기출 creat system call function

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0. Problem

Explain the creat system call function.

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1. Introduction

The creat system call is a traditional Unix system call used to create a new file or rewrite an existing file. Originating in the early Unix operating systems in the 1970s, its primary purpose is to simplify the creation of files by opening them in write-only mode and truncating existing files to zero length.

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2. Main Body

2.1 Definition and Prototype

The creat function has the following prototype in C:

int creat(const char *pathname, mode_t mode);

• pathname: Path to the file to be created.
• mode: File permission bits (e.g., 0644), which specify the access permissions for the newly created file.
• Return value: On success, returns a non-negative file descriptor; on failure, returns -1 and sets errno.

2.2 Operation and Mechanism

• If the file specified by pathname does not exist, creat creates it with the specified permissions.
• If the file exists, it truncates the file size to zero (clearing all content).
• The file is opened in write-only mode.
• Internally, creat is equivalent to calling:

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

Here,

• O_CREAT: create the file if it does not exist,
• O_WRONLY: open for write only,
• O_TRUNC: truncate the file to zero length if it exists.

2.3 Features, Advantages, and Disadvantages

Feature	Advantages	Disadvantages
Simple file creation & truncation	Convenient for quick file creation & clearing	Only supports write-only mode
Permission setting via mode	Can specify file permission bits	Limited flexibility compared to open
Legacy Unix system call	Easy to use and widely supported	Deprecated in favor of open syscall

2.4 Comparison with Related Functions

• The open() syscall is more versatile and recommended for modern applications because it allows for multiple flags (read/write, append, sync, etc.).
• creat can be viewed as a shorthand for a specific use of open.

2.5 Practical Use in Real-World

• Often used in simple scripts or programs that only need to create or overwrite a file quickly.
• Not recommended for applications requiring complex file access modes or advanced permission handling.
• Programs needing both reading and writing should prefer open.

2.6 Latest Trends

• Modern POSIX standards and Linux programming discourage direct use of creat.
• Most developers now use open() with appropriate flags for better clarity and control.

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3. Conclusion

The creat system call is a legacy Unix function that creates or truncates a file, opening it in write-only mode. While it simplifies file creation, it lacks flexibility compared to the more general open system call. Contemporary software development prefers open for file creation due to its enhanced options and clearer semantics.

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Child-friendly Summary

creat is like a simple tool in the computer that makes a new empty file or wipes clean an old one so you can write fresh stuff. But nowadays, people mostly use a better tool called open that can do many more things.

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Summary Table

Item	Description
Function Name	creat
Purpose	Create a new file or truncate existing one
Prototype	int creat(const char *pathname, mode_t mode)
Internal Equivalent	`open(pathname, O_CREAT	O_WRONLY	O_TRUNC, mode)`
Returns	File descriptor (>=0) on success, -1 on error
Permissions	Yes, via mode parameter
Advantages	Simple file creation & truncation
Disadvantages	Only write-only mode, less flexible
Modern Usage	Generally replaced by open syscall