c - Is writing 3 instructions separated by comma `,` undefined behaviour? -

i think saw somewhere writing more 1 instruction separated comma , undefined behavior.

so following code generate undefined behavior?

for (i=0, j=3, k=1; i<3 && j<9 && k<5; i++, j++, k++) {     printf("%d %d %d\n", i, j, k); } 

because there 3 instructions separated comma , :

i++, j++, k++ 

writing more 1 instruction separated comma , undefined behaviour.

nope, it's not general case.

in case, i++, j++, k++ valid.

fwiw, per c11, chapter §6.5.17, comma operator (emphasis mine)

the left operand of comma operator evaluated void expression; there sequence point between evaluation , of right operand. right operand evaluated; [...]

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[note]: might have got confused seeing along line of

  printf("%d %d %d", i++, ++i, i); 

kind of statement, note, there , not comma operator altogether (rather, separator supplied arguments) , sequencing not happen. so, kind of statements are ub.

again, referring standard, footnote 3 same chapter

^{as indicated syntax, comma operator (as described in subclause) cannot appear in contexts comma used separate items in list (such arguments functions or lists of initializers).}