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<a name="1204684"> </a><font color="#003366" face="Verdana, Arial, Helvetica, sans-serif">17.9 Limitations</font>
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<a name="1204685"> </a><font size=2 face="Verdana, Arial, Helvetica, sans-serif">Although the ST20 implementation of the POSIX I/O library is based on the POSIX standard ISO/IEC 9945-1: 1990 (IEEE Std. 1003.1-1990), the current implementation has a number of limitations which it is necessary to be aware of when porting code from other systems. Most of these limitations are designed to reduce the memory or CPU overheads of the system, but may cause problems if a system is expecting strict POSIX compatibility.</font>
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<p> <font size=2 face="Verdana, Arial, Helvetica, sans-serif"><li ><a name="1204686"> </a>In most POSIX systems, file descriptors are local to a single process, however, the ST20 does not implement processes, so file descriptors are shared by all threads. Thus a thread can access a file which it did not open.</font></p>
<p> <font size=2 face="Verdana, Arial, Helvetica, sans-serif"><li ><a name="1204687"> </a>Care should be taken when working with duplicated file handles. The ST20 run-time library does not count how many times an open file descriptor has been duplicated using the <font size=2 face=Courier><strong>dup()</strong></font> function. This means that after a <font size=2 face=Courier><strong>dup()</strong></font> call there are multiple file handles referring to the same open file, but only one of these file handles needs to be closed to close all of them.</font></p>
<p> <font size=2 face="Verdana, Arial, Helvetica, sans-serif"><li ><a name="1204688"> </a>The concept of a current working directory is not implemented by the POSIX code, although individual device drivers may choose to.</font></p>
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