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 (file descriptor or fd).
2. The pointer to a buffer where the data is stored (buf).
3. The number of bytes to write from the buffer (nbytes).

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(int fildes, const void *buf, size_t nbyte);

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.

• fwrite
• getchar
• fprintf
• read (system call)
• sync (Unix)

• POSIX write
• C_Programming/C_Reference/stdio.h/fwrite at Wikibooks