Non-existent physical address error with malloc

Question

I'm trying to figure out how to use malloc correctly in C, and have run into an error that I'm having trouble with.

My code:

#include <stdio.h>
#include <stdlib.h>

int main() {

    char * str;

    str = (char *)malloc(10);

    str = "Hello World";

    str[0] = 'R';

    return EXIT_SUCCESS;
}

My Valgrind output:

==23136== Process terminating with default action of signal 10 (SIGBUS)
==23136==  Non-existent physical address at address 0x100000F92
==23136==    at 0x100000F66: main (test.c:12)

I know that the issue is due to me trying to allocate the letter 'R' to str, but I was under the impression that the advantage of using malloc in this situation (as opposed to char str[10] = "Hello World"), was the ability to edit the contents of my string.

Thanks!

what is length of string `Hello World`? It's more then you actually allocate and you have to copy string using `strcpy`. — Jayesh Bhoi, Sep 14 '14 at 08:45

score 4 · Accepted Answer · answered Sep 14 '14 at 08:48

4

str = "Hello World"; makes str point to a constant char string "Hello World", and the memory you have malloced will become memory leak.

answered Sep 14 '14 at 08:48

noark

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Also, the crash is due to attempt to write to a string literal – M.M Sep 14 '14 at 09:03

score 1 · Answer 2 · edited May 23 '17 at 12:12

1

You copy a string with strcpy from <string.h>, not by re-assigning a pointer.

But take care that the target buffer will actually hold strlen(source) + 1 characters (0-terminator). "Hello World" is 11+1.

Also, trying to modify that improperly assigned string literal is UB.

Anyway, Don't cast the result of malloc (and friends).

Finally, return EXIT_SUCCESS is superfluous (since C99 main has an implicit return 0; at the end).

edited May 23 '17 at 12:12

Community

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answered Sep 14 '14 at 08:45

Deduplicator

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The implicit `return 0;` is only available since C99. – Yu Hao Sep 14 '14 at 08:55
@YuHao: Yes... Added reference to minimum standard. – Deduplicator Sep 14 '14 at 08:59
`stdlib.h` is required for `malloc` – M.M Sep 14 '14 at 09:04

score 0 · Answer 3 · answered Sep 14 '14 at 08:46

0

You are throwing away the return value of malloc. Instead you are setting it to a value in read only memory

Try replacing

str = "Hello World";

with

strcpy(str, "Hello World");

You will need to include the appropriate header file

answered Sep 14 '14 at 08:46

Ed Heal

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If doing this then OP needs to allocate more than 10 bytes – M.M Sep 14 '14 at 09:04
@MattMcNabb - I did not bother to count the characters so you right – Ed Heal Sep 14 '14 at 09:10

Non-existent physical address error with malloc

3 Answers3