The Fortran library contains a set of subroutines that convert between data formats of various vendors. These routines are callable from any supported programming language. For complete details, see the individual man pages for each routine. These subroutines provide an efficient way to convert data that was read into system central memory.

The following table lists these conversion routines.

See the individual man pages for details about the syntax and arguments for each routine.

Implicit data conversion in Fortran requires no explicit action by the program to convert the data in the I/O stream other than using the assign command to instruct the libraries to perform conversion. For details, see the assign(1) man page.

The implicit data conversion process is performed in two steps:

Record format conversion interprets or converts the internal record blocking structures in the data stream to gain record-level access to the data. The data contained in the records can then be converted.

Using implicit conversion, you can select record blocking or deblocking alone, or you can request that the data items be converted automatically. When enabled, record format conversion and data item conversion occur transparently and simultaneously. Changes are usually not required in your Fortran code.

To enable conversion of foreign record formats, specify the appropriate record type with the assign -F command. The -N (numeric conversion) and -C (character conversion) assign options control conversion of data contained in a record. If -F is specified, but -N and -C are not, the libraries interpret the record format, but they do not convert data. You can obtain information about the type of data that will be converted (and, therefore, the type of conversion that will be performed) from the Fortran I/O list.

If -N is used and -C is not, an appropriate character conversion type is selected by default, as shown in the following table.

Supported implicit data conversion includes conversion of the supported tape and disk formats and data types through standard Fortran formatted, unformatted list-directed, and Namelist I/O and through BUFFER IN and BUFFER OUT statements. Generally, read, write, and rewind are supported for all record formats.

If you select the -N option, the libraries perform data conversion for Fortran unformatted statements and BUFFER IN and BUFFER OUT I/O statements. Data is converted according to its Fortran data type. Table 11-3 describes the conversion performed for each of the conversion types.

For numeric data conversions, most foreign data elements are defined with fewer bits than their corresponding native data elements. If the value in a native element is too large to fit in the foreign element, the foreign element is set to the largest or smallest possible value; no error is generated. When converting from a native element to a smaller foreign element, precision is also lost due to truncation of the floating-point mantissa.

If the assign -N user or assign -N site command is specified, the user or site must provide site numeric data conversion routines. They follow the same calling conventions as the other explicit routines.

For implicit conversion, specify format characteristics on an assign command.

Files can be converted to one of the following:

When a Fortran I/O operation is performed on the file, the appropriate file format and data conversions are performed during the I/O operation. Data conversion is performed on each data item, based on the type of the Fortran variable in the I/O list.

For example, if the first read of a foreign format file is the following, the library interprets any blocking structures in the file that precede the first data record:

READ (10) INT,FLOAT1,FLOAT2

These vary depending on the file type and record format. The first 32 bits of data (in IBM format, for example) are extracted, sign-extended, and stored in the INT Fortran variable. The next 32 bits are extracted, converted to native floating-point format, and stored in the FLOAT1 Fortran variable.

The next 32 bits are extracted, converted, and stored into the FLOAT2 Fortran variable. The library then skips to the end of the foreign logical record. When writing from a native system to a foreign format (for example, if in the previous example WRITE(10) was used), precision is lost when converting from a 64-bit representation to 32-bit representation.

As with any software process, the various options for data conversion have advantages and disadvantages, which are discussed in this section. As a set, various data conversion options provide choices in methods of file processing for front-end systems. No one option is best for all applications.

IRIX systems use 32-bit IEEE standard floating point, as do many workstations and personal computers. These workstations often use a dialect of UNIX software as the operating system, with twos-complement arithmetic and the ASCII character set. The logical values in these implementations are usually the same for Fortran and C. They use zero for false and nonzero for true. It is also common to see the f77 record blocking used by the Fortran run-time library on unformatted sequential files.

No IEEE record format exists, but the IEEE implicit and explicit data conversion routine facilities are provided with the assumption that many of these things are true.

Most computer systems that use the IEEE data formats run operating systems based on UNIX software and use f77 record blocking. You can use the rcp or ftp commands to transfer files. In most cases, the following command should work for implicit conversion:

assign -F f77 -N ieee fort.1

When writing files in the f77 format, remember that you can gain a large performance boost by ensuring that the records being written fit in the working buffer of the f77 layer.

SGI MIPS systems use IEEE floating-point representation, so IEEE conversion is usually unnecessary when reading or writing IEEE data on these systems.

On MIPS systems, data types can be declared as 64-bits in size and can then be read or written directly. This is the most direct and efficient method to read or write data files for IEEE systems. The user can either alter the declarations of the variables used in the Fortran I/O list to declare them as KIND=8 or as REAL*8 (or INTEGER*8), or all the variables in the program can be resized by compiling with the -r8 (or -i8) compiler option.

The following are other IEEE data conversion variants; not all variants are available on all systems: