08. C Pro File Operation

// 一、常用的文件操作函数
#define MAX 50
void main() {
int num, i, array[MAX];
FILE *fp;
long offset;
for (i = 0; i < MAX; i++) { array[i] = i+10; }

if ((fp = fopen("123.txt", "wb")) == NULL) { // 写指针
printf("Error for open file to write\n");
exit(1);
}
if (fwrite(array, sizeof(int), MAX, fp) != MAX) { // 写入失败
printf("Error for writing data to file\n");
exit(1);
}
fclose(fp);

if ((fp = fopen("123.txt", "rb")) == NULL) { // 读指针
printf("Open file to Read : Error!\n");
exit(1);
}
printf("Please input offset or -1 to quit:");
scanf("%ld", &offset); // offset == 跳过的数字个数
if (offset < 0) exit(1); // 其实负数表示从当前位置 -> 反向偏移)
while (offset // fseek(fp, offset, SEEK_SET) == 0,表示成功定位到第offset个字节
if (fseek(fp, (offset * sizeof(int)), SEEK_SET) != 0) {
printf("偏移量位置，定位失败\n");
exit(1);
}
// fread(*buffer, 数据块的字节数, 数据块的块数，fp)
fread(&num, sizeof(int), 1, fp);
printf("The offset is %ld, its value is %d.Location:%ld \n", offset, num, ftell(fp));
offset += 1;
}
fclose(fp);
}

// 二、file copy
void main() {
FILE *oldf, *newf;
if ((oldf = fopen("123.txt", "rb")) == NULL || (newf = fopen("123_copy.txt", "wb")) == NULL) {
exit(1);
}
while (!feof(oldf)) {
fputc(fgetc(oldf), newf);
}
fclose(oldf) && fclose(newf);
printf("copy done\n");
}

// 三、有符号整数强制转换位无符号整数
void main() {
unsigned int a = 6;
int b = -20;
printf("a+b=%u\n", a + b);
(a + b == -14) ? puts("True") : puts("False");
}

// 四、malloc/ calloc/ realloc
int main(void) {
int i;
char *buffer;
printf("input number of chars:");

scanf("%d", &i);
if ((buffer = (char*)malloc(sizeof(i + 1))) == NULL) exit(1);

for (int j = 0; j < i; j++) buffer[j] = rand() % 26 + 'a';
buffer[i] = '\0';

printf("%s\n", buffer);
free(buffer);

getchar(); return 0;
}