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標(biāo)  題: C51 Primer (12) Library Functions 

發(fā)信站: 南京大學(xué)小百合站 (Wed Nov 24 12:04:43 1999), 轉(zhuǎn)信 

  

  

12 C51 Library Functions 

One of the main characteristics of C is its ability to allow complex functio 

ns to be constructed from the basic commands. To save programmer effort many 

 common mathematical and string functions are supplied ready compiled in the 

 form of library files. 

12.1 Library Function Calling 

Library functions are called as per user-defined functions, i.e 

    #include ctype.h 

    { 

    char test_byte ; 

    result = isdigit(test_byte) ; 

    } 

where "isdigit()" is a function that returns value 1 (true) if the test_byte 

 is an ASCII character in the range 0 to 9. 

The declarations of the library functions are held in files with a ".h" exte 

nsion - see the above code fragment. 

Examples are: 

ctype.h, 

stdio.h, 

string.h etc.. 

These are included at the top of the module which uses a library function. 

Many common mathematical functions are available such as ln, log, exp, 10x, 

sin, cos, tan (and the hyperbolic equivalents). These all operate on floatin 

g point numbers and should therefore be used sparingly! The include file con 

taining the mathematical function prototypes is "math.h". 

Library files contain many discrete functions, each of which can be used by 

a C program. They are actually retrieved by the linker utility covered in se 

ction 8. These files are treated as libraries by virtue of their structure r 

ather than their extension. The insertion or removal of functions from such 

a file is performed by a library manager called LIB51. 

12.2 Memory-Model Specific Libraries 

Each of the possible memory models requires a different run-time library fil 

e. Obviously if the LARGE model is used the code required will be different 

for a SMALL model program. 

Thus with C51, 6 different library files are provided: 

C51S.LIB - SMALL model 

C51C.LIB - COMPACT model 

C51L.LIB - LARGE model 

plus three additional files containing floating point routines as well as th 



e integer variety. 

C51 library functions are registerbank independent. This means that library 

functions can be used freely without regard to the current REGISTERBANK() or 

 USING status. This is a major advantage as it means that library functions 

can be used freely within interrupt routines and background functions withou 

t regard to the current register bank. 

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