Convert Char * buffer to hex for printing in c

Question

Hi i tried to print the value in char buffer as hex value as ("0X%x"),

char buffer[1024]
printf("recived (0x%x) from the client\n",buffer);

but there are warning when i compile the code :

server.c:105:5: warning: format ‘%x’ expects argument of type ‘unsigned int’, but argument 2 has type ‘char *’ [-Wformat]

so how to solve this problem ????

In what sense do you want to "convert" it? Print each byte as hex? You will need a loop for that. (Studying basic algorithms and data structures would be good before you get into programming. This is something so fundamental that if you can't solve it yourself, then you should probably not yet be writing a full-fledged client-server application.) — , Oct 24 '13 at 21:39
can you provide some examples of buffer contents an expected output? — jev, Oct 24 '13 at 22:03
maybe duplicate of: http://stackoverflow.com/questions/8060170/printing-hexadecimal-characters-in-c — jev, Oct 24 '13 at 22:23

score 1 · Answer 1 · answered Oct 24 '13 at 21:46

1

To print the first char in the buffer:

printf("0x%02x", ((unsigned char) buffer[0]));

To print the entire buffer - same thing in a loop.

answered Oct 24 '13 at 21:46

Alexander L. Belikoff

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score 1 · Accepted Answer · answered Oct 24 '13 at 23:34

Here is how I would do it:

#include <stdio.h>
int main(void) {
  unsigned char buffer[1024];
  int ii;
  // put some dummy values in the buffer:
  for(ii=0; ii<1024; ii++) buffer[ii] = ii%256;

  // printf("0x"); -- uncomment if you want to start with "0x"

  for(ii = 0; ii < 1024; ii++) {
    printf("%02x", (unsigned int)(buffer[ii]));
    if(ii%4 == 3) printf(" ");    // groups of 8: makes more readable
                                  // uncomment if you want "all one line"
    if(ii%32 == 31) printf("\n"); // ditto
  }

  printf("\n");
}

Note this doesn't prefix the output with 0x - since it's one big long number, it seems silly to do so. But you could add it at the beginning. Instead I chose to add some formatting (spaces, returns) to make the output easier to read:

00010203 04050607 08090a0b 0c0d0e0f 10111213 14151617 18191a1b 1c1d1e1f
20212223 24252627 28292a2b 2c2d2e2f 30313233 34353637 38393a3b 3c3d3e3f
40414243 44454647 48494a4b 4c4d4e4f 50515253 54555657 58595a5b 5c5d5e5f
60616263 64656667 68696a6b 6c6d6e6f 70717273 74757677 78797a7b 7c7d7e7f
80818283 84858687 88898a8b 8c8d8e8f 90919293 94959697 98999a9b 9c9d9e9f
a0a1a2a3 a4a5a6a7 a8a9aaab acadaeaf b0b1b2b3 b4b5b6b7 b8b9babb bcbdbebf
c0c1c2c3 c4c5c6c7 c8c9cacb cccdcecf d0d1d2d3 d4d5d6d7 d8d9dadb dcdddedf
e0e1e2e3 e4e5e6e7 e8e9eaeb ecedeeef f0f1f2f3 f4f5f6f7 f8f9fafb fcfdfeff
00010203 04050607 08090a0b 0c0d0e0f 10111213 14151617 18191a1b 1c1d1e1f
20212223 24252627 28292a2b 2c2d2e2f 30313233 34353637 38393a3b 3c3d3e3f
40414243 44454647 48494a4b 4c4d4e4f 50515253 54555657 58595a5b 5c5d5e5f
60616263 64656667 68696a6b 6c6d6e6f 70717273 74757677 78797a7b 7c7d7e7f
80818283 84858687 88898a8b 8c8d8e8f 90919293 94959697 98999a9b 9c9d9e9f
a0a1a2a3 a4a5a6a7 a8a9aaab acadaeaf b0b1b2b3 b4b5b6b7 b8b9babb bcbdbebf
c0c1c2c3 c4c5c6c7 c8c9cacb cccdcecf d0d1d2d3 d4d5d6d7 d8d9dadb dcdddedf
e0e1e2e3 e4e5e6e7 e8e9eaeb ecedeeef f0f1f2f3 f4f5f6f7 f8f9fafb fcfdfeff
00010203 04050607 08090a0b 0c0d0e0f 10111213 14151617 18191a1b 1c1d1e1f
20212223 24252627 28292a2b 2c2d2e2f 30313233 34353637 38393a3b 3c3d3e3f
40414243 44454647 48494a4b 4c4d4e4f 50515253 54555657 58595a5b 5c5d5e5f
60616263 64656667 68696a6b 6c6d6e6f 70717273 74757677 78797a7b 7c7d7e7f
80818283 84858687 88898a8b 8c8d8e8f 90919293 94959697 98999a9b 9c9d9e9f
a0a1a2a3 a4a5a6a7 a8a9aaab acadaeaf b0b1b2b3 b4b5b6b7 b8b9babb bcbdbebf
c0c1c2c3 c4c5c6c7 c8c9cacb cccdcecf d0d1d2d3 d4d5d6d7 d8d9dadb dcdddedf
e0e1e2e3 e4e5e6e7 e8e9eaeb ecedeeef f0f1f2f3 f4f5f6f7 f8f9fafb fcfdfeff
00010203 04050607 08090a0b 0c0d0e0f 10111213 14151617 18191a1b 1c1d1e1f
20212223 24252627 28292a2b 2c2d2e2f 30313233 34353637 38393a3b 3c3d3e3f
40414243 44454647 48494a4b 4c4d4e4f 50515253 54555657 58595a5b 5c5d5e5f
60616263 64656667 68696a6b 6c6d6e6f 70717273 74757677 78797a7b 7c7d7e7f
80818283 84858687 88898a8b 8c8d8e8f 90919293 94959697 98999a9b 9c9d9e9f
a0a1a2a3 a4a5a6a7 a8a9aaab acadaeaf b0b1b2b3 b4b5b6b7 b8b9babb bcbdbebf
c0c1c2c3 c4c5c6c7 c8c9cacb cccdcecf d0d1d2d3 d4d5d6d7 d8d9dadb dcdddedf
e0e1e2e3 e4e5e6e7 e8e9eaeb ecedeeef f0f1f2f3 f4f5f6f7 f8f9fafb fcfdfeff

Note also that I use an unsigned char buffer - otherwise, the numbers greater than 127 are interpreted as negative numbers, and for example, 128, instead of being printed as 80, would be printed as FFFFFF80 - in other words, the format specifier would be ignored.

You could achieve the same thing by leaving buffer as char, and changing the printf to

printf("%02x", (unsigned int)(unsigned char)buffer[ii]);

but it does get rather clumsy...

Convert Char * buffer to hex for printing in c

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