Write (system call)


The write is one of the most basic routines provided by a Unix-like operating system kernel. It writes data from a buffer declared by the user to a given device, such as a file. This is the primary way to output data from a program by directly using a system call. The destination is identified by a numeric code. The data to be written, for instance a piece of text, is defined by a pointer and a size, given in number of bytes.
write thus takes three arguments:
  1. The file code.
  2. The pointer to a buffer where the data is stored.
  3. The number of bytes to write from the buffer.

    POSIX usage

The write call interface is standardized by the POSIX specification. Data is written to a file by calling the write function. The function prototype is:
ssize_t write;
In above syntax, ssize_t is a typedef. It is a signed data type defined in stddef.h. Note that write does not return an unsigned value; it returns -1 if an error occurs so it must return a signed value.
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 an exceptional condition is encountered, see section on [|errors] below.

Usage Example


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int main

Errors encountered during operation

Listed below are some errors that could be encountered during writing to a file. The errors are macros listed in errno.h.
Error NumbersErrorMeaning
The system call was interrupted.
EIO
Low-level errors, often concerned with hardware read/write operations.
EBADF
The file descriptor fd is not valid, or an attempt is being made to write into a file opened in 'read-only' mode.
The user does not have the necessary permissions to write into the file.
EFAULT
The address specified in the function is an invalid address.
EINVAL
The argument passed with the function is invalid.
The file size specified in nbytes is too large, and is greater than that allowed by the system.
ENOSPC
No space available for writing onto the storage device.
The pipe is either broken, or the file at the other end of the pipe is not open for I/O purposes.

Higher level I/O functions calling write

The write system call is not an ordinary function, in spite of the close resemblance. For example, in Linux with the x86 architecture, the system call uses the instruction INT 80H, in order to transfer control over to the kernel. 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 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.
With the use of this assembly the higher level functions can collect bytes of data and then write the required data into a file.