C stdlib library functions can be divided in these groups depending on their utility:
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conversion
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atof, atoi, atol, mblen, mbstowcs, mbtowc, strtod, strtol, strtoul, wctomb, wcstombs
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dynamic memory allocation/deallocation
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calloc, free, malloc, realloc
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process control and environment variables
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abort, atexit, exit, getenv, system
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sorting and searching
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bsearch, lfind, lsearch, qsort
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mathematical operations
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abs, div, labs, ldiv, rand, srand, etc.
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NULL - NULL is a defined constant used to express null pointers, that is, an unassigned pointer or a pointer that points nowhere. It is defined as:
#define NULL 0 -
size_t - Defined type used as arguments for some functions that require sizes or counts specifications. This represents an unsigned value generally defined in header files as unsigned int:
typedef unsigned int size_t;