The assertion is not applying padding rules. The difference between sizeof(http_parser)=32 and offsetof(http_parser, data)=24 shows a 4-byte pointer and 4-bytes of padding after it. I haven't found a succinct source, but a stack overflow post on Structure padding and packing asserts:

For struct, other than the alignment need for each individual member, the size of whole struct itself will be aligned to a size divisible by size of largest individual member, by padding at end.

Which is consistent with this struct being padded to a multiple of 8-bytes, since it contains an 8-byte uint64_t content_length.

./test_g
http_parser v2.9.4 (0x020904)
sizeof(http_parser) = 32
sizeof(void *) = 4
offsetof(http_parser, nread) = 4
offsetof(http_parser, content_length) = 8
offsetof(http_parser, http_major) = 16
offsetof(http_parser, http_minor) = 18
offsetof(http_parser, data) = 24
test_g: test.c:4230: main: Assertion `sizeof(http_parser) == 4 + 4 + 8 + 2 + 2 + 4 + sizeof(void *)' failed.

With a small patch to show some offsets

diff -ur source/test.c source-debug/test.c
--- source/test.c	1970-01-01 00:00:01.000000000 +0000
+++ source-debug/test.c	2020-04-29 18:31:08.678915685 +0000
@@ -4221,6 +4221,12 @@
   printf("http_parser v%u.%u.%u (0x%06lx)\n", major, minor, patch, version);

   printf("sizeof(http_parser) = %u\n", (unsigned int)sizeof(http_parser));
+  printf("sizeof(void *) = %u\n", (unsigned int)sizeof(void*));
+  printf("offsetof(http_parser, nread) = %u\n", (unsigned int)offsetof(http_parser, nread));
+  printf("offsetof(http_parser, content_length) = %u\n", (unsigned int)offsetof(http_parser, content_length));
+  printf("offsetof(http_parser, http_major) = %u\n", (unsigned int)offsetof(http_parser, http_major));
+  printf("offsetof(http_parser, http_minor) = %u\n", (unsigned int)offsetof(http_parser, http_minor));
+  printf("offsetof(http_parser, data) = %u\n", (unsigned int)offsetof(http_parser, data));
   assert(sizeof(http_parser) == 4 + 4 + 8 + 2 + 2 + 4 + sizeof(void *));

Uh oh!

Uh oh!

armv7hl: Assertion `sizeof(http_parser) == 4 + 4 + 8 + 2 + 2 + 4 + sizeof(void *)' failed #507

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