【C语言】C tips

1. How to solve warning: cast to pointer from integer of different size and vice versa:

1. #include <stdio.h>
3. int main() {
4. void *p = NULL;
5. int a = 3;
6. p = (void *)a;
7. a = (int)p;
9. printf("%ld\n", p);
10. printf("%ld\n", a);
12. return 0;
13. }

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The above code has the following warnings when compiling: 

1. /tmp/0pubXc3fmS.c: In function 'main':
2. /tmp/0pubXc3fmS.c:7:9: warning: cast to pointer from integer of different size [-Wint-to-pointer-cast]
3. 7 | p = (void *)a;
4. | ^
5. /tmp/0pubXc3fmS.c:8:9: warning: cast from pointer to integer of different size [-Wpointer-to-int-cast]
6. 8 | a = (int)p;
7. | ^
8. /tmp/0pubXc3fmS.o

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Converting pointer/data to (unsigned) long before casting to required data types can eliminate this warning: (intptr_t works the same)

1. #include <stdio.h>
3. int main() {
4. void *p = NULL;
5. int a = 3;
6. p = (void *)(long)a;
7. a = (int)(long)p;
9. printf("%ld\n", p);
10. printf("%ld\n", a);
12. return 0;
13. }

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2. Access to anonymous union in a struct

If a union in a structure that has no name, you can directly access the fields in the union without the union name:

1. #include <stdio.h>
2. #include <stdio.h>
3. #include <string.h>
4. #include <stdint.h>
6. struct ifreq {
7. union {
8. short ifru_flags;
9. int ifru_ivalue;
10. int ifru_mtu;
11. };
12. union {
13. short ifru_flags2;
14. int aaa;
15. int bbb;
16. };
17. };
19. int main()
20. {
21. printf("%d\n", sizeof(struct ifreq));
23. struct ifreq ifr = {0};
24. printf("ifr.ifru_flags:%.4X\n", ifr.ifru_flags);
26. ifr.ifru_flags = 0x55A5;
27. ifr.ifru_flags2 = 0x3344;
28. ifr.aaa = 0xAABBCCDD;
30. printf("ifr.ifru_flags:%.4X\n", ifr.ifru_flags);
31. printf("ifr.ifru_flags2:%.4X\n", ifr.ifru_flags2);
32. dg_frame_dump((uint8_t *)&ifr, sizeof(ifr));
34. return 0;
35. }

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