A sequence point is a point in C evaluation where specified preceding evaluations are complete before subsequent evaluation proceeds. It serializes execution without necessarily choosing an order between two expressions. section 17.2
3 unsigned add(unsigned* x, unsigned const* y) {
4 return *x += *y;
5 }
6 int main(void) {
7 unsigned a = 3;
8 unsigned b = 5;
9 printf("a = %u, b = %u\n", add(&a, &b), add(&b, &a));
10 }
!@%STYLE%@!
{"css":"{\"css\": \"font-weight: bold;\"}","target":"[[{\"line\":0,\"ch\":1},{\"line\":0,\"ch\":2}],[{\"line\":4,\"ch\":20},{\"line\":4,\"ch\":21}],[{\"line\":0,\"ch\":5},{\"line\":0,\"ch\":13}],[{\"line\":4,\"ch\":7},{\"line\":4,\"ch\":15}],[{\"line\":5,\"ch\":7},{\"line\":5,\"ch\":15}],[{\"line\":0,\"ch\":5},{\"line\":0,\"ch\":13}],[{\"line\":4,\"ch\":7},{\"line\":4,\"ch\":15}],[{\"line\":5,\"ch\":7},{\"line\":5,\"ch\":15}],[{\"line\":0,\"ch\":31},{\"line\":0,\"ch\":45}],[{\"line\":1,\"ch\":1},{\"line\":1,\"ch\":2}],[{\"line\":1,\"ch\":7},{\"line\":1,\"ch\":13}],[{\"line\":2,\"ch\":1},{\"line\":2,\"ch\":2}],[{\"line\":5,\"ch\":20},{\"line\":5,\"ch\":21}],[{\"line\":3,\"ch\":1},{\"line\":3,\"ch\":2}],[{\"line\":3,\"ch\":5},{\"line\":3,\"ch\":8}],[{\"line\":6,\"ch\":9},{\"line\":6,\"ch\":12}],[{\"line\":3,\"ch\":9},{\"line\":3,\"ch\":13}],[{\"line\":3,\"ch\":14},{\"line\":3,\"ch\":18}],[{\"line\":4,\"ch\":1},{\"line\":4,\"ch\":2}],[{\"line\":0,\"ch\":5},{\"line\":0,\"ch\":13}],[{\"line\":4,\"ch\":7},{\"line\":4,\"ch\":15}],[{\"line\":5,\"ch\":7},{\"line\":5,\"ch\":15}],[{\"line\":5,\"ch\":1},{\"line\":5,\"ch\":2}],[{\"line\":0,\"ch\":5},{\"line\":0,\"ch\":13}],[{\"line\":4,\"ch\":7},{\"line\":4,\"ch\":15}],[{\"line\":5,\"ch\":7},{\"line\":5,\"ch\":15}],[{\"line\":6,\"ch\":1},{\"line\":6,\"ch\":2}],[{\"line\":6,\"ch\":7},{\"line\":6,\"ch\":13}],[{\"line\":7,\"ch\":0},{\"line\":7,\"ch\":2}]]"}
!@%STYLE%@!
Consider two calls that modify shared variables inside one printf expression. The function calls impose sequence points, but their relative order remains unspecified. section 17.2
One expression, two valid outcomes
Starting with a = 3 and b = 5, evaluating add(&a, &b) first produces a = 8 and b = 13, yielding “a = 8, b = 13”. Evaluating add(&b, &a) first instead yields “a = 11, b = 8”. Both outcomes are well defined because sequencing requires an order, not a particular order. section 17.2
