Print a struct in C

Question

I am trying to print a struct that is coming as an argument in a function in order to do some debugging.

Is there anyway I could print a structure's contents without knowing what it looks like, i.e. without printing each field explicitly? You see, depending on loads of different #defines the structure may look very differently, i.e. may have or not have different fields, so I'd like to find an easy way to do something like print_structure(my_structure).

NetBeans' debugger can do that for me, but unfortunately the code is running on a device I can't run a debugger on.

Any ideas? I suppose it's not possible, but at least there may be some macro to do that at compilation time or something?

Thanks!

This is now possible in the clang compiler. See [my response to a similar question](https://stackoverflow.com/a/58721823/219710). (Not sure if SO etiquette is to post a duplicate answer here, or link to the other one) — Jonathan Fuerth, Nov 18 '19 at 01:03

Fred Foo · Accepted Answer · 2011-03-18T09:15:20.690

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You can always do a hex dump of the structure:

#define PRINT_OPAQUE_STRUCT(p)  print_mem((p), sizeof(*(p)))

void print_mem(void const *vp, size_t n)
{
    unsigned char const *p = vp;
    for (size_t i=0; i<n; i++)
        printf("%02x\n", p[i]);
    putchar('\n');
};

edited Mar 18 '11 at 09:15

answered Mar 18 '11 at 09:08

Fred Foo

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This code will also display the contents of the padding bytes. They may contain any value and may be located anywhere inside the struct. – Lundin Mar 18 '11 at 09:13
1

@Lundin: that's true. But with some knowledge of the compiler, this may still contain useful information. You'd have to know where to look. – Fred Foo Mar 18 '11 at 09:14
when I compile this function,it tells print_struct.cpp: In function ‘void print_mem(const void*, size_t)’: print_struct.cpp:14:30: error: invalid conversion from ‘const void*’ to ‘const unsigned char*’ [-fpermissive] – kuafu Jun 08 '13 at 15:04
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@young001: that's because it's written in C. In C++, you need an explicit cast to `unsigned char const *`. – Fred Foo Jun 08 '13 at 19:16
1

slight better, with grouping and line wrapping: `printf("%02x%s", p[i], i==0 ? "" : i % 16 == 15 ? "\n" : i % 4 == 3 ? " " : "");` – sehari24jam Sep 10 '15 at 06:30
3

I strongly disagree with the notion that cramming multiple ternary operators into a parameter is "better." – 3Dave Nov 29 '16 at 14:41

score 3 · Answer 2 · edited Jul 17 '12 at 14:21

3

There is nothing like RTTI in C, only solution (apart from hex dump like above) is to #define dump function together with other #defines, ie.

#if _DEBUG

struct { ..... }
#define STRUCT_DUMP(x) printf(.....)

#else

struct { ..... } // other version
#define STRUCT_DUMP(x) printf(.....)    // other version dump

#endif

edited Jul 17 '12 at 14:21

Shiplu Mokaddim

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answered Mar 20 '11 at 21:21

pma_

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Print a struct in C

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