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:
The write 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:
| Argument | Description | |
|---|---|---|
fildes | The file descriptor obtained from a call to open. It is an integer value. The values 0, 1, 2 can also be given, for standard input, standard output & standard error, respectively . | |
buf | Points to a character array, with content to be written to the file pointed to by filedes. | |
nbyte | Specifies the number of bytes to be written from the character array, buf, into the file pointed to by filedes. |
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 file, which may at times be less than the specified nbytes. It returns -1 if an exceptional condition is encountered, see section on errors below.
On Linux, write is system call number 1.[4]