "Revati S Misra"  
published: 2023-07-25  
updated: 2023-07-25  
canonical: https://www.mindstick.com/forum/159270/when-can-i-call-a-function-in-c-as-a-system-call  
category: "C Language"  
tags: ["functional programming", "programming language"]  
reading_time: 2 minutes  

---

# When can I call a function in C as a system call?

When can I call a [function](https://www.mindstick.com/articles/13001/multi-statement-table-valued-user-defined-function-in-sql-server) in C as a [system](https://www.mindstick.com/articles/23411/the-most-effective-method-to-find-the-perfect-small-business-phone-system-for-your-business) call?

## Replies

### Reply by Aryan Kumar

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

```plaintext
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

```plaintext
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.


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