Shell C中的动态分配和分段故障

Question

I want to dynamic allocate memory for line and argv and realloc if needed but i get errors when i do it like this:

char* argv = malloc(sizeof(char)*BUFFER); assignment makes integer from pointer without a cast in many lines like strcmp. Error log : https://i.imgur.com/qsYNinq.jpg

Also i want it to print nothing than $(space) when someone hits ENTER but i get Segmentation fault(Core dumped) with the code or not scanf("%s",line); if(line == "\\n") printf("$ "); scanf("%s",line); if(line == "\\n") printf("$ ");

Thanks in advance.

Answer 1

If this

char* argv = malloc(sizeof(char)*BUFFER);

results in an assignment makes integer from pointer without a cast warning, the actual code you are compiling probably misses an

#include <stdlib.h>

directive at the start of the file. GCC still implements implicit function definitions (long removed from the C standard), so it automatically supplies a declaration of malloc like this:

int malloc();

But this is definition is useless on 64-bit architectures because it clips to upper 32 bits of the pointer value (casting the return value back to char * will not restore them).

Current versions of GCC will warn about the missing declaration of malloc . You really should compile with -Wall to get this warning (it was not enabled by default with older GCC versions).

Answer 2

In the current code, argv is a array of pointers, but it the proposed code char* argv = malloc(sizeof(char)*BUFFER); it becomes an array of characters .

When you later try to do argv[i]=token; , you try to assign a pointer ( token ) to a mere character, which causes the warning assignment makes integer from pointer without a cast .

The correct way is to let argv still be a pointer to pointers:

int cmd_argc = 10;
char **argv = malloc(cmd_argc * sizeof(char *));

If you later need more parameters in argv array, you will be able to realloc:

cmd_argc *= 2;
if (NULL == realloc(argv, cmd_argc * sizeof(char *))) {
    // memory allocation error
    ...
}

But by convention , argc and argv are used for the parameters of the current program, so you should use another name like for example cmd_argv .