From Free Pascal wiki

The procedure writeStr formats arguments using the same syntax as write/writeLn, but the destination is a string variable. It is an extended Pascal extension, but the FPC’s default RTL supports this procedure regardless of the current compiler compatibility mode, not just {$mode extendedPascal}.


WriteStr needs at least two arguments. The first parameter has to be a string-assignment-compatible variable, or, if allowed, a proper “writable constant”. The second and following arguments have the same type requirements and format specification syntax as for write/writeLn.


WriteStr behaves like write/writeLn, but does not need to open an external file for processing.

If the destination parameter, the first argument, has a smaller capacity than the concatenated formatted strings, the result is clipped to the left, i. e. only the first x characters will be stored.


Note: As an extension, the FPC allows referencing the destination variable as a source variable. The extended Pascal states “It shall be an error if any of the write-parameters accesses the referenced string-variable.” The FPC ignores this requirement and does not yield an error.


WriteStr is the go-to method of standard, yet simple formatting tasks. A few examples are:

writeStr(myString, someInteger:1); is equivalent to myString := intToStr(someInteger); (or myString := someInteger.toString();), but the latter requires the sysUtils unit. Note, the minimum width specifier :1 is optional in non-ISO-compliant compiler modes.

In non-ISO-compliant modes writeStr(tableCell, finding:24); is equivalent to tableCell := padLeft(finding, 24); (or tableCell := finding.padLeft(24); if finding is an ANSI string), but padLeft requires the strUtils unit.
For reference, in ISO-compliant modes writeStr(tableCell, caption:10); is equivalent to the more complicated tableCell := caption.subString(0, 10).padLeft(10);.

see also

  • readStr performs the reverse operation
  • str, similar to writeStr but less versatile (only one numeric or ordinal type argument permitted)