Segmentation fault strcpy/strcat

Question

I am writing a program to open a file (say "input_file"), manipulate its contents and then output it to another file ("manipulated-input_file")

I have done this with strcpy and strcat in the following way:

char t-filename[]="Manipulated-", filename[50], td-filename[50];
memset ( filename, '\0', FILE_NAME);

printf("Please enter the filename - \n");
scanf( "%30s", filename );
strcpy(td-filename,filename);
strcat(t-filename,td-filename);
printf("%s\n", t-filename);

Now printf functions prints the t-filename as "Manipulated-input_file"

After this part, I have a section where I open "input_file" and do something.

fptr = fopen(filename, "r");
while ( fgets (line, sizeof line, fptr) != NULL)
{
 ...do something...
}
fclose(fptr);

Later I wanted to open a file with name 't-filename' at the end of the code:

tptr = fopen(t-filename, "w");
fprintf(tptr,"something");
fclose(tpr);

When I compile this code I am getting "Segmentation fault (core dumped)".

I do not know what went wrong. Can somebody help?

Can you post the real code instead of this? Given that `t-` etc are not valid identifiers in the C language and this code can never compile. — Lundin, Jun 16 '20 at 10:15
Please try to create a proper [mcve], preferably one that we can copy-paste (as is without modifications) and try ourselves. — Some programmer dude, Jun 16 '20 at 10:15
As a hint though: Think about the size of `t-filename`. How many elements do you think that array will have? — Some programmer dude, Jun 16 '20 at 10:16
From the edit. As commented `transfilename` has no spare elements into which `strcat(transfilename,translatedfilename);` can work. It immediately overflows. — Weather Vane, Jun 16 '20 at 10:24

score 4 · Accepted Answer · answered Jun 16 '20 at 10:29

4

transfilename doesn't have enough space to hold additional items added there by strcat(transfilename,translatedfilename);. It's already full with "Translated-" since [] gives you the exact amount of characters in the initializing string, plus a null terminator.

You need to either change it to char transfilename [LARGE_ENOUGH] or use dynamic memory allocation to change the size in run-time.

answered Jun 16 '20 at 10:29

Lundin

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Can you tell me how can I set dynamic memory in this case? Does it mean that I have to use char transfilename[SIZE] after reading the 'filename'? – JohnP Jun 16 '20 at 10:50

score 2 · Answer 2 · answered Jun 16 '20 at 10:30

strcat(t-filename,td-filename);

t-filename is not large enough to accommodate the result and it is an Undefined Behaviour.

same here

strcat(transfilename,translatedfilename);

transfilename is not large enough to accommodate the result and it is an Undefined Behaviour.

Segmentation fault strcpy/strcat

2 Answers2