Linux System Programming: File Read and Write

1. API

open() Function

Prototype:

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

Parameters:

• pathname: The path of the file to be opened.

• flags: The mode and options for opening the file, which can be a combination of the following flags:

• O_RDONLY: Open the file in read-only mode.

• O_WRONLY: Open the file in write-only mode.

• O_RDWR: Open the file in read-write mode.

• O_CREAT: Create a new file if it does not exist.

• O_TRUNC: If the file exists, truncate its length to 0.

• O_APPEND: Append data to the end of the file when writing.

• mode (optional): When the O_CREAT flag is set, this parameter specifies the permission mode of the file.

Return value: Returns a non-negative file descriptor on success; returns -1 and sets errno to indicate an error on failure. read() Function

read() Function

Used to read data from a file descriptor.

Prototype:

ssize_t read(int fd, void *buf, size_t count);

Parameters:

• fd: File descriptor.

• buf: Pointer to the buffer for storing the read data.

• count: The number of bytes to read.

Return value: Returns the number of bytes read on success; returns 0 if the end of the file is reached; returns -1 and sets errno on failure.

write() Function

The write() function is used to write data to a file descriptor.

Prototype:

ssize_t write(int fd, const void *buf, size_t count);

Parameters:

• fd: File descriptor.

• buf: Pointer to the buffer containing the data to be written.

• count: The number of bytes to write.

Return value: Returns the number of bytes written on success; returns -1 and sets errno on failure.

close() Function

The close() function is used to close a file descriptor.

Prototype:

int close(int fd);

Parameters:

• fd: The file descriptor to be closed.

Return value: Returns 0 on success; returns -1 and sets errno on failure.

lseek() Function

The lseek() function is used to reposition the read/write location of a file descriptor.

Prototype:

off_t lseek(int fd, off_t offset, int whence);

Parameters:

• fd: File descriptor.

• offset: The displacement relative to whence.

• whence: The starting position for addressing, which can be one of the following values:

• SEEK_SET: Beginning of the file.

• SEEK_CUR: Current position.

• SEEK_END: End of the file.

Return value: Returns the new file position (byte offset) on success; returns (off_t)-1 and sets errno on failure.

Error Handling

When these functions fail, errno is usually set to indicate the specific error. errno is a global variable that contains the error code. You can use perror() or strerror() to print or get the error description.

Example:

if (read(fd, buf, sizeof(buf)) == -1) {
    perror("Read error");
    // or
    fprintf(stderr, "Read error: %s\n", strerror(errno));
}

Basic Steps for File Read and Write

• Open File: Use the open() function to open a file.

• Read and Write File: Use the read() and write() functions for file read and write operations.

• Close File: Use the close() function to close the file.

Open File

#include <fcntl.h> // Include open() function
#include <unistd.h> // Include close() and read() functions
#include <stdio.h>  // Include perror() and printf() functions

int main() {
    int fd;
    fd = open("example.txt", O_RDWR | O_CREAT, 0666);
    if (fd == -1) {
        perror("Error opening file");
        return -1;
    }
    // File opened successfully, fd is the file descriptor
    return 0;
}

Read and Write File

#include <string.h> // Include string for writing data

// Write to file
char *text = "Hello, World!\n";
write(fd, text, strlen(text));

// Read from file
char buffer[1024];
int bytes_read = read(fd, buffer, sizeof(buffer));
if (bytes_read == -1) {
    perror("Error reading from file");
} else {
    printf("Read %d bytes: %s", bytes_read, buffer);
}

Close File

if (close(fd) == -1) {
    perror("Error closing file");
    return -1;
}

Complete Example

#include <fcntl.h>
#include <unistd.h>
#include <string.h>
#include <stdio.h>

int main() {
    int fd;
    char *text = "Hello, World!\n";
    char buffer[1024];

    // Open file
    fd = open("example.txt", O_RDWR | O_CREAT, 0666);
    if (fd == -1) {
        perror("Error opening file");
        return -1;
    }

    // Write to file
    if (write(fd, text, strlen(text)) == -1) {
        perror("Error writing to file");
        return -1;
    }

    // Read from file
    lseek(fd, 0, SEEK_SET); // Move file pointer to the beginning of the file
    int bytes_read = read(fd, buffer, sizeof(buffer));
    if (bytes_read == -1) {
        perror("Error reading from file");
    } else {
        printf("Read %d bytes: %s", bytes_read, buffer);
    }

    // Close file
    if (close(fd) == -1) {
        perror("Error closing file");
        return -1;
    }

    return 0;
}

Notes

• The file descriptor fd is the integer value used to identify the file after it is opened.

• O_RDWR indicates opening the file in read-write mode, O_CREAT indicates creating the file if it does not exist.

• 0666 is the permission mode for the file, indicating that the file owner, group, and other users have read and write permissions.

• Using the lseek() function allows moving the file pointer to any position in the file.

• Always check the return value of system calls to determine if the operation was successful.

• It is very important to use the close() function to close the file after reading and writing to release system resources.