c - use printf to print character string in hex format, distorted results -

i want print character string in hex format,

on machine , like

ori_mesg = gen_rdm_bytestream (1400,seed) sendto(machine b, ori_mesg, len(mesg)) 

on machine b

 recvfrom(machine a, mesg)  mesg_check = gen_rdm_bytestream (1400, seed)              for(i=0;i<20;i++){                     printf("%02x ", *(mesg+i)& 0xff);             }             printf("\n");             for(i=0;i<20;i++){                     printf("%02x ", *(mesg_check+i));             }             printf("\n"); 

seed varies among 1, 2 3....

the bytes generation funcion is:

u_char *gen_rdm_bytestream (size_t num_bytes, unsigned int seed) {     u_char *stream = malloc (num_bytes+4);     size_t i;      u_int16_t seq = seed;     seq = htons(seq);     u_int16_t tail = num_bytes;     tail = htons(tail);     memcpy(stream, &seq, sizeof(seq));     srand(seed);     (i = 3; < num_bytes+2; i++){             stream[i] = rand ();             }     memcpy(stream+num_bytes+2, &tail, sizeof(tail));    return stream; } 

but got results printf like:

00 01 00 67 c6 69 73 51 ff 4a ec 29 cd ba ab f2 fb e3 46 7c  00 01 00 67 ffffffc6 69 73 51 ffffffff 4a ffffffec 29 ffffffcd ffffffba ffffffab fffffff2 fffffffb ffffffe3 46 7c 

or

00 02 88 fa 7f 44 4f d5 d2 00 2d 29 4b 96 c3 4d c5 7d 29 7e  00 02 00 fffffffa 7f 44 4f ffffffd5 ffffffd2 00 2d 29 4b ffffff96 ffffffc3 4d ffffffc5 7d 29 7e  

why there many fffff mesg_check?

are there potential reasons phenomenon? thanks!

here's small program illustrates problem think might having:

#include <stdio.h> int main(void) {     char arr[] = { 0, 16, 127, 128, 255 };     (int = 0; < sizeof arr; ++) {         printf(" %2x", arr[i]);     }     putchar('\n');     return 0; } 

on system (on plain char signed), output:

  0 10 7f ffffff80 ffffffff 

the value 255, when stored in (signed) char, stored -1. in printf call, it's promoted (signed) int -- "%2x" format tells printf treat unsigned int, displays fffffffff.

make sure mesg , mesg_check arrays defined arrays of unsigned char, not plain char.

update: rereading answer more year later, realize it's not quite correct. here's program works correctly on system, , work on reasonable system:

#include <stdio.h> int main(void) {     unsigned char arr[] = { 0, 16, 127, 128, 255 };     (int = 0; < sizeof arr; ++) {         printf(" %02x", arr[i]);     }     putchar('\n');     return 0; } 

the output is:

 00 10 7f 80 ff 

an argument of type unsigned char promoted (signed) int (assuming int can hold values of type unsigned char, i.e., int_max >= uchar_max, case on practically systems). argument arr[i] promoted int, while " %02x" format requires argument of type unsigned int.

the c standard implies, doesn't quite state directly, arguments of corresponding signed , unsigned types interchangeable long they're within range of both types -- case here.

to completely correct, need ensure argument of type unsigned int:

printf("%02x", (unsigned)arr[i]);