C程序出现段错误与指针比较警告，如何解决？

核心错误：strcmp调用的语法错误 #

你的段错误和编译器警告完全是因为getUserInput()函数里的strcmp参数写法错误：
原错误代码：

if(strcmp(createPass, userInput == 0)){
    storeServiceEmailPass();
} else if (strcmp(readAllPass, userInput == 0)){
    readAllData();
}

这里的userInput == 0是将数组与空指针做比较（数组永远不可能为NULL，因此编译器抛出警告），随后把这个布尔值（0）当作指针传给strcmp的第二个参数，属于非法指针操作，直接触发段错误。

正确写法：先调用strcmp，再判断返回值是否为0：

if(strcmp(createPass, userInput) == 0){
    storeServiceEmailPass();
} else if (strcmp(readAllPass, userInput) == 0){
    readAllData();
}

其他需要修复的问题 #

1. 读取文件的模式错误
   readAllData()里用"a"（追加写模式）打开文件不符合读取逻辑，应改为"r"（只读模式）：

FILE * passfile = fopen("Passwords.txt", "r");

2. 未检查文件操作的返回值
   所有fopen调用都必须检查是否成功，避免空指针引发的未定义行为，比如createPassDoc()修改为：

void createPassDoc() {
    FILE * passfile = fopen("Passwords.txt", "w");
    if (passfile == NULL) {
        perror("Failed to create file");
        exit(EXIT_FAILURE);
    }
    fclose(passfile);
    printf("File Created.\n");
}

storeServiceEmailPass()和readAllData()里的fopen也需要添加同样的检查逻辑。

3. 重复换行问题
   fgets会读取输入的换行符，原代码中printf("%s\n", savedData)会导致输出重复换行，改为printf("%s", savedData)即可。

完整修正后的代码 #

#include <stdio.h> 
#include <stdlib.h>
#include <string.h>

void createPassDoc() {
    FILE * passfile = fopen("Passwords.txt", "w");
    if (passfile == NULL) {
        perror("Failed to create file");
        exit(EXIT_FAILURE);
    }
    fclose(passfile);
    printf("File Created.\n");
}

void storeServiceEmailPass() {
    char ServiceEmailPassword[300];
    FILE * passfile = fopen("Passwords.txt", "a");
    if (passfile == NULL) {
        perror("Failed to open file for appending");
        exit(EXIT_FAILURE);
    }
    printf("Please enter the following: Service, Email, Password.\n");
    fgets(ServiceEmailPassword, 300, stdin);
    fprintf(passfile, "%s", ServiceEmailPassword);
    fclose(passfile); 
}

void readAllData() {
    char savedData[5000];
    FILE * passfile = fopen("Passwords.txt", "r");
    if (passfile == NULL) {
        perror("Failed to open file for reading");
        exit(EXIT_FAILURE);
    }
    printf("Here is your saved Data:\n");
    while(fgets(savedData, 100, passfile)) {
        printf("%s", savedData);
    }
    fclose(passfile);
}

void getUserInput() {
    char userInput[255];
    char createPass[] = "Store Service\n";
    char readAllPass[] = "Show Passwords\n";

    printf("What would you like to do?\n");
    fgets(userInput, 255, stdin);
    if(strcmp(createPass, userInput) == 0){
        storeServiceEmailPass();
    } else if (strcmp(readAllPass, userInput) == 0){
        readAllData();
    } else {
        printf("Please enter a command such as: Store Service or Show Passwords.\n");
    }
}

int main() {
    createPassDoc();
    getUserInput();
    return 0;
}

内容的提问来源于stack exchange，提问作者WhatsEndoDoing