Question

基本上我想要做的是从文件中读取一个数字，将值递增1，然后将数字写回同一个文件。使用fork（）应该让两个进程都访问文件但是使用锁来轮流。我一直遇到分段错误。

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

void appendValue(FILE *, int *);
int readValue(FILE *, int *);
void lockFile(FILE *);
void unlockFile(FILE *);
void whatProcess(pid_t *pID);

int main(void) {
    pid_t pID;
    pID = fork();
    int value = 0, counter = 0;
    int *valPtr = &value;
    pid_t *pidPtr = &pID;
    FILE *file = fopen("output.txt", "a+");
    lockFile(file);

    while(counter < 1000) {
        whatProcess(pidPtr);
        value = readValue(file, valPtr);
        value++;
        appendValue(file, valPtr);
        rewind(file);
        counter++;
    }
    unlockFile(file);       
    fclose(file);
    printf("\n");
    return 0;
}

void whatProcess(pid_t *pID) {
    if(*pID > 0) {
        printf("\n --- In Parent ---");
    } else if(*pID == 0) {
        printf("\n --- In Child ---");
    } else {
            printf("\n --- fork() Failed ---");
    }
}

void lockFile(FILE *file) {
    int lock;
    lock = lockf(fileno(file), F_LOCK, 0);
        while(lock != 0) {}
        if(lock == 0) {
            printf("\nPID %d: Lock Successful", getpid());
        } else {
            printf("\nPID %d: Lock Unsuccessful", getpid());
        }   
}

void unlockFile(FILE *file) {
    int lock;
    lock = lockf(fileno(file), F_LOCK, 0);
        while(lock != 0) {}
        if(lock == 0) {
            printf("\nPID %d: Unlock Successful", getpid());
        } else {
            printf("\nPID %d: Unlock Unsuccessful", getpid());
        }   
}

void appendValue(FILE *file, int *value) {
    fprintf(file, "%d\n", *value);
}

int readValue(FILE *file, int *value) {
    while(!feof(file)) {
        fscanf(file, "%d", value);
    }
    return *value;
}

Answer 1

fscanf中的readValue正在写入未分配的位置。您将value作为指针传递，因此无需使用运算符的地址。

int readValue(FILE *file, int *value) {
    fscanf(file, "%d", value);
    printf("\nreadValue(): %d", *value);
    return *value;
}

或者，甚至更好：

int readValue(FILE *file, int *value) {
    if (fscanf(file, "%d", value) == 1) {
        printf("\nreadValue(): %d", *value);
        return 0;
    }
    return -1;
}

您当前的功能并不表示它是成功还是失败。添加状态返回值（零，-1很常见）或省略错误检查并改为执行此操作：

int readValue(FILE *file) {
    int buf;
    if (fscanf(file, "%d", &buf) == 1) {
        printf("\nreadValue(): %d", buf);
    } else {
        perror("readValue(): fscanf failed");
    }
    return buf; /* could be a garbage value, use at own risk */
}

在C中使用fork（）时出现分段错误

1 个答案: