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Writes data to a file.


int _write(
   int fd,
   const void *buffer,
   unsigned int count


File descriptor of file into which data is written.

Data to be written.

Number of bytes.

Return value

If successful, _write returns the number of bytes written. If the actual space remaining on the disk is less than the size of the buffer the function is trying to write to the disk, _write fails and doesn't flush any of the buffer's contents to the disk. A return value of -1 indicates an error. If invalid parameters are passed, this function invokes the invalid parameter handler, as described in Parameter validation. If execution is allowed to continue, the function returns -1 and errno is set to one of three values: EBADF, which means the file descriptor is invalid or the file isn't opened for writing; ENOSPC, which means there isn't enough space left on the device for the operation; or EINVAL, which means that buffer was a null pointer, or that an odd count of bytes was passed in Unicode mode.

For more information about these and other return codes, see errno, _doserrno, _sys_errlist, and _sys_nerr.

If the file is opened in text mode, each line feed character is replaced with a carriage return-line feed pair in the output. The replacement doesn't affect the return value.

When the file is opened in Unicode translation mode—for example, if fd is opened by using _open or _sopen and a mode parameter that includes _O_WTEXT, _O_U16TEXT, or _O_U8TEXT, or if it's opened by using fopen and a mode parameter that includes ccs=UNICODE, ccs=UTF-16LE, or ccs=UTF-8, or if the mode is changed to a Unicode translation mode by using _setmodebuffer is interpreted as a pointer to an array of wchar_t that contains UTF-16 data. An attempt to write an odd number of bytes in this mode causes a parameter validation error.


The _write function writes count bytes from buffer into the file associated with fd. The write operation begins at the current position of the file pointer (if any) associated with the given file. If the file is open for appending, the operation begins at the current end of the file. After the write operation, the file pointer is increased by the number of bytes written.

When it writes to files opened in text mode, _write treats a CTRL+Z character as the logical end of file. When it writes to a device, _write treats a CTRL+Z character in the buffer as an output terminator.

By default, this function's global state is scoped to the application. To change this behavior, see Global state in the CRT.


Routine Required header
_write <io.h>

For more compatibility information, see Compatibility.


// crt__write.c
// This program opens a file for output and uses _write to write
// some bytes to the file.

#include <io.h>
#include <stdio.h>
#include <stdlib.h>
#include <fcntl.h>
#include <sys/types.h>
#include <sys/stat.h>
#include <errno.h>
#include <share.h>

char buffer[] = "This is a test of '_write' function";

int main( void )
   int         fileHandle = 0;
   unsigned    bytesWritten = 0;

   if ( _sopen_s(&fileHandle, "write.o", _O_RDWR | _O_CREAT,
                  _SH_DENYNO, _S_IREAD | _S_IWRITE) )
      return -1;

   if (( bytesWritten = _write( fileHandle, buffer, sizeof( buffer ))) == -1 )
         case EBADF:
            perror("Bad file descriptor!");
         case ENOSPC:
            perror("No space left on device!");
         case EINVAL:
            perror("Invalid parameter: buffer was NULL!");
            // An unrelated error occurred
            perror("Unexpected error!");
      printf_s( "Wrote %u bytes to file.\n", bytesWritten );
   _close( fileHandle );
Wrote 36 bytes to file.

See also

Low-level I/O
_open, _wopen