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Asked: 2026-06-14T00:05:24+00:00

I am having trouble deciphering the Valgrind output that I am getting. For example

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I am having trouble deciphering the Valgrind output that I am getting. For example I have an error that says I am doing an illegal write of size 1. So I added one to the malloc and that doesn’t change anything. Since I have never used Valgrind before, I thought it would be best to get some help after searching online yielded no responses. Please be aware that I am not looking for the answer so I have not reproduced my code. I would like to be pushed in the correct direction so that I can combat future errors.
Thanks!

==28881== Memcheck, a memory error detector
==28881== Copyright (C) 2002-2009, and GNU GPL'd, by Julian Seward et al.
==28881== Using Valgrind-3.6.0.SVN-Debian and LibVEX; rerun with -h for copyright info
==28881== Command: ./sws root
==28881==
==28892== Conditional jump or move depends on uninitialised value(s)
==28892==    at 0x4C286D9: __GI_strlen (mc_replace_strmem.c:284)
==28892==    by 0x4040B7: get_headers (html.c:280)
==28892==    by 0x402D77: read_page (networking.c:341)
==28892==    by 0x402B8E: get_page (networking.c:310)
==28892==    by 0x40268E: header_parse (networking.c:232)
==28892==    by 0x401D96: retrieve_msg (networking.c:62)
==28892==    by 0x401AAB: main (sws.c:139)
==28892==
==28892== Conditional jump or move depends on uninitialised value(s)
==28892==    at 0x4C286D9: __GI_strlen (mc_replace_strmem.c:284)
==28892==    by 0x4040C5: get_headers (html.c:280)
==28892==    by 0x402D77: read_page (networking.c:341)
==28892==    by 0x402B8E: get_page (networking.c:310)
==28892==    by 0x40268E: header_parse (networking.c:232)
==28892==    by 0x401D96: retrieve_msg (networking.c:62)
==28892==    by 0x401AAB: main (sws.c:139)
==28892==
==28892== Conditional jump or move depends on uninitialised value(s)
==28892==    at 0x4C286D9: __GI_strlen (mc_replace_strmem.c:284)
==28892==    by 0x4040D3: get_headers (html.c:280)
==28892==    by 0x402D77: read_page (networking.c:341)
==28892==    by 0x402B8E: get_page (networking.c:310)
==28892==    by 0x40268E: header_parse (networking.c:232)
==28892==    by 0x401D96: retrieve_msg (networking.c:62)
==28892==    by 0x401AAB: main (sws.c:139)
==28892==
==28892== Conditional jump or move depends on uninitialised value(s)
==28892==    at 0x4C286D9: __GI_strlen (mc_replace_strmem.c:284)
==28892==    by 0x4040E1: get_headers (html.c:280)
==28892==    by 0x402D77: read_page (networking.c:341)
==28892==    by 0x402B8E: get_page (networking.c:310)
==28892==    by 0x40268E: header_parse (networking.c:232)
==28892==    by 0x401D96: retrieve_msg (networking.c:62)
==28892==    by 0x401AAB: main (sws.c:139)
==28892==
==28892== Invalid write of size 1
==28892==    at 0x4C2874C: strcpy (mc_replace_strmem.c:311)
==28892==    by 0x4041E1: get_headers (html.c:299)
==28892==    by 0x402E00: read_page (networking.c:347)
==28892==    by 0x402B8E: get_page (networking.c:310)
==28892==    by 0x40268E: header_parse (networking.c:232)
==28892==    by 0x401D96: retrieve_msg (networking.c:62)
==28892==    by 0x401AAB: main (sws.c:139)
==28892==  Address 0x53be030 is 0 bytes after a block of size 32 alloc'd
==28892==    at 0x4C274A8: malloc (vg_replace_malloc.c:236)
==28892==    by 0x402D84: read_page (networking.c:341)
==28892==    by 0x402B8E: get_page (networking.c:310)
==28892==    by 0x40268E: header_parse (networking.c:232)
==28892==    by 0x401D96: retrieve_msg (networking.c:62)
==28892==    by 0x401AAB: main (sws.c:139)
==28892==
==28892== Invalid write of size 1
==28892==    at 0x4C2875F: strcpy (mc_replace_strmem.c:311)
==28892==    by 0x4041E1: get_headers (html.c:299)
==28892==    by 0x402E00: read_page (networking.c:347)
==28892==    by 0x402B8E: get_page (networking.c:310)
==28892==    by 0x40268E: header_parse (networking.c:232)
==28892==    by 0x401D96: retrieve_msg (networking.c:62)
==28892==    by 0x401AAB: main (sws.c:139)
==28892==  Address 0x53be034 is 4 bytes after a block of size 32 alloc'd
==28892==    at 0x4C274A8: malloc (vg_replace_malloc.c:236)
==28892==    by 0x402D84: read_page (networking.c:341)
==28892==    by 0x402B8E: get_page (networking.c:310)
==28892==    by 0x40268E: header_parse (networking.c:232)
==28892==    by 0x401D96: retrieve_msg (networking.c:62)
==28892==    by 0x401AAB: main (sws.c:139)
==28892==
==28892== Invalid read of size 1
==28892==    at 0x4C28374: strcat (mc_replace_strmem.c:176)
==28892==    by 0x4041F7: get_headers (html.c:300)
==28892==    by 0x402E00: read_page (networking.c:347)
==28892==    by 0x402B8E: get_page (networking.c:310)
==28892==    by 0x40268E: header_parse (networking.c:232)
==28892==    by 0x401D96: retrieve_msg (networking.c:62)
==28892==    by 0x401AAB: main (sws.c:139)
==28892==  Address 0x53be030 is 0 bytes after a block of size 32 alloc'd
==28892==    at 0x4C274A8: malloc (vg_replace_malloc.c:236)
==28892==    by 0x402D84: read_page (networking.c:341)
==28892==    by 0x402B8E: get_page (networking.c:310)
==28892==    by 0x40268E: header_parse (networking.c:232)
==28892==    by 0x401D96: retrieve_msg (networking.c:62)
==28892==    by 0x401AAB: main (sws.c:139)
==28892==
==28892== Invalid write of size 1
==28892==    at 0x4C2838C: strcat (mc_replace_strmem.c:176)
==28892==    by 0x4041F7: get_headers (html.c:300)
==28892==    by 0x402E00: read_page (networking.c:347)
==28892==    by 0x402B8E: get_page (networking.c:310)
==28892==    by 0x40268E: header_parse (networking.c:232)
==28892==    by 0x401D96: retrieve_msg (networking.c:62)
==28892==    by 0x401AAB: main (sws.c:139)
==28892==  Address 0x53be034 is 4 bytes after a block of size 32 alloc'd
==28892==    at 0x4C274A8: malloc (vg_replace_malloc.c:236)
==28892==    by 0x402D84: read_page (networking.c:341)
==28892==    by 0x402B8E: get_page (networking.c:310)
==28892==    by 0x40268E: header_parse (networking.c:232)
==28892==    by 0x401D96: retrieve_msg (networking.c:62)
==28892==    by 0x401AAB: main (sws.c:139)
==28892==
==28892== Invalid write of size 1
==28892==    at 0x4C2839F: strcat (mc_replace_strmem.c:176)
==28892==    by 0x4041F7: get_headers (html.c:300)
==28892==    by 0x402E00: read_page (networking.c:347)
==28892==    by 0x402B8E: get_page (networking.c:310)
==28892==    by 0x40268E: header_parse (networking.c:232)
==28892==    by 0x401D96: retrieve_msg (networking.c:62)
==28892==    by 0x401AAB: main (sws.c:139)
==28892==  Address 0x53be061 is 15 bytes before a block of size 40 alloc'd
==28892==    at 0x4C274A8: malloc (vg_replace_malloc.c:236)
==28892==    by 0x403E53: get_headers (html.c:224)
==28892==    by 0x402E00: read_page (networking.c:347)
==28892==    by 0x402B8E: get_page (networking.c:310)
==28892==    by 0x40268E: header_parse (networking.c:232)
==28892==    by 0x401D96: retrieve_msg (networking.c:62)
==28892==    by 0x401AAB: main (sws.c:139)
==28892==
==28892== Invalid read of size 1
==28892==    at 0x4C28374: strcat (mc_replace_strmem.c:176)
==28892==    by 0x40420D: get_headers (html.c:301)
==28892==    by 0x402E00: read_page (networking.c:347)
==28892==    by 0x402B8E: get_page (networking.c:310)
==28892==    by 0x40268E: header_parse (networking.c:232)
==28892==    by 0x401D96: retrieve_msg (networking.c:62)
==28892== Invalid read of size 1
==28892==    at 0x4C28374: strcat (mc_replace_strmem.c:176)
==28892==    by 0x40420D: get_headers (html.c:301)
==28892==    by 0x402E00: read_page (networking.c:347)
==28892==    by 0x402B8E: get_page (networking.c:310)
==28892==    by 0x40268E: header_parse (networking.c:232)
==28892==    by 0x401D96: retrieve_msg (networking.c:62)
==28892==    by 0x401AAB: main (sws.c:139)
==28892==  Address 0x53be030 is 0 bytes after a block of size 32 alloc'd
==28892==    at 0x4C274A8: malloc (vg_replace_malloc.c:236)
==28892==    by 0x402D84: read_page (networking.c:341)
==28892==    by 0x402B8E: get_page (networking.c:310)
==28892==    by 0x40268E: header_parse (networking.c:232)
==28892==    by 0x401D96: retrieve_msg (networking.c:62)
==28892==    by 0x401AAB: main (sws.c:139)
==28892==
==28892== Invalid write of size 1
==28892==    at 0x4C2838C: strcat (mc_replace_strmem.c:176)
==28892==    by 0x40420D: get_headers (html.c:301)
==28892==    by 0x402E00: read_page (networking.c:347)
==28892==    by 0x402B8E: get_page (networking.c:310)
==28892==    by 0x40268E: header_parse (networking.c:232)
==28892==    by 0x401D96: retrieve_msg (networking.c:62)
==28892==    by 0x401AAB: main (sws.c:139)
==28892==  Address 0x53be061 is 15 bytes before a block of size 40 alloc'd
==28892==    at 0x4C274A8: malloc (vg_replace_malloc.c:236)
==28892==    by 0x403E53: get_headers (html.c:224)
==28892==    by 0x402E00: read_page (networking.c:347)
==28892==    by 0x402B8E: get_page (networking.c:310)
==28892==    by 0x40268E: header_parse (networking.c:232)
==28892==    by 0x401D96: retrieve_msg (networking.c:62)
==28892==    by 0x401AAB: main (sws.c:139)
==28892==
==28892== Invalid read of size 1
==28892==    at 0x4C28374: strcat (mc_replace_strmem.c:176)
==28892==    by 0x404223: get_headers (html.c:302)
==28892==    by 0x402E00: read_page (networking.c:347)
==28892==    by 0x402B8E: get_page (networking.c:310)
==28892==    by 0x40268E: header_parse (networking.c:232)
==28892==    by 0x401D96: retrieve_msg (networking.c:62)
==28892==    by 0x401AAB: main (sws.c:139)
==28892==  Address 0x53be030 is 0 bytes after a block of size 32 alloc'd
==28892==    at 0x4C274A8: malloc (vg_replace_malloc.c:236)
==28892==    by 0x402D84: read_page (networking.c:341)
==28892==    by 0x402B8E: get_page (networking.c:310)
==28892==    by 0x40268E: header_parse (networking.c:232)
==28892==    by 0x401D96: retrieve_msg (networking.c:62)
==28892==    by 0x401AAB: main (sws.c:139)
==28892==
==28892== Invalid read of size 1
==28892==    at 0x4C28374: strcat (mc_replace_strmem.c:176)
==28892==    by 0x404239: get_headers (html.c:303)
==28892==    by 0x402E00: read_page (networking.c:347)
==28892==    by 0x402B8E: get_page (networking.c:310)
==28892==    by 0x40268E: header_parse (networking.c:232)
==28892==    by 0x401D96: retrieve_msg (networking.c:62)
==28892==    by 0x401AAB: main (sws.c:139)
==28892==  Address 0x53be030 is 0 bytes after a block of size 32 alloc'd
==28892==    at 0x4C274A8: malloc (vg_replace_malloc.c:236)
==28892==    by 0x402D84: read_page (networking.c:341)
==28892==    by 0x402B8E: get_page (networking.c:310)
==28892==    by 0x40268E: header_parse (networking.c:232)
==28892==    by 0x401D96: retrieve_msg (networking.c:62)
==28892==    by 0x401AAB: main (sws.c:139)
==28892==
==28892== Invalid write of size 1
==28892==    at 0x4C2838C: strcat (mc_replace_strmem.c:176)
==28892==    by 0x404239: get_headers (html.c:303)
==28892==    by 0x402E00: read_page (networking.c:347)
==28892==    by 0x402B8E: get_page (networking.c:310)
==28892==    by 0x40268E: header_parse (networking.c:232)
==28892==    by 0x401D96: retrieve_msg (networking.c:62)
==28892==    by 0x401AAB: main (sws.c:139)
==28892==  Address 0x53be098 is 0 bytes after a block of size 40 alloc'd
==28892==    at 0x4C274A8: malloc (vg_replace_malloc.c:236)
==28892==    by 0x403E53: get_headers (html.c:224)
==28892==    by 0x402E00: read_page (networking.c:347)
==28892==    by 0x402B8E: get_page (networking.c:310)
==28892==    by 0x40268E: header_parse (networking.c:232)
==28892==    by 0x401D96: retrieve_msg (networking.c:62)
==28892==    by 0x401AAB: main (sws.c:139)
==28892==
==28892== Invalid read of size 1
==28892==    at 0x4C286E4: __GI_strlen (mc_replace_strmem.c:284)
==28892==    by 0x402E0F: read_page (networking.c:348)
==28892==    by 0x402B8E: get_page (networking.c:310)
==28892==    by 0x40268E: header_parse (networking.c:232)
==28892==    by 0x401D96: retrieve_msg (networking.c:62)
==28892==    by 0x401AAB: main (sws.c:139)
==28892==  Address 0x53be030 is 0 bytes after a block of size 32 alloc'd
==28892==    at 0x4C274A8: malloc (vg_replace_malloc.c:236)
==28892==    by 0x402D84: read_page (networking.c:341)
==28892==    by 0x402B8E: get_page (networking.c:310)
==28892==    by 0x40268E: header_parse (networking.c:232)
==28892==    by 0x401D96: retrieve_msg (networking.c:62)
==28892==    by 0x401AAB: main (sws.c:139)
==28892==
==28892== Syscall param socketcall.sendto(msg) points to unaddressable byte(s)
==28892==    at 0x5121052: send (send.c:28)
==28892==    by 0x40276D: send_msg (networking.c:245)
==28892==    by 0x402E29: read_page (networking.c:348)
==28892==    by 0x402B8E: get_page (networking.c:310)
==28892==    by 0x40268E: header_parse (networking.c:232)
==28892==    by 0x401D96: retrieve_msg (networking.c:62)
==28892==    by 0x401AAB: main (sws.c:139)
==28892==  Address 0x53be030 is 0 bytes after a block of size 32 alloc'd
==28892==    at 0x4C274A8: malloc (vg_replace_malloc.c:236)
==28892==    by 0x402D84: read_page (networking.c:341)
==28892==    by 0x402B8E: get_page (networking.c:310)
==28892==    by 0x40268E: header_parse (networking.c:232)
==28892==    by 0x401D96: retrieve_msg (networking.c:62)
==28892==    by 0x401AAB: main (sws.c:139)
==28892==
==28892==
==28892== HEAP SUMMARY:
==28892==     in use at exit: 337 bytes in 10 blocks
==28892==   total heap usage: 19 allocs, 9 frees, 3,307 bytes allocated
==28892==
==28892== LEAK SUMMARY:
==28892==    definitely lost: 337 bytes in 10 blocks
==28892==    indirectly lost: 0 bytes in 0 blocks
==28892==      possibly lost: 0 bytes in 0 blocks
==28892==    still reachable: 0 bytes in 0 blocks
==28892==         suppressed: 0 bytes in 0 blocks
==28892== Rerun with --leak-check=full to see details of leaked memory
==28892==
==28892== For counts of detected and suppressed errors, rerun with: -v
==28892== Use --track-origins=yes to see where uninitialised values come from
==28892== ERROR SUMMARY: 348 errors from 17 contexts (suppressed: 4 from 4)
==28881==
==28881== HEAP SUMMARY:
==28881==     in use at exit: 0 bytes in 0 blocks
==28881==   total heap usage: 0 allocs, 0 frees, 0 bytes allocated
==28881==
==28881== All heap blocks were freed -- no leaks are possible
==28881==
==28881== For counts of detected and suppressed errors, rerun with: -v
==28881== ERROR SUMMARY: 0 errors from 0 contexts (suppressed: 4 from 4)

217 int get_headers(char* time, char* last_modified, char* server, char* content_type, size_t msglen, char* header, int flag)
218 {
219         char* header1 = NULL;
220         char* header2 = NULL;
221         char* header3 = NULL;
222         char* header4 = NULL;
223         char* header5 = NULL;
224
225         if ((header1 = (char*)malloc(strlen("Date: ")+strlen(time)+2) ) ==NULL)
226         {
227                 fprintf(stderr, "%s: No more memory\n", getprogname());
228                 exit(EXIT_FAILURE);
229         }
230
231
232         strcpy(header1, "Date: ");
233         strcat(header1, time);
234         strcat(header1, "\n");
235
236
237         if ((header2 = (char*)malloc(strlen("Last-Modified: ")+strlen(last_modified)+5) ) ==NULL)
238         {
239                 fprintf(stderr, "%s: No more memory\n", getprogname());
240                 exit(EXIT_FAILURE);
241         }
242
243
244         strcpy(header2, "Last-Modified: ");
245         strcat(header2, last_modified);
246         strcat(header2, "\n");
247
248
249         if ((header3 = (char*)malloc(strlen("Server: ")+strlen(server)+5) ) ==NULL)
250         {
251                 fprintf(stderr, "%s: No more memory\n", getprogname());
252                 exit(EXIT_FAILURE);
253         }
254
255
256         strcpy(header3, "Server: ");
257         strcat(header3, server);
258         strcat(header3, "\n");
259
260
261         if ((header4 = (char*)malloc(strlen("Content-Type: ")+strlen(content_type)+5) ) ==NULL)
262         {
263                 fprintf(stderr, "%s: No more memory\n", getprogname());
264                 exit(EXIT_FAILURE);
265         }
266
267
268         strcpy(header4, "Content-Type: ");
269         strcat(header4, content_type);
270         strcat(header4, "\n");
271
272
273         int content_length = strlen(header1)+strlen(header2)+strlen(header3)+strlen(header4)+strlen("Content-Length: ")+msglen+5;
274         char temp[10] = {' '};
275         snprintf(temp, sizeof(msglen),"%d", content_length);
276
277         if ((header5 = (char*)malloc(strlen("Content-Length: ")+strlen(temp)+5) ) ==NULL)
278         {
279                 fprintf(stderr, "%s: No more memory\n", getprogname());
280                 exit(EXIT_FAILURE);
281         }
282
283
284         strcpy(header5, "Content-Length: ");
285         strncat(header5, temp, strlen(temp));
286         strcat(header5, "\n\n");
287
288
289         if(flag == 1)
290         {
291                 strcpy(header, header1);
292                 strcat(header, header2);
293                 strcat(header, header3);
294                 strcat(header, header4);
295                 strcat(header, header5);
296         }
297
298         return content_length;
299         free(header1);
300         free(header2);
301         free(header3);
302         free(header4);
303         free(header5);
304 }
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1 Answer

  1. Editorial Team

    If flag is 0, then you never initialize any of the data in the header1..4 strings, so when you try to get their lengths with strlen, you read in uninitialized garbage. You should make sure to always initialize the header values to empty strings regardless of the flag setting.

    Also, there’s no need to strcat(..., "\0") onto the string — strcpy and strcat always null-terminate the strings (in fact, "\0" is indistinguishable from the empty string "", since both of their first bytes are the NUL byte). And beware of Shlemiel the Painter algorithms when concatenating a bunch of strings together.

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