strcat causing segmentation fault

Question

This causes a segmentation fault:

char str1[60];
char**array;

array=malloc( str_nos * sizeof(char *) );
array[i]=malloc( str_len * sizeof(char *) );

strcat(array[i],str1);
strcat(array[i]," ");

str1 is taken from scanf and it's shorter than 60 characters. array[i] is from a dynamic array of strings.

Do you have any idea of what causes the problem? It happens only for a great amount of scanfs.

Either `array[i]` is too small to hold the extra data, or one of the strings is not null-terminated. — Seth Carnegie, Nov 23 '11 at 21:26
You should give more information on how you allocate array's strings and how you create str1. It might be that str1 is too large for array[i]. — Teudimundo, Nov 23 '11 at 21:27
please provide more code. probably you attempt to write more data to the buffer than would fit into the buffer. — moooeeeep, Nov 23 '11 at 21:27

Oliver Charlesworth · Answer 1 · 2011-11-23T21:34:24.817

3

At least two possibilities:

If the buffer pointed to by array[i] doesn't hold enough space, then you will overwrite the end of the buffer, which will often result in a seg-fault.
One of the strings isn't properly null-terminated, so strcat just starts walking through memory.

edited Nov 23 '11 at 21:34

answered Nov 23 '11 at 21:25

Oliver Charlesworth

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@harp: How big is the buffer? How long is the original string in the buffer? How long is the string pointed to by `str1`. – Oliver Charlesworth Nov 23 '11 at 21:33

AnT stands with Russia · Answer 2 · 2013-05-11T22:20:57.513

1

Either array[i] is pointing to nowhere, or the length of the buffer pointed by array[i] is insufficient.

EDIT: According to the code you posted, the buffer pointed by array[i] initially contains uninitialized garbage. You can't apply strcat to a destination buffer that contains uninitialized garbage.

Either make your buffer to hold an empty string before trying to strcat anything to it

array[i][0] = '\0';
strcat(array[i],str1);
strcat(array[i]," ");

or, alternatively, start with strcpy and then do strcat

strcpy(array[i],str1);
strcat(array[i]," ");

edited May 11 '13 at 22:20

answered Nov 23 '11 at 21:26

AnT stands with Russia

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You *can* apply `strcat()` to an uninitialized buffer? Didn't you want to write "You **can't** apply `strcat()` to an uninitialized buffer"? – Flow May 11 '13 at 19:20
@Flow: Yes, it was typo. Fixed now. Thank you. – AnT stands with Russia May 11 '13 at 22:21

score 0 · Answer 3 · answered Nov 23 '11 at 23:27

0

array[i]=malloc( str_len * sizeof(char *) );

What's i here? If i is not in the range [0,str_len), then you are accessing memory that you may not have permission to use...

answered Nov 23 '11 at 23:27

dmckee --- ex-moderator kitten

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strcat causing segmentation fault

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