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 file code (file descriptor or fd).
- The pointer to a buffer where the data is stored (buf).
- The number of bytes to write from the buffer (nbytes).
POSIX usage
The write call interface123 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);| 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.
Linux
On Linux, write is system call number 1.4
See also
External links
- POSIX write
- C_Programming/C_Reference/stdio.h/fwrite at Wikibooks
References
http://www.unix.com/man-page/FreeBSD/2/write/ Manual page for Write http://www.unix.com/man-page/FreeBSD/2/write/ ↩
https://www.gnu.org/s/hello/manual/libc/I_002fO-Primitives.html#I_002fO-Primitives I/O Primitives https://www.gnu.org/s/hello/manual/libc/I_002fO-Primitives.html#I_002fO-Primitives ↩
"Write". http://pubs.opengroup.org/onlinepubs/007904875/functions/write.html ↩
"Linux/Arch/X86/Entry/Syscalls/Syscall_64.TBL at master · torvalds/Linux". GitHub. https://github.com/torvalds/linux/blob/master/arch/x86/entry/syscalls/syscall_64.tbl ↩