Thread: Unpredicted outcome using char array, strncpy, and strncat

Hello,

I am trying to write a program that saves multiple newly created data files that are named with a prefix that is included as one of the arguments. The aim is to create a string array of the output filename.

See the relevant piece of code below, with added printf statements I have used to try and debug:

size_t prefixLength = strlen(argv[3]);
printf("\nprefix length is %d\n",prefixLength);

char fout1[prefixLength+11];
char fout2[prefixLength+11];
char fout3[prefixLength+11];
char fout4[prefixLength+11];
char fout5[prefixLength+11];
char fout6[prefixLength+11];

strncpy(fout1,argv[3],prefixLength);
strncpy(fout2,argv[3],prefixLength);
strncpy(fout3,argv[3],prefixLength);
strncpy(fout4,argv[3],prefixLength);
strncpy(fout5,argv[3],prefixLength);
strncpy(fout6,argv[3],prefixLength);
printf("\n%s",fout1);
printf("\n%s",fout2);
printf("\n%s",fout3);
printf("\n%s",fout4);
printf("\n%s",fout5);
printf("\n%s\n",fout6);

strncat(fout1,".scale1.nii",(prefixLength+11));
strncat(fout2,".scale2.nii",(prefixLength+11));
strncat(fout3,".scale3.nii",(prefixLength+11));
strncat(fout4,".scale4.nii",(prefixLength+11));
strncat(fout5,".scale5.nii",(prefixLength+11));
strncat(fout6,".scale6.nii",(prefixLength+11));

printf("\n%s",fout1);
printf("\n%s",fout2);
printf("\n%s",fout3);
printf("\n%s",fout4);
printf("\n%s",fout5);
printf("\n%s\n",fout6);

I execute the program with "killme" as the 3rd argument and get the following output from the printf statements;

prefix length is 6

killme
killme
killme��ժ���p[=
killme����<���<<l��W�killme��ժ���p[=
killme
killme

killme.scale1.nii
e4.nii
killme��ժ���p[=.scale3.nii.scale4.nii
killme����<���<<l��W�killme��ժ���p[=.scale3.nii.scale4.nii
killme.scale5.nii
killme.scale6.nii

As you can see outputs 1,5, and 6 are correct, but, 2,3, and 4 are not. I do not understand why this is happening, but feel I must be making a basic error. Can anybody please explain this to me>

Thank you for any advice.

Joe

It appears that you're not reserving enough room for the end of string characters. Also have you considered using sprintf() to piece together the strings?

Originally Posted by jimblumberg

It appears that you're not reserving enough room for the end of string characters. Also have you considered using sprintf() to piece together the strings?

[QUOUTE=Salem;]Well strncpy doesn't automatically append a \0.
You have to make sure it's a proper string.[/QUOTE]

Thanks both. I didn't consider the use of sprintf, but perhaps that might be easier.

Joe