Write (system call)
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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 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:
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.
See also
- fwrite
- getchar
- fprintf
- read (system call)
- sync (Unix)
References
External links
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