C用fseek实现替换文件中字符

使用fseek和ftell来移动到行首，替换字符串。代码如下

#include 
#include 
#include 
#define	BUFF_SIZE 256
int showHelp(){
    fputs("Usage:n", stdout);
    fputs("progname [filename] [oldstr] [newstr]n", stdout);
    fputs("n", stdout);
    return -1;
}
int replace(char *line, char *oldstr, char *newstr){
    char buf[BUFF_SIZE];
    char *p;
    int iLen = 0;
    p = strstr(line, oldstr);
    if(p) {
        memset(buf, 0x00, sizeof(buf));
        strcpy(buf, p + strlen(oldstr));
    }else{
        return 0;
    }
    iLen = strlen(buf);
    if ( buf[iLen-1] == 'n')
        buf[iLen]=0;
    *p = '';
    strcat(p, newstr);
    strcat(p, buf);
    printf("p=%sn", p);
    return 1;
}
int main(int argc, char **argv)
{
    char buf[BUFF_SIZE];
    FILE *fp;
    int iNum = 0;
    int i = 0;
    long sttell, endtell;
    if(argc != 4) {
        showHelp();
        return -1;
    }
    fp = fopen(argv[1],"r+");
    if(fp == NULL) {
        perror("open file error");
        return -1;
    }
    while( ! feof(fp) ) {
        sttell = ftell(fp);
        fgets(buf, 256, fp);
        if(replace(buf, argv[2], argv[3]) != 0) {
            endtell = ftell(fp);
            fseek(fp, sttell - endtell, 1);
            fputs(buf, fp);
            fseek(fp, sttell - endtell, 1);
            iNum++;
        }
    }
    fclose(fp);
    printf("num=[%d]n", iNum);
    return 0;
}