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charset: Fix padding of USSD messages in 7bit GSM alphabet 

USSD messages are sent in pages of 82 bytes, and there is no way to
indicate a shorter payload length inside the page.  Hence, we always
must pad up to the end of the page, using <CR> characters.

Change-Id: I9950431e920579e6c3a0d12348573f51d21739ec
This commit is contained in:
Harald Welte 2021-01-23 21:52:25 +01:00
parent 1d78b2e73d
commit f97cd9672d
1 changed files with 30 additions and 3 deletions
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33
src/charset.c
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@ -26,13 +26,40 @@
#include "charset.h"
/* pad the entire "remainder" of a buffer with repeated instances of the given pad character */
static void pad_with_septets(uint8_t *buf, size_t buf_len, int num_septets, char pad_char)
{
unsigned int bit_offset;
for (bit_offset = num_septets * 7; bit_offset + 7 <= buf_len * 8; bit_offset += 7) {
unsigned int byte_offset = bit_offset / 8;
unsigned int bits = bit_offset % 8;
/* put one more septet */
buf[byte_offset] |= ((pad_char << bits) & 0xff);
if (bits > 1)
buf[byte_offset+1] = (pad_char) >> (8-bits);
}
}
/* return number of output bytes written */
int charset_utf8_to_gsm7(uint8_t *out, size_t out_len, const char *in, size_t in_len)
{
int octets;
int octets, num_septets, num_bits, num_bytes_used;
/* FIXME: implement this for 'escape' characters outside 7bit alphabet */
gsm_7bit_encode_n_ussd(out, out_len, in, &octets);
return octets;
num_septets = gsm_7bit_encode_n(out, out_len, in, &octets);
num_bits = num_septets * 7;
/* we need to pad the entire remainder of the message with <CR> */
pad_with_septets(out, out_len, num_septets, '\r');
/* return actual number of output octets used, excluding any padding */
num_bytes_used = num_bits/8;
if (num_bits % 8)
num_bytes_used++;
return num_bytes_used;
}
/* return number of output bytes written */