Advanced Programming in the UNIX Environment: Second Edition [Electronic resources] نسخه متنی

We show the third argument as ..., which is the ISO C way to specify that the number and types of the remaining arguments may vary. For this function, the third argument is used only when a new file is being created, as we describe later. We show this argument as a comment in the prototype.

The

pathname is the name of the file to open or create. This function has a multitude of options, which are specified by the

oflag argument. This argument is formed by ORing together one or more of the following constants from the <fcntl.h> header:

Most implementations define O_RDONLY as 0, O_WRONLY as 1, and O_RDWR as 2, for compatibility with older programs.

One and only one of these three constants must be specified. The following constants are optional:

In earlier releases of System V, the O_NDELAY (no delay) flag was introduced. This option is similar to the O_NONBLOCK (nonblocking) option, but an ambiguity was introduced in the return value from a read operation. The no-delay option causes a read to return 0 if there is no data to be read from a pipe, FIFO, or device, but this conflicts with a return value of 0, indicating an end of file. SVR4-based systems still support the no-delay option, with the old semantics, but new applications should use the nonblocking option instead.

The following three flags are also optional. They are part of the synchronized input and output option of the Single UNIX Specification (and thus POSIX.1):

The O_DSYNC and O_SYNC flags are similar, but subtly different. The O_DSYNC flag affects a file's attributes only when they need to be updated to reflect a change in the file's data (for example, update the file's size to reflect more data). With the O_SYNC flag, data and attributes are always updated synchronously. When overwriting an existing part of a file opened with the O_DSYNC flag, the file times wouldn't be updated synchronously. In contrast, if we had opened the file with the O_SYNC flag, every write to the file would update the file's times before the write returns, regardless of whether we were writing over existing bytes or appending to the file.

Section 3.12 with the dup2 function.

Filename and Pathname Truncation

What happens if NAME_MAX is 14 and we try to create a new file in the current directory with a filename containing 15 characters? Traditionally, early releases of System V, such as SVR2, allowed this to happen, silently truncating the filename beyond the 14th character. BSD-derived systems returned an error status, with errno set to ENAMETOOLONG. Silently truncating the filename presents a problem that affects more than simply the creation of new files. If NAME_MAX is 14 and a file exists whose name is exactly 14 characters, any function that accepts a

pathname argument, such as open or stat, has no way to determine what the original name of the file was, as the original name might have been truncated.

With POSIX.1, the constant _POSIX_NO_TRUNC determines whether long filenames and long pathnames are truncated or whether an error is returned. As we saw in Chapter 2, this value can vary based on the type of the file system.

Whether or not an error is returned is largely historical. For example, SVR4-based systems do not generate an error for the traditional System V file system, S5. For the BSD-style file system (known as UFS), however, SVR4-based systems do generate an error.

As another example, see Figure 2.19. Solaris will return an error for UFS, but not for PCFS, the DOS-compatible file system, as DOS silently truncates filenames that don't fit in an 8.3 format.

BSD-derived systems and Linux always return an error.

If _POSIX_NO_TRUNC is in effect, errno is set to ENAMETOOLONG, and an error status is returned if the entire pathname exceeds PATH_MAX or any filename component of the pathname exceeds NAME_MAX.