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From 6.3.1.5 of the C99 standard:

If a double is demoted to a float or a long double is demoted to a double or a float, ...and the

...

value

...

being converted is outside the range of values that can be represented, the behavior is undefined.

Non-Compliant Code Example

This non-compliant code illustrates possible undefined behavior associated with demoting floating point represented numbers.

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In the assignments above, it is possible that the variable d1 exceeds the maximum value is outside the range of values that can be stored represented by a float or that the variable ld exceeds the maximum value is outside the range of values that can be represented as either a float or a double.

Compliant Code Example

This compliant code properly checks to see whether the values to be stored are too large to be represented.

#include <float.h>

...

long double ld;
double d1;
double d2;
float f1;
float f2;

...

if(d1 > FLT_MAX || d1 < -FLT_MAX) {
	/* Handle error condition */
} else {
	f1 = (float)d1;
}
if(ld > FLT_MAX || ld < -FLT_MAX) {
	/* Handle error condition */
} else {
	f2 = (float)ld;
}
if(ld > DBL_MAX || ld > -DBL_MAX) {
	/* Handle error condition */
} else {
	d2 = (double)ld;
}

References

\[[ISO/IEC 9899-1999:TC2|AA. C References#ISO/IEC 9899-1999TC2]\] Section 6.3.1.5, "Real floating types"