C application falling over on strcat

Question

My application falls over when I make the following call to strcat any ideas why?

char *encryptedLine[255] = {0};  
char encryptedLString[8];        
sprintf(encryptedLString, "%08lX", L);
strcat(*encryptedLine, encryptedLString);

do you understand what char *encryptedLine[255] construction is? And what is L, that you use in sprintf? — Alecs, Mar 02 '12 at 14:37
What's with all the down votes? I clearly gave enough information for the question to be answered? — Dunc, Mar 02 '12 at 15:31

hmjd · Accepted Answer · 2012-03-02T14:42:31.393

3

*encryptedLine is NULL pointer: it needs to point a char buffer.

*encyptedLine is equivalent to encryptedLine[0], which is the first entry in the array of char* encryptedLine:

char *encryptedLine[255] = { 0 };

which is a NULL pointer.

To fix, either change to:

char encryptedLine[255] = { 0 };

strcat(encryptedLine, encryptedString);

or:

encryptedLine[0] = malloc(255);
strcat(*encryptedLine, encryptedString);

I think you also need to increase the size of encryptedLString by 1 as:

sprintf(encryptedLString, "%08lX", L);

will attempt to write 9 characters: 8 specified by the format plus 1 for the null terminator:

char encryptedLString[9];

edited Mar 02 '12 at 14:42

answered Mar 02 '12 at 14:37

hmjd

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So do you mean I should remove the = {0}? That doesn't seem to work – Dunc Mar 02 '12 at 14:38
2

Most likely you want to remove * – Alecs Mar 02 '12 at 14:39

score 1 · Answer 2 · answered Mar 02 '12 at 14:37

1

encryptedLine is an array of 255 pointers initialized to NULL. *encryptedLine is a NULL pointer. You are passing a NULL pointer as the first argument to strcat, so it fails.

answered Mar 02 '12 at 14:37

William Pursell

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score 1 · Answer 3 · answered Mar 02 '12 at 14:37

1

You are trying to strcat() with 0 as first parameter. *encryptedLine is encryptedLine[0] and you assigned {0} to it

answered Mar 02 '12 at 14:37

Eregrith

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score 0 · Answer 4 · answered Mar 02 '12 at 14:37

0

You have an array of char pointers

char *encryptedLine[255]

You haven't allocated any memory and assigned it to any of them. Crash.

answered Mar 02 '12 at 14:37

Brian Roach

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score 0 · Answer 5 · answered Mar 02 '12 at 14:46

/* LINE 1*/ char *encryptedLine[255] = {0};  
char encryptedLString[8];        
sprintf(encryptedLString, "%08lX", L);
strcat(*encryptedLine, encryptedLString);

Look at LINE1, it allocates an array of 255 pointers to char that are all initialised to 0, then you do a strcat, which will try to write the value from encryptedLString into the location pointed to by the pointer *encryptedLine which is NULL, which isn't allowed.

Just allocate some memory for encryptedLine's pointers, and this program won't crash.

C application falling over on strcat

5 Answers5