Cstructures and unions
(Reek, Ch.10)
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CS 3090: Safety Critical Programming in C
C structures: aggregate, yet scalar
CS 3090: Safety Critical Programming in C
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aggregate in that they hold multiple data items at one timenamedmembershold data items of various typeslike the notion of class/field in C or C++– but without the data hiding featuresscalar in that C treats each structure as a unitas opposed to the “array” approach: a pointer to a collection of members in memoryentire structures (not just pointers to structures) may be passed as function arguments, assigned to variables, etc.Interestingly, they cannot be compared using==(rationale: too inefficient)
Structure declarations
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Combined variable and type declarationstructtag {member-list} variable-list;Any one of the three portions can be omittedstruct{inta, b; char *p;} x, y; /* omit tag */variablesx, ydeclared with members as described:intmembersa, bandcharpointerp.xandyhave same type, but differ from all others –even if there is another declaration:struct{inta, b; char *p;} z;/* z has different type from x, y */
Structure declarations
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structS {inta, b; char *p;};/* omitvariables */No variables are declared, but there is now a typestructSthat can be referred to laterstructS z; /* omit members */Given an earlier declaration ofstructS, this declares a variable of that typetypedefstruct{inta, b; char *p;} S;/* omit both tag and variables*/This creates a simple type nameS(more convenient thanstructS)
Recursively defined structures
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Obviously, you can’t have a structure that contains an instance of itself as a member – such a data item would be infinitely largeBut within a structure you canreferto structures of the same type, via pointersstructTREENODE {char *label;structTREENODE *leftchild, *rightchild;}
Recursively defined structures
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When two structures refer to each other, one must be declared in incomplete (prototype) fashionstructHUMAN;structPET {char name[NAME_LIMIT];char species[NAME_LIMIT];structHUMAN *owner;}fido= {″Fido″, ″Canislupusfamiliaris″};structHUMAN {char name[NAME_LIMIT];structPET pets[PET_LIMIT];}sam= {″Sam″, {fido}};
Direct access operators.msubscript and dot operators have same precedence and associate left-to-right, so we don’t need parentheses forsam.pets[0].speciesIndirect accesss->m: equivalent to(*s).mDereference a pointer to a structure, then return a member of that structureDot operator has higher precedence than indirection operator , so parentheses are needed in (*s).m(*fido.owner).nameorfido.owner->name
Member access
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structCOST {intamount;charcurrency_type[2]; }structPART { char id[2];structCOSTcost;intnum_avail; }layout ofstructPART:Here, the system uses 4-byte alignment of integers,soamountandnum_availmust be alignedFour bytes wasted for each structure!
Memory layout
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id
amount
num_avail
cost
currency_type
A better alternative (from a space perspective):structCOST {intamount;charcurrency_type; }structPART {structCOSTcost;char id[2];intnum_avail;}
Memory layout
CS 3090: Safety Critical Programming in C
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id
amount
num_avail
cost
currency_type
Bit fields
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If space is a serious concern, you can select the number of bits used for each memberstructCHAR { unsignedch: 7;unsigned font: 6;unsigned size: 19; };Layout possibilities (machine-dependent):
ch
font
size
ch
font
size
Bit fields
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Portability is an issue:Do any bit field sizes exceed the machine’sintsize?Is there any pointer manipulation in your code that assumes a particular layout?Bit fields are “syntactic sugar” for more complex shifting/maskinge.g. to getfontvalue, mask off thechandsizebits, then shift right by 19This is whatactually happensin the object code –bit fields just make it look simpler at the source level
Structures as function arguments
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Structures are scalars, so they can be returned and passed as arguments – just likeints,charsstructBIGchangestruct(structBIG s);Call by value: temporary copy of structure is createdCaution: passing large structures is inefficient– involves a lot of copyingavoid by passing a pointer to the structure instead:voidchangestruct(structBIG *s);What if thestructargument is read-only?Safe approach: useconstvoidchangestruct(structBIGconst *s);
Unions
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Like structures, but every member occupies the same region of memory!Structures: members are “and”edtogether: “name and species and owner”Unions: members are “xor”edtogetherunion VALUE {float f;inti;char *s;};/* either a floatxoranintxora string */
Unions
CS 3090: Safety Critical Programming in C
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Up to programmer to determine how to interpret a union (i.e. which member to access)Often used in conjunction with a “type” variable that indicates how to interpret the union valueenumTYPE { INT, FLOAT, STRING };structVARIABLE {enumTYPEtype;union VALUEvalue;};
Unions
CS 3090: Safety Critical Programming in C
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Storagesize of union is the size of its largest memberavoid unions with widely varying member sizes;for the larger data types, consider using pointers insteadInitializationUnion may only be initialized to a value appropriate for the type of its first member
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