Strcat初始化值

Question

I have code like this:

char *all_arguments = (char *) malloc(sizeof(char)*argc);

for(int i=1; i<argc; i++) {
    strcat(all_arguments, argv[i]);
}

valgrind output:

==20425== Conditional jump or move depends on uninitialised value(s)
==20425==    at 0x4C30C0A: strcat (in /usr/lib/valgrind/vgpreload_memcheck-amd64-linux.so)
==20425==    by 0x40065B: main (main.c:15)

What is wrong? I know that I should initialize all_arguments, but when I do "all_arguments = NULL;" I got a segmentation fault.

Answer 1

Two problems:

1) You don't allocate enough memory for all_arguments . Even if each argument is only a single character, you still don't have space for the string terminator. And presumably, at least some of the arguments are larger.

2) Since all_arguments is not a string when it's first allocated, you can't pass it to strcat . The arguments to strcat must both be strings. The first time you call strcat , all_arguments is not valid string.

Answer 2

argc holds number of parameters received to function main I assume.

So, before you copy new memory, simply realloc (extend) your memory for next entry.

char * all_parameters = malloc(1);

*all_parameters = 0; //Create valid string
for (int i = 1; i < argc; i++) {
    //Extend to new size
    all_parameters = realloc(all_parameters, strlen(all_parameters) + 1 + strlen(argv[i]);
    //Copy together
    strcat(all_parameters, argv[i]);
}

Another option is to sum all lengths first and then use malloc only once.

int total_sum=0; // remember to initialize variable 
for (int i = 1; i < argc; i++) {
    total_sum += strlen(argv[i]);
}

char * all_parameters = malloc(total_sum + 1);
*all_parameters = 0;
for (int i = 1; i < argc; i++) {
    strcat(all_parameters, argv[i]);
}