C防止输入超过30个字符

Question

Hi I'm having an issue with my code. I need it to run so that if the user enters more than 30 characters for the "fileName" variable it will "return;" but the code I have below doesn't work. Any ideas?

FILE *packetFile; //initialize file handle
char fileName[30]; //initialize
int i;
int length = 0;

length = strlen(fileName);
getchar();

printf("Please enter a file name (Maxium 30 characters): ");
fgets(fileName, 30, stdin);
sscanf(fileName, "%s", fileName);

if(fileName[0] == '\n'){
    return;
}

if(strlen(fileName) > 30)
    return;

if((packetFile = fopen(fileName,"w"))==NULL) {

    printf("Unable to open the file: %s\n",fileName);

} else {

    printf("%d packets have been saved to file named %s", pCount, &fileName);

    for(i=0;i<pCount;i++) {

     fprintf(packetFile,"%04i:%04i:%04i:%04i:%s\r\n",pRecords[i].source,pRecords[i].destination,pRecords[i].type,pRecords[i].port,pRecords[i].data);

    }

        fclose(packetFile);

}

Answer 1

Multiple reasons. You're only allocating room for 30 characters, so a strlen will never return a result > 30 in your case. You're also only reading max 30 chars in your fgets call.

A possible fix is to allocate more space to begin with and use strlen . Another solution is to keep the limited allocation, and use a loop to read individual characters from stdin - if you don't reach a newline character in time, you just break out and return.

On POSIX systems, getline provides a neat solution. It reads a whole line, and it returns the length. Only thing to remember is that getline allocates memory on the heap, so you need to free it when you're done.

Something along the lines of:

char *filename = NULL;
size_t linecap = 0; // These initial values tells getline to allocate as much space as needed
ssize_t linelen = getline(&filename, &linecap, stdin);
if (linelen < 0)
    ...
if (linelen > 30)
    // Treat as error or whatever

free(filename);

(shameless borrowing from the man page)

Answer 2

    char fileName[30+2];//+1 for '\n', +1 for '\0'
    int i;
    int length = 0;

    printf("Please enter a file name (Maxium 30 characters): ");
    fgets(fileName, sizeof(fileName), stdin);

    if(fileName[0] == '\n'){
        return;
    }
    length = strlen(fileName);
    if(fileName[length-1]!='\n'){
        return;
    }