Create string.out from string.in in C - c

Question

I want to read in from the arguments a file with a name of format filename.in and output a file with the exact same filename but with .out extension instead of .in (filename.out). The key is that filename can be anything.
This code compiles correctly and does the job I want, but when I run it with valgrind I get a bunch of errors. Can anyone tell me what's causing the errors?
int main(int argc, char * argv[])
{
int i, dotIndex;
char extOut[] = ".out";
char *filenameIn, *filenameOut, *aux;
FILE *fpIn, *fpOut;
filenameIn = argv[1];
aux = strchr(filenameIn, '.');
dotIndex = aux - filenameIn;
aux = (char *)malloc((strlen(filenameIn) - 1)*sizeof(char));
for(i = 0; i < dotIndex; i++)
aux[i] = filenameIn[i];
filenameOut = (char *)malloc((strlen(aux) + 5)*sizeof(char));
strcat(aux, extOut);
strcpy(filenameOut, aux);
/* open input file */
fpIn = fopen(filenameIn,"r");
if(fpIn == NULL) {
printf("Open error of input file\n");
exit(2);
}
/* open output file */
fpOut = fopen(filenameOut,"w");
if(fpOut == NULL) {
printf("Open error of output file\n");
exit(3);
}
fclose(fpOut);
fclose(fpIn);
free(aux);
free(filenameOut);
exit(0);
}
The valgrind report:
Command: ./doorsmaze text.in
==3493==
==3493== Invalid write of size 1
==3493== at 0x402C36B: strcat (in /usr/lib/valgrind/vgpreload_memcheck-x86-linux.so)
==3493== by 0x80487AB: main (main.c:64)
==3493== Address 0x41ef02e is 0 bytes after a block of size 6 alloc'd
==3493== at 0x402BE68: malloc (in /usr/lib/valgrind/vgpreload_memcheck-x86-linux.so)
==3493== by 0x8048716: main (main.c:55)
==3493==
==3493== Invalid write of size 1
==3493== at 0x402C390: strcat (in /usr/lib/valgrind/vgpreload_memcheck-x86-linux.so)
==3493== by 0x80487AB: main (main.c:64)
==3493== Address 0x41ef030 is 2 bytes after a block of size 6 alloc'd
==3493== at 0x402BE68: malloc (in /usr/lib/valgrind/vgpreload_memcheck-x86-linux.so)
==3493== by 0x8048716: main (main.c:55)
==3493==
==3493== Invalid read of size 1
==3493== at 0x402C6C5: strcpy (in /usr/lib/valgrind/vgpreload_memcheck-x86-linux.so)
==3493== by 0x80487BF: main (main.c:65)
==3493== Address 0x41ef02e is 0 bytes after a block of size 6 alloc'd
==3493== at 0x402BE68: malloc (in /usr/lib/valgrind/vgpreload_memcheck-x86-linux.so)
==3493== by 0x8048716: main (main.c:55)
==3493==
==3493==
==3493== HEAP SUMMARY:
==3493== in use at exit: 0 bytes in 0 blocks
==3493== total heap usage: 4 allocs, 4 frees, 719 bytes allocated
==3493==
==3493== All heap blocks were freed -- no leaks are possible
==3493== Command: ./doorsmaze text.in
==3493==
==3493== Invalid write of size 1
==3493== at 0x402C36B: strcat (in /usr/lib/valgrind/vgpreload_memcheck-x86-linux.so)
==3493== by 0x80487AB: main (main.c:64)
==3493== Address 0x41ef02e is 0 bytes after a block of size 6 alloc'd
==3493== at 0x402BE68: malloc (in /usr/lib/valgrind/vgpreload_memcheck-x86-linux.so)
==3493== by 0x8048716: main (main.c:55)
==3493==
==3493== Invalid write of size 1
==3493== at 0x402C390: strcat (in /usr/lib/valgrind/vgpreload_memcheck-x86-linux.so)
==3493== by 0x80487AB: main (main.c:64)
==3493== Address 0x41ef030 is 2 bytes after a block of size 6 alloc'd
==3493== at 0x402BE68: malloc (in /usr/lib/valgrind/vgpreload_memcheck-x86-linux.so)
==3493== by 0x8048716: main (main.c:55)
==3493==
==3493== Invalid read of size 1
==3493== at 0x402C6C5: strcpy (in /usr/lib/valgrind/vgpreload_memcheck-x86-linux.so)
==3493== by 0x80487BF: main (main.c:65)
==3493== Address 0x41ef02e is 0 bytes after a block of size 6 alloc'd
==3493== at 0x402BE68: malloc (in /usr/lib/valgrind/vgpreload_memcheck-x86-linux.so)
==3493== by 0x8048716: main (main.c:55)
==3493==
==3493==
==3493== HEAP SUMMARY:
==3493== in use at exit: 0 bytes in 0 blocks
==3493== total heap usage: 4 allocs, 4 frees, 719 bytes allocated
==3493==
==3493== All heap blocks were freed -- no leaks are possible
==3493==
==3493== For counts of detected and suppressed errors, rerun with: -v
==3493== ERROR SUMMARY: 6 errors from 3 contexts (suppressed: 0 from 0)
==3493== For counts of detected and suppressed errors, rerun with: -v
==3493== ERROR SUMMARY: 6 errors from 3 contexts (suppressed: 0 from 0)

Answer 1

You have some issues in your code
You might not get the last . in the filename
aux = strchr(filenameIn, '.');
better use strrchr. You should also check for NULL, just in case the filename doesn't include an extension.
You allocate memory large enough to hold filename.in (excluding the nul byte)
aux = (char *)malloc((strlen(filenameIn) - 1)*sizeof(char));
You do your own version of strncpy(aux, filenameIn, dotIndex)
for(i = 0; i < dotIndex; i++)
aux[i] = filenameIn[i];`
You write beyond the allocated buffer, because you miss the space for the last character plus the nul byte
strcat(aux, extOut);
To fix this, you must allocate enough memory, which is the length of filename.in plus 1 char ("out" is one longer than "in") plus nul byte
aux = (char *)malloc((strlen(filenameIn) + 2)*sizeof(char));