write (system call)

The write system call is one of the most basic routines provided by the kernel. It writes data, in bytes as specified by the caller, from a buffer declared by the user in the program and then writes it into the file supplied by the calling process. In most modern operating systems, a program that needs to write data to a file stored in a filesystem uses the Write system call. The file is identified by the file descriptor that is obtained from a previous call to open.

Write, thus, takes three arguments:

1. The file descriptor of the file (fd).
2. The buffer from where data is to be written into the file (buf).
3. The number of bytes to be read from the buffer (nbytes).

POSIX usage

The write system call interface^[1][2][3] is standardized by the POSIX specification. Data is written to a file by calling the write function. The function prototype is :

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

In above syntax, size_t is a typedef. It is unsigned data type defined in stddef.h. sizeof operator produces an integer value which is of this type.^[4]
The write function returns the number of bytes successfully written into the array, which may at times be less than the specified nbytes. It returns -1 if EOF is encountered.

Usage Example

#include <stdio.h>
#include <string.h>
#include <stdlib.h>
#include <fcntl.h>
#include <unistd.h>
 
int main (int argc, char *argv[])
{
    int fd1;
    char buf[128];
    fd1 = open(argv[1], O_WRONLY);
    if (fd1 == -1) {
        perror("File cannot be opened");
        return EXIT_FAILURE;
    }
 
    /* Enter the data to be written into the file */
    scanf("%127s", buf);
 
    write(fd1, buf, strlen(buf)); /* fd1 is the file descriptor, buf is the character array used to 
 hold the data, strlen(buf) informs the function that the number of bytes equal to the length of the 
 string in the buffer need to be copied */
 
    return 0;
}

Errors encountered during operation

Listed below are some errors^[5][6] that could be encountered during writing to a file. The errors are macros listed in errno.h .

Higher level I/O functions calling write

The write system call is not an ordinary function, in spite of the close resemblance . In Linux, the system call uses the operating code INT 80H of the assembly language, in order to transfer control over to the kernel^[7]. The write system call, and its counterpart read, being low level functions, are only capable of understanding bytes. Write cannot be used to write records, like classes. Thus, higher level input-output functions (like printf) are required. Often, the high-level interface is preferred, as compared to the cluttered low-level interface. These functions call other functions internally, and these in turn can make calls to write, giving rise to a layered assembly of functions^[8][9].

With the use of this assembly the higher level functions can collect bytes of data and then write the required data into a file.

See Also

• fwrite
• getchar
• fprintf
• sync (Unix)

References

External links

• POSIX write