Concatenating file extension to base name in C

For some reason the following is not working:

int i;
for(i = 1; i < argc; i++) // Create thread for each dataset.
{
    filename = strcat(argv[i], ".sdx"); // Concatenate file-extension '.sdx' to basename.
    pthread_attr_init(&attr); // Set the attribute of the thread (default).
    pthread_create(&tid[i], &attr, start_routine, filename); // Create thread.
    pthread_join(tid[i],NULL); // Join thread after it completed.
}

It works if I only pass in one file, but more than that gives a segmentation fault. I dont understand, everything works if I dont concatenate the file extension, and instead pass in the full filename (including its extension) as a command line argument.

Best How To :

You should not modify argv[i] directly. Copy it into local buffer instead.

int i;
for(i = 1; i < argc; ++i)
{
    char *filename = malloc(strlen(argv[i]) + 4 + 1);
    sprintf(filename, "%s.sdx", argv[i]);
    pthread_attr_init(&attr); // Set the attribute of the thread (default).
    pthread_create(&tid[i], &attr, start_routine, filename); // Create thread.
    pthread_join(tid[i],NULL); // Join thread after it completed.
    free(filename);
}

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